1
Which of the following is a chemical change?
💡 Hint:
In which one does a new substance (gas, ash) form?
Explanation:
Coal burns to form CO2 and ash — new substances, so it is chemical. In the other three the substance stays the same.
Class 10 · Science · Chapter 1 · Bihar Board (BSEB)CBSE · NCERT 2026-27
Chemical Reactions and Equations
रासायनिक अभिक्रियाएँ एवं समीकरण
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What is a chemical reaction? Physical vs chemical change
रासायनिक अभिक्रिया क्या है? भौतिक बनाम रासायनिक परिवर्तन · NCERT Introduction
Look around you — milk left out in summer turns sour, an iron pan or nail rusts in moist air, grapes ferment, food gets cooked, our body digests food and we breathe. In all of these, the nature and identity of the starting substance changes.
Definition
When a new substance forms, we call it a chemical change, and the process by which it happens is called a chemical reaction.
| Basis | Physical change | Chemical change |
|---|---|---|
| Does a new substance form? | No — the substance stays the same | Yes — a new substance forms |
| Reversing it | Usually easy (ice ↔ water) | Usually hard (curd does not turn back into milk) |
| Examples | Ice/wax melting, salt dissolving, folding paper | Milk → curd, burning coal, rusting, digestion |
Remember
Ice → water = a physical change (H2O stays the same). Milk → curd = a chemical change (lactose, the sugar in milk, turns into lactic acid).
Worked example — Identify — physical or chemical?Example
Burning a candle — it has both! Wax melting = physical; wax burning at the wick to form CO2 and water vapour = chemical.
Dissolving sugar in water — physical (evaporate the water and you get the sugar back).
Digestion of food — chemical (food breaks down into new substances such as glucose).
10-second revision
- New substance formed = chemical; only the state/shape changes = physical
- Milk → curd, rusting, digestion, respiration, burning coal — chemical
- Ice/wax melting, salt dissolving, folding paper — physical
Board tip · BSEBBoard tip
Objective questions often ask “Which is a chemical change?” — check whether a new substance forms. Milk to curd, rusting, digestion = chemical; ice melting, water boiling = physical.
Board tip · CBSEBoard tip
CBSE asks situation-based questions: “A cut apple turns brown — physical or chemical change? Give a reason.” Always write the evidence (change in colour / smell / gas / temperature).
Check your understandingall correct = mastery ★
2
Ice melting into water is a chemical reaction.
Explanation:
H2O changed from solid to liquid — same substance, only the state changed. This is a physical change.
3
Why is milk turning into curd a chemical change?
💡 Hint:
Ask — did a new substance form or not?
Explanation:
The taste (sour) and texture of curd change because a new substance forms; it cannot be turned back into milk.
Activity 1.1 — burning a magnesium ribbon
क्रियाकलाप 1.1 — मैग्नीशियम रिबन का जलना · NCERT Activity 1.1 · In-text Q 1
Experiment (under the teacher's supervision)Activity
Clean a 3–4 cm long magnesium ribbon by rubbing it with sandpaper, hold it with tongs, burn it over a spirit lamp or burner and collect the ash in a watch-glass.
What do you see? The ribbon burns with a dazzling white flame and a white powder is left — this is magnesium oxide (MgO).
2Mg(s) + O2(g) → 2MgO(s)
Why rub it? Magnesium kept in air slowly gets coated with a layer of MgO. This layer stops Mg from meeting oxygen, so the ribbon does not burn properly. Rubbing removes the layer and exposes shiny Mg.
What type of reaction is this?Notes
- Combination — two substances (Mg and O2) give one product (MgO)
- Exothermic — heat and light are given out
- Oxidation — Mg gains oxygen
Remember
Safety: hold the ribbon with tongs, keep the burning ribbon away from your eyes and do not stare at its bright light for long.
Worked example — NCERT in-text question 1Example
Question: Why should a magnesium ribbon be cleaned before burning it in air?
Answer: Oxygen in the air forms a layer of magnesium oxide on the magnesium ribbon, which hinders burning. Rubbing with sandpaper removes this layer, so pure Mg reacts directly with oxygen and burns easily.
10-second revision
- Rubbing = removing the MgO layer
- 2Mg(s) + O2(g) → 2MgO(s)
- Combination + exothermic + oxidation of Mg
Board tip · BSEBBoard tip
For the 2-mark question “Why is a magnesium ribbon cleaned with sandpaper before burning?” remember one line: to remove the layer of MgO / carbonate so that Mg combines directly with oxygen.
Board tip · CBSEBoard tip
Write it as observation → conclusion: dazzling white flame + white powder → a new substance, MgO, has formed. Add the balanced equation 2Mg + O₂ → 2MgO.
Check your understandingall correct = mastery ★
1
The white powder left after burning a Mg ribbon is —
Explanation:
Mg combines with oxygen in the air to form magnesium oxide (MgO) — a white powder.
2
Why is a Mg ribbon rubbed with sandpaper before burning?
💡 Hint:
Which layer forms on a ribbon kept in air?
Explanation:
The MgO layer stops oxygen from reaching Mg; rubbing removes the layer and the ribbon burns properly.
3
Balance the equation by filling in the coefficients (blank = 1).
Mg + O2 → MgO
💡 Hint:
There are 2 O atoms on the left — what will you put before MgO on the right? Then equalise Mg.
Explanation:
O: 2 = 2, Mg: 2 = 2 ✓
Activities 1.2 & 1.3 — signs that a reaction has happened
क्रियाकलाप 1.2, 1.3 और अभिक्रिया के संकेत · NCERT Activity 1.2, 1.3
Activity 1.2Activity
Add a colourless solution of potassium iodide to a colourless solution of lead nitrate — a bright yellow precipitate forms at once. This is lead iodide (PbI2).
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
Activity 1.3Activity
Take zinc granules in a conical flask and add dilute sulphuric acid (or dilute hydrochloric acid) — bubbles (H2 gas) form around the zinc and the flask becomes warm. Bring a burning splinter near it: a “pop” sound = the test for hydrogen.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
How do we know a chemical reaction has taken place?Notes
- Change in state
- Change in colour
- Evolution of a gas
- Change in temperature
- Formation of a precipitate (an insoluble solid)
Worked example — Board-style question: write four observations (2 marks)Example
(i) Change in colour — when an iron nail is put in CuSO4 solution, the blue colour turns pale green.
(ii) Evolution of a gas — bubbles of H2 when dilute H2SO4 is poured on Zn.
(iii) Change in temperature — the beaker becomes hot when water is added to CaO.
(iv) Formation of a precipitate — yellow solid PbI2 from Pb(NO3)2 + KI.
10-second revision
- Pb(NO₃)₂ + KI → yellow PbI₂ precipitate
- Zn + dilute acid → H₂ (pop test) + the flask gets warm
- Signs: state · colour · gas · temperature · precipitate
Board tip · BSEBBoard tip
The list of four signs of a reaction (change of state, change of colour, gas evolved, change in temperature) helps in both objective and short answers — remember one example of each.
Board tip · CBSEBoard tip
A question may say: “The test tube became warm and bubbles appeared — what do you conclude?” → an exothermic reaction and a gas formed. Link each sign to its reason.
Check your understandingall correct = mastery ★
1
When solutions of lead nitrate and potassium iodide are mixed, the colour of the precipitate is —
Explanation:
Lead iodide (PbI2) is a bright yellow insoluble solid.
2
How do we test the gas given off when dilute H₂SO₄ is poured on zinc?
💡 Hint:
Hydrogen gas burns in air...
Explanation:
H₂ burns with a “pop” sound near a burning splinter. Lime water turning milky is the test for CO2.
3
Simply cutting a candle into two pieces is a sign of a chemical reaction.
Explanation:
Cutting only changed the shape; no new substance formed — a physical change.
Writing a chemical equation — word, formula and skeletal
रासायनिक समीकरण — शब्द से सूत्र तक · NCERT 1.1, 1.1.1
A short and clear way of writing a reaction = a chemical equation.
1. Word equationNotes
magnesium + oxygen → magnesium oxide
- Left side of the arrow = reactants (the substances that change)
- Right side of the arrow = products (the new substances formed)
- If there are several reactants or products, put + between them; the arrow points towards the products
2. Formula equation — write chemical formulae instead of words:
Mg + O2 → MgO
Now count the atoms — 2 O atoms on the left, only 1 on the right. The atoms are not equal on both sides, so the equation is unbalanced. Such an unbalanced formula equation is called a skeletal chemical equation.
Worked example — Zn + dilute H₂SO₄ (Activity 1.3)Example
Words: zinc + sulphuric acid → zinc sulphate + hydrogen
Formulae: Zn + H2SO4 → ZnSO4 + H2
Count: Zn 1 = 1 · H 2 = 2 · S 1 = 1 · O 4 = 4 → it is already balanced.
10-second revision
- Reactants on the left → products on the right
- Unbalanced formula equation = skeletal equation
Board tip · BSEBBoard tip
Questions that turn a word equation into a formula equation come up directly. Reactants on the left, products on the right, an arrow (→) and “+” in between — never use “=”.
Board tip · CBSEBoard tip
Learn to spot a skeletal equation — CBSE may ask “Why does this equation not obey the law of conservation of mass?” → the atoms are not equal on both sides.
Check your understandingall correct = mastery ★
1
In a chemical equation, the substances written to the left of the arrow are called —
Explanation:
Reactants on the left, products on the right.
2
What is a skeletal equation?
Explanation:
A formula equation in which the number of atoms is not equal on both sides is a skeletal equation.
3
In Mg + O2 → MgO there are ______ atoms of oxygen on the left.
💡 Hint:
Look at the small number written below in O2.
Explanation:
O2 = 2 atoms of oxygen; only 1 in MgO on the right — so it is unbalanced.
Balancing — law of conservation of mass and the hit-and-trial method
संतुलित समीकरण — क्यों और कैसे? · NCERT 1.1.2
Law of conservation of massDefinition
mass can neither be created nor destroyed in a chemical reaction. That means atoms are neither made nor lost; they only join up in new ways. So the number of atoms of each element must be equal on both sides. Such an equation is called a balanced chemical equation.
3 rules of balancingNotes
- Imagine a box around every formula — do not change anything inside the box.
- Only put coefficients in front of formulae: 4H2O ✓ · H2O4 ✗ · (H2O)4 ✗
- Start with the compound that has the most atoms, and within it the element with the most atoms.
Guess → count → correct — this is called the hit-and-trial method. Coefficients must always be the smallest whole numbers.
Worked example — Fe + steam → Fe₃O₄ + H₂ (NCERT example)Example
Fe + H2O → Fe3O4 + H2
| Element | Left | Right |
|---|---|---|
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
Step 1 — O (biggest compound Fe3O4, O = 4 in it): 4 in front of H₂O → Fe + 4H2O → Fe3O4 + H2
Step 2 — H: now H = 8 on the left, so 4 in front of H₂ on the right → Fe + 4H2O → Fe3O4 + 4H2
Step 3 — Fe: Fe = 3 on the right, so 3Fe on the left →
3Fe + 4H2O → Fe3O4 + 4H2
Check: Fe 3 = 3 · H 8 = 8 · O 4 = 4 ✓ balanced.
10-second revision
- Atoms are neither made nor destroyed → balance the equation
- Change only coefficients; never subscripts
- 3Fe + 4H2O → Fe3O4 + 4H2
Board tip · BSEBBoard tip
“Why must a chemical equation be balanced?” — answer: the law of conservation of mass; atoms are neither created nor destroyed. This is a favourite 2-mark question.
Board tip · CBSEBoard tip
While balancing, never change a formula (subscript), change only the coefficients — CBSE assertion–reason questions are built on exactly this.
Check your understandingall correct = mastery ★
1
Balancing a chemical equation is based on which law?
Explanation:
Conservation of mass: the total mass (and the number of atoms) does not change in a reaction.
2
How do you correctly write 4 molecules of water?
💡 Hint:
Nothing inside the formula changes — the number goes in front of the formula.
Explanation:
The coefficient goes in front of the formula: 4H2O. H2O4 would be a completely different substance.
3
Balance the equation by filling in the coefficients (blank = 1).
Fe + H2O → Fe3O4 + H2
💡 Hint:
First equalise O (4 O in Fe3O4), then H, and finally Fe.
Explanation:
Fe 3 = 3, H 8 = 8, O 4 = 4 ✓
Balancing drills — easy, harder and word statements
संतुलन का अभ्यास — आसान से कठिन · NCERT In-text Q 2 · Exercise 5, 6
Easy (NCERT in-text question 2)Notes
H2 + Cl2 → 2HCl
2Na + 2H2O → 2NaOH + H2
Harder — trickNotes
if groups like SO4, NO3, OH stay the same on both sides, count each as one “block”.
3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(SO₄ block: 3 on the left, 3 on the right · Ba 3 = 3 · Al 2 = 2 · Cl 6 = 6)
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
2NaOH + H2SO4 → Na2SO4 + 2H2O
BaCl2 + H2SO4 → BaSO4 + 2HCl
Statement → equation (NCERT exercise 5)Notes
first the word equation, then formulae, then balancing.
- Nitrogen + hydrogen → ammonia: N2 + 3H2 → 2NH3
- Burning of hydrogen sulphide in air: 2H2S + 3O2 → 2H2O + 2SO2
- Potassium + water: 2K + 2H2O → 2KOH + H2
Worked example — Step by step: HNO₃ + Ca(OH)₂Example
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
- Ca: 1 = 1 ✓
- NO₃ block: 1 on the left, 2 on the right → 2 in front of HNO3
- H: on the left 2 (from HNO₃) + 2 (from OH) = 4; on the right 2 in H₂O → 2 in front of H2O
- Check O: left 6 + 2 = 8, right 6 + 2 = 8 ✓
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
Formula = a closed box
Subscripts never change
Group = one block
Count SO₄, NO₃, OH, CO₃ as one unit
H and O last
They are spread over several compounds
Smallest whole numbers
Not 4, 6, 4 → 2, 3, 2
10-second revision
- Groups (SO₄, NO₃, OH) = blocks
- Statement → word equation → formulae → balance
Board tip · BSEBBoard tip
Whenever you write an equation in the exam, write it balanced — unbalanced equations lose marks. At the end, count the atoms of each element on both sides.
Board tip · CBSEBoard tip
Reactions given in words (“barium chloride + sodium sulphate …”) must be converted to formulae and balanced yourself — practise two such statements every day.
Check your understandingall correct = mastery ★
1
Balance the equation by filling in the coefficients (blank = 1).
Na + H2O → NaOH + H2
💡 Hint:
For the 2 H of H2, both NaOH and H₂O must become 2.
Explanation:
Na 2 = 2, H 4 = 4, O 2 = 2 ✓
2
Balance the equation by filling in the coefficients (blank = 1).
BaCl2 + Al2(SO4)3 → BaSO4 + AlCl3
💡 Hint:
Treat SO₄ as one block: Al2(SO4)3 has 3 SO₄ → 3BaSO₄.
Explanation:
Ba 3 = 3, SO₄ 3 = 3, Al 2 = 2, Cl 6 = 6 ✓
3
Balance the equation by filling in the coefficients (blank = 1).
N2 + H2 → NH3
💡 Hint:
2 atoms of N → 2NH₃; then count H.
Explanation:
N 2 = 2, H 6 = 6 ✓
State symbols (s)(l)(g)(aq), ↑ ↓ and reaction conditions
अवस्था संकेत और तीर पर लिखी शर्तें · NCERT 1.1.2 (step VII) · In-text Q 3
To make an equation more informative, we write the physical state of each substance next to it:
| Symbol | Meaning | Example |
|---|---|---|
| (s) | solid | Fe(s), MgO(s) |
| (l) | liquid | H2O(l) |
| (g) | gas | O2(g), steam = H2O(g) |
| (aq) | aqueous solution — dissolved in water | NaCl(aq), CuSO4(aq) |
| ↑ | gas given off | H2↑ |
| ↓ | precipitate formed | BaSO4↓ |
Note — water in the form of steam (a gas) is written as H2O(g):
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
State symbols are usually written only when necessary or when the question asks for them.
Conditions above/below the arrowNotes
conditions such as temperature, pressure, a catalyst or light are written on the arrow —
CO(g) + 2H2(g) 340 atm→ CH3OH(l)
6CO2(aq) + 12H2O(l) sunlight→chlorophyll C6H12O6(aq) + 6O2(aq) + 6H2O(l)
Worked example — NCERT in-text question 3 — write with state symbolsExample
(i) Aqueous solutions of barium chloride and sodium sulphate → insoluble barium sulphate + a solution of sodium chloride:
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(ii) Sodium hydroxide solution + hydrochloric acid solution → sodium chloride solution + water:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Trick: “insoluble / precipitate” → (s) · “solution” → (aq) · pure water → (l) · steam → (g)
10-second revision
- (s) solid · (l) liquid · (g) gas · (aq) aqueous solution
- ↑ gas · ↓ precipitate · conditions above/below the arrow
Board tip · BSEBBoard tip
The meanings of (s), (l), (g), (aq) are asked in objective questions — (aq) = aqueous solution. Remember to write temperature, pressure or catalyst above/below the arrow.
Board tip · CBSEBoard tip
If a full equation with state symbols is asked, put a symbol with every substance, e.g. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
Check your understandingall correct = mastery ★
1
How do you write water in the form of steam in an equation?
Explanation:
Steam is a gas, so H2O(g) — as in 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
2
In BaCl2(aq) + Na2SO4(aq) → BaSO4 + 2NaCl(aq) the state of BaSO4 will be —
💡 Hint:
BaSO₄ does not dissolve in water — it forms a precipitate.
Explanation:
An insoluble precipitate = solid, so BaSO4(s).
3
For the reaction CO + 2H₂ → CH₃OH, the condition “340 atm” is written above the arrow.
Explanation:
Conditions such as temperature, pressure, a catalyst or light are written above/below the arrow.
Types of reactions · combination reaction · whitewashing
अभिक्रियाओं के प्रकार और संयोजन अभिक्रिया · NCERT 1.2, 1.2.1 · Activity 1.4 · In-text Q (substance X)
In a reaction, atoms are neither destroyed nor created, and one element does not change into another — only the bonds between atoms break and new ones form. Reactions are classified on this basis (see the figure below).
Combination reactionDefinition
when two or more substances (elements or compounds) combine to form a single product.
Activity 1.4 (teacher demonstration)Activity
Take a little calcium oxide (quicklime) in a beaker and slowly add water — the beaker becomes very hot and slaked lime (calcium hydroxide) forms.
CaO(s) + H2O(l) → Ca(OH)2(aq) + heat
WhitewashingNotes
a solution of slaked lime is applied to walls. It slowly reacts with CO2 in the air and forms a thin layer of calcium carbonate on the wall — after 2–3 days the wall starts to shine. (The chemical formula of marble is also CaCO3.)
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
More examplesBoard tip
- Burning of coal: C(s) + O2(g) → CO2(g)
- Formation of water from H₂ and O₂: 2H2(g) + O2(g) → 2H2O(l)
Worked example — NCERT in-text question — a solution of substance ‘X’ is used for whitewashingExample
(i) X = calcium oxide (quicklime), formula CaO.
(ii) Reaction with water — slaked lime forms and a lot of heat is released:
CaO(s) + H2O(l) → Ca(OH)2(aq) + heat
Note: the solution applied to the wall is slaked lime, Ca(OH)2; so some books write “X = slaked lime”. But part (ii) asks for “the reaction of X with water” — which only applies to CaO.
10-second revision
- Combination: many → one
- CaO(s) + H2O(l) → Ca(OH)2(aq) + heat — combination + exothermic
- The shine of whitewash = a layer of CaCO3
Board tip · BSEBBoard tip
The whitewash question (quicklime + water) comes again and again: CaO + H₂O → Ca(OH)₂ + heat; on the wall, CO₂ forms a shiny layer of CaCO₃.
Board tip · CBSEBoard tip
In the “substance X” puzzle, X = CaO and slaked lime is Ca(OH)₂ — write both equations along with the identification.
Check your understandingall correct = mastery ★
1
The chemical formula of quicklime is —
Explanation:
Quicklime = CaO (calcium oxide); slaked lime = Ca(OH)2.
2
2–3 days after whitewashing, the wall starts to shine because this forms on it —
💡 Hint:
Which gas in the air does Ca(OH)₂ slowly react with?
Explanation:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) — a thin layer of CaCO₃ gives the shine.
3
C(s) + O2(g) → CO2(g)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
💡 Hint:
Two → one; heat is given out on burning; C gains O.
Explanation:
Burning of coal = combination + exothermic; carbon is oxidised (gain of O).
Exothermic reactions — respiration, burning natural gas, compost
ऊष्माक्षेपी अभिक्रियाएँ — श्वसन, दहन, कंपोस्ट · NCERT 1.2.1 · Exercise 10
Definition
Reactions in which heat is given out along with the formation of products are called exothermic reactions. The beaker getting hot in the CaO + water reaction is an example.
Everyday examplesBoard tip
- Burning of natural gas (methane): CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
- Respiration: food is digested into glucose; in our cells glucose combines with oxygen and provides energy — C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
- Vegetable waste decomposing to form compost
- Burning of coal/wood, burning of a magnesium ribbon
Worked example — NCERT exercise 10 — why is respiration called exothermic?Example
Food (carbohydrates in rice, potatoes, bread) is broken down into glucose during digestion. In the cells, glucose reacts with oxygen to form CO2 and water, and energy is released — this energy lets the body work and keeps it warm. Since energy is given out, respiration is an exothermic reaction.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
10-second revision
- Heat given out = exothermic 🔥
- Respiration, burning, compost, CaO + water
Board tip · BSEBBoard tip
“Why is respiration called an exothermic reaction?” — glucose is oxidised and energy is released: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Board tip · CBSEBoard tip
Connect exothermic examples to daily life — compost heating up, natural gas burning; case-based questions ask exactly this.
Check your understandingall correct = mastery ★
1
Which of the following is an exothermic reaction?
💡 Hint:
In which one does the container get hot by itself?
Explanation:
A lot of heat is released in CaO + water. The other three are decompositions that need energy (endothermic).
2
Respiration is an endothermic reaction.
Explanation:
In respiration, oxidation of glucose releases energy — exothermic.
3
Balance the equation by filling in the coefficients (blank = 1).
CH4 + O2 → CO2 + H2O
💡 Hint:
First equalise H (4 H of CH4 → 2H₂O), then count O.
Explanation:
C 1 = 1, H 4 = 4, O 4 = 4 ✓
Decomposition by heat — FeSO₄, limestone, lead nitrate
वियोजन (अपघटन) — ऊष्मा से · NCERT 1.2.2 · Activity 1.5, 1.6
Definition
When a single reactant breaks down into two or more simpler products, it is called a decomposition reaction. Decomposition caused by heating = thermal decomposition.
Activity 1.5Activity
Heat about 2 g of green ferrous sulphate crystals (FeSO4·7H2O) in a dry test tube. First the water of crystallisation is lost and the colour changes; on further heating it breaks down — there is a smell of burning sulphur.
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
(Fe2O3 = ferric oxide, a reddish-brown solid)
Decomposition of limestone (very useful in industry)Notes
heating calcium carbonate gives quicklime, one use of which is in making cement.
CaCO3(s) heat→ CaO(s) + CO2(g)
Activity 1.6 (teacher demonstration — the fumes are poisonous)Activity
heating lead nitrate powder gives off brown fumes — this is nitrogen dioxide (NO2).
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
Worked example — How do you recognise decomposition?Example
One reactant on the left + two or more products on the right = decomposition.
CaCO3(s) heat→ CaO(s) + CO2(g) — 1 → 2 ✓
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g) — 1 → 3 ✓
(A coefficient of 2 does not make it “two” reactants — it is still one substance.)
10-second revision
- One → many = decomposition; by heat = thermal decomposition
- FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (smell of sulphur)
- CaCO₃ → CaO + CO₂ (cement) · Pb(NO₃)₂ → brown NO₂
Board tip · BSEBBoard tip
Heating ferrous sulphate turns it green → white/brown with a smell of burning sulphur — this observation question is common. With lead nitrate, remember the brown fumes (NO₂).
Board tip · CBSEBoard tip
Practise one balanced equation for each thermal decomposition: CaCO₃ → CaO + CO₂ (used in the lime industry).
Check your understandingall correct = mastery ★
1
The brown fumes given off on heating lead nitrate are of which gas?
Explanation:
Nitrogen dioxide (NO2) is a brown, poisonous gas; O2 is colourless.
2
On heating ferrous sulphate crystals, where does the smell of burning sulphur come from?
Explanation:
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g) — the smell of the oxides of sulphur.
3
CaCO3(s) heat→ CaO(s) + CO2(g)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
💡 Hint:
One → two; heat has to be supplied; nothing gains or loses O.
Explanation:
Thermal decomposition, endothermic; no oxidation or reduction.
Decomposition by electricity and light · opposite of combination
वियोजन — विद्युत और प्रकाश से · NCERT Activity 1.7, 1.8 · In-text Q 2 · Exercise 11, 12
Activity 1.7 — electrolysis of waterActivity
Fix carbon electrodes in a plastic mug and connect them to a 6 V battery. Fill the mug with water + a few drops of dilute sulphuric acid and invert test tubes filled with water over both electrodes. When the current flows, bubbles form at both electrodes.
2H2O(l) electric current→ 2H2(g) + O2(g)
Why is there twice as much gas in one tube? In water (H2O) every O atom has two H atoms, so the volume of hydrogen formed is double that of oxygen (2 : 1). Tests: H2 — a burning splinter burns with a “pop”; O2 — a candle burns more brightly.
Activity 1.8 — by lightActivity
Keep white silver chloride in a china dish in sunlight — after some time it turns grey, because silver metal forms.
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
2AgBr(s) sunlight→ 2Ag(s) + Br2(g)
These reactions are used in black-and-white photography — that is why AgCl/AgBr are kept in dark-coloured bottles.
Decomposition = the opposite of combinationNotes
combination is many → one; decomposition is one → many.
| Combination (many → one) | Decomposition (one → many) |
|---|---|
| 2H2 + O2 → 2H2O | 2H2O → 2H2 + O2 |
| CaO + CO2 → CaCO3 | CaCO3 → CaO + CO2 |
Worked example — NCERT exercise 12 — three forms of energy, three equationsExample
- Heat: CaCO3(s) heat→ CaO(s) + CO2(g)
- Light: 2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
- Electricity: 2H2O(l) electric current→ 2H2(g) + O2(g)
10-second revision
- Electricity: 2H₂O → 2H₂ + O₂ (H₂ : O₂ = 2 : 1)
- Light: 2AgCl → 2Ag + Cl₂ (grey) — photography
- Decomposition = the opposite of combination
Board tip · BSEBBoard tip
In the electrolysis of water, H₂ at the cathode and O₂ at the anode, volume ratio 2 : 1 — this was asked in the 2025 board paper too. Remember the decomposition of AgBr / AgCl by light in black-and-white photography.
Board tip · CBSEBoard tip
Data question: “20 mL of gas at the cathode — how much at the anode?” → 10 mL of O₂. Combination and decomposition are opposites — this comparison is asked.
Check your understandingall correct = mastery ★
1
The ratio of the volumes of H₂ and O₂ formed in the electrolysis of water is —
💡 Hint:
How many H and O are there in the formula H₂O?
Explanation:
In H₂O, H : O = 2 : 1, so the volume of H₂ is double that of O₂.
2
Why does white silver chloride turn grey in sunlight?
Explanation:
Decomposition by light: 2AgCl(s) sunlight→ 2Ag(s) + Cl2(g) — the silver formed looks grey.
3
The exact opposite of a combination reaction is a ______ reaction.
💡 Hint:
One in which a single substance breaks down into many.
Explanation:
Combination: many → one; decomposition: one → many.
Endothermic reactions · exothermic vs endothermic · group activity
ऊष्माशोषी अभिक्रियाएँ और सामूहिक क्रियाकलाप · NCERT 1.2.2 · Exercise 9 · group activity
Definition
In decomposition, energy has to be supplied as heat, light or electricity to break the reactant. Reactions in which energy is absorbed (taken in) are called endothermic reactions.
Try it (NCERT)Activity
Take about 2 g of barium hydroxide in a test tube, add 1 g of ammonium chloride and stir with a glass rod. Touch the bottom of the test tube — it feels cold! Heat was taken from the surroundings = endothermic.
Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH3 + 2H2O
| Exothermic 🔥 | Endothermic ❄️ |
|---|---|
| Heat is given out | Energy is taken in |
| The container gets hot | The container gets cold (or energy must be supplied continuously) |
| CaO + water, burning, respiration, compost | Decomposition of CaCO₃, AgCl in sunlight, electrolysis of water, photosynthesis |
Group activity (NCERT)Activity
Take four beakers A, B, C, D — 25 mL of water in A, B, C and copper sulphate solution in D. Note the temperature. Then add potassium sulphate to A, ammonium nitrate to B, anhydrous copper sulphate to C and fine iron powder to D, stir, and note the temperature again.
| Beaker | What was added | What is usually seen | Conclusion |
|---|---|---|---|
| A | K2SO4 | temperature falls slightly | endothermic |
| B | NH4NO3 | temperature falls a lot (cold) | endothermic |
| C | anhydrous CuSO4 | temperature rises (hot) | exothermic |
| D | Fe powder (in CuSO4 solution) | temperature rises | exothermic |
Write down what your thermometer actually showed — the change in A may be very small. (In A, B, C a salt dissolves/gets hydrated; D is a displacement reaction: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s))
Worked example — NCERT exercise 9 — model answerExample
Exothermic: reactions in which heat is given out along with the products. Example: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Endothermic: reactions in which energy is absorbed. Example: CaCO3(s) heat→ CaO(s) + CO2(g)
10-second revision
- Energy taken in = endothermic ❄️ (most decompositions)
- Ba(OH)₂ + NH₄Cl → the test tube gets cold
Board tip · BSEBBoard tip
Write the difference between exothermic and endothermic as a table (2 marks) — examples: respiration (exothermic), photosynthesis / decomposition of CaCO₃ (endothermic).
Board tip · CBSEBoard tip
Experiment-based question: “The beaker became cold when barium hydroxide and ammonium chloride were mixed” → endothermic. The temperature change is the evidence.
Check your understandingall correct = mastery ★
1
The test tube becomes cold when barium hydroxide and ammonium chloride are mixed. This reaction is —
Explanation:
Heat was taken from the surroundings (your hand), so it feels cold — endothermic.
2
Decomposition reactions are generally endothermic.
Explanation:
Heat, light or electrical energy has to be supplied to break the reactant.
3
What type of reaction is photosynthesis?
💡 Hint:
Do plants take in or give out the energy of sunlight?
Explanation:
Plants absorb the energy of sunlight to make glucose — endothermic.
Displacement reactions · reactivity · metal + dilute acid
विस्थापन अभिक्रिया और सक्रियता श्रेणी · NCERT 1.2.3 · Activity 1.9 · In-text Q 1 · Exercise 2, 3, 7, 14
Definition
When a more reactive element removes a less reactive element from its compound, it is called a displacement reaction: A + BC → AC + B
Activity 1.9Activity
Dip iron nails cleaned with sandpaper in blue copper sulphate solution for about 20 minutes. A brown coating (copper) forms on the nails and the blue colour of the solution fades to pale green (ferrous sulphate forms).
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
More examplesBoard tip
- Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
- Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)
- Refining of silver (NCERT exercise 14): Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
- Copper wire in FeSO4 solution → no reaction (Cu is less reactive than Fe)
Metal + dilute acid → salt + hydrogen (this is also displacement — the metal displaces H from the acid):
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
MCQ trap (NCERT exercise 2)Caution
Fe2O3 + 2Al → Al2O3 + 2Fe — Al removes Fe from iron oxide = a displacement reaction (also redox: Al is oxidised, Fe2O3 is reduced). It is not combination or double displacement.
Worked example — NCERT in-text question — why does the colour of CuSO₄ solution change when a nail is dipped in it?Example
Iron is more reactive than copper. It displaces copper from CuSO4 and forms ferrous sulphate, whose solution is pale green; so the blue colour fades and brown copper deposits on the nail.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
10-second revision
- A more reactive metal displaces a less reactive one
- Fe + CuSO₄: blue → pale green, brown Cu on the nail
- Fe₂O₃ + 2Al → Al₂O₃ + 2Fe = displacement (MCQ)
Board tip · BSEBBoard tip
Iron nail + copper sulphate: blue colour fades / turns pale green, brown coating on the nail — remember both the observation and the equation Fe + CuSO₄ → FeSO₄ + Cu.
Board tip · CBSEBoard tip
Reason with reactivity: “Can Cu displace Fe from FeSO₄?” → no, Cu is less reactive. Such reasoning questions are common in CBSE.
Check your understandingall correct = mastery ★
1
Which metal can displace copper from copper sulphate solution?
💡 Hint:
Which one is above Cu in the reactivity series?
Explanation:
Zn is more reactive than Cu: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s). Ag, Au, Pt are less reactive than Cu.
2
When a copper wire is put in FeSO₄ solution, iron separates out of the solution.
Explanation:
Cu is less reactive than Fe, so it cannot displace Fe — no reaction.
3
Fe2O3 + 2Al → Al2O3 + 2Fe
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
💡 Hint:
What did Al push out? What gained O and what lost O?
Explanation:
Al displaced Fe = displacement; a lot of heat is given out (thermite, used to join rails); Al is oxidised, Fe₂O₃ is reduced = redox.
Double displacement · precipitation · neutralisation
द्विविस्थापन, अवक्षेपण और उदासीनीकरण · NCERT 1.2.4 · Activity 1.10 · In-text Q 2 · Exercise 13, 15
Activity 1.10Activity
Mix barium chloride solution with sodium sulphate solution — a white precipitate forms. This is barium sulphate, which is insoluble in water; sodium chloride stays dissolved.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Here Ba2+ and SO42- ions combine to form BaSO4 — the two reactants exchanged ions. Such a reaction = double displacement: AB + CD → AD + CB
A reaction in which a precipitate (an insoluble solid) forms is called a precipitation reaction — precipitation reactions produce insoluble salts.
More examplesBoard tip
- Silver nitrate + salt solution (white precipitate): AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
- Recall Activity 1.2 (yellow precipitate — also double displacement): Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
- BaCl2(aq) + K2SO4(aq) → BaSO4(s) + 2KCl(aq)
Neutralisation is also double displacementNotes
acid + base → salt + water (the full topic is in Chapter 2)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
| Displacement | Double displacement |
|---|---|
| One element removes another element from a compound | Two compounds exchange ions |
| A + BC → AC + B | AB + CD → AD + CB |
| Fe + CuSO4 → FeSO4 + Cu | Na2SO4 + BaCl2 → BaSO4 + 2NaCl |
Worked example — Recall — three questions on Activity 1.2 (NCERT)Example
(i) The precipitate is yellow; the compound is lead iodide (PbI2).
(ii) Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(iii) Yes, it is double displacement — Pb2+ and K+ exchanged their partner ions (NO3- and I-).
10-second revision
- Double displacement = exchange of ions
- Precipitation → insoluble salts (BaSO₄ white, AgCl white, PbI₂ yellow)
- Neutralisation (acid + base) is also double displacement
Board tip · BSEBBoard tip
“Explain a precipitation reaction with an example” — this came in the 2025 board paper. Write Na₂SO₄ + BaCl₂ → BaSO₄↓ (white) + 2NaCl.
Board tip · CBSEBoard tip
In double displacement, show the exchange of ions; do not forget ↓ or (s) for the precipitate.
Check your understandingall correct = mastery ★
1
What happens in a double displacement reaction?
Explanation:
AB + CD → AD + CB — exchange of ions.
2
The white precipitate formed on mixing solutions of AgNO3 and NaCl is —
💡 Hint:
Which product gets (s)?
Explanation:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) — AgCl is an insoluble white precipitate; NaNO3 stays dissolved.
3
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
💡 Hint:
Acid + base → salt + water = neutralisation; ions are exchanged.
Explanation:
Neutralisation = double displacement; heat is given out; no oxidation/reduction by O or H.
Oxidation, reduction and redox (oxygen and hydrogen rules)
उपचयन (ऑक्सीकरण), अपचयन और रेडॉक्स · NCERT 1.2.5 · Activity 1.11 · In-text Q 3 · Exercise 1, 16, 17
Activity 1.11Activity
Heat about 1 g of copper powder in a china dish — the shiny brown copper turns black. Copper gains oxygen and copper(II) oxide forms:
2Cu(s) + O2(g) heat→ 2CuO(s)
Now pass hydrogen gas over this hot black CuO — the black coating turns brown again, because copper is formed back:
CuO(s) + H2(g) heat→ Cu(s) + H2O(l)
Here CuO is losing oxygen = reduction; H2 is gaining oxygen = oxidation. One substance oxidised and another reduced at the same time = an oxidation–reduction (redox) reaction.
| In terms of oxygen | In terms of hydrogen | |
|---|---|---|
| Oxidation | gain of oxygen (+O) | loss of hydrogen (−H) |
| Reduction | loss of oxygen (−O) | gain of hydrogen (+H) |
More examples (NCERT)Board tip
- ZnO + C → Zn + CO — C is oxidised (CO forms), ZnO is reduced (Zn forms)
- MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 — HCl is oxidised (loses H to form Cl₂), MnO₂ is reduced (loses O to form MnCl₂)
- Burning of Mg 2Mg + O2 → 2MgO — Mg is oxidised
Extra information (common in board guide-books, not in the NCERT text)Board tip
the substance that is itself reduced while oxidising another = the oxidising agent; the one that is itself oxidised while reducing another = the reducing agent. In CuO + H2 → Cu + H2O: CuO = oxidising agent, H2 = reducing agent.
Worked example — NCERT exercise 1 — avoid the trapExample
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
- PbO lost O → lead oxide is reduced (statement d is correct)
- C gained O → carbon is oxidised (statement c is correct)
- “Lead is getting reduced” ✗ — it is PbO that is reduced; Pb is a product
- “Carbon dioxide is getting oxidised” ✗ — CO₂ is also a product
The incorrect statements are (a) and (b) → answer (i)
10-second revision
- Oxidation = +O or −H · reduction = −O or +H
- CuO + H₂ → Cu + H₂O: CuO reduced, H₂ oxidised
- 2Cu + O₂ → 2CuO (brown → black)
Board tip · BSEBBoard tip
Objective questions on identifying the substance oxidised / reduced appear almost every year — rule: gain of oxygen / loss of hydrogen = oxidation; the reverse = reduction.
Board tip · CBSEBoard tip
For an equation like ZnO + C → Zn + CO, write all four: what is oxidised, what is reduced, the oxidising agent and the reducing agent.
Check your understandingall correct = mastery ★
1
In CuO + H2 → Cu + H2O which substance is oxidised?
💡 Hint:
Which one gained oxygen?
Explanation:
H₂ gained O (formed H₂O) = oxidation; CuO lost O = reduction.
2
In terms of hydrogen, “oxidation” means —
Explanation:
Oxidation = gain of O or loss of H (as in HCl → Cl₂).
3
In 4Na(s) + O2(g) → 2Na2O(s) —
Explanation:
Na gained oxygen — Na is oxidised (NCERT in-text question 3(i)).
Corrosion — rusting of iron, damage and prevention
संक्षारण — लोहे में जंग और बचाव · NCERT 1.3.1 · Exercise 18, 20(a)
Definition
New iron things are shiny, but after some time a reddish-brown flaky layer forms on them — this is called rusting. When a metal is slowly eaten away by substances around it — moisture, acids and so on — it is called corrosion.
- Rust on iron: a reddish-brown layer (hydrated iron(III) oxide, Fe2O3·xH2O)
- The black coating on silver and the green coating on copper — these are also corrosion
Rusting needs both air (oxygen) and moisture. In simple form:
4Fe(s) + 3O2(g) + 2xH2O(l) → 2Fe2O3·xH2O(s)
DamageNotes
car bodies, bridges, iron railings, ships and all things made of iron become weak; a lot of money is spent every year to replace rusted iron.
Prevention — why paint iron? (NCERT exercise 18)Notes
paint forms a layer on the iron surface and keeps it away from air and moisture, so it does not rust. Other methods: applying oil/grease, galvanisation (a coating of zinc), chrome plating, making alloys (more in Chapter 3).
Worked example — NCERT exercise 20(a) — corrosion (with an example)Example
Corrosion: a metal being slowly destroyed by moisture, air, acids, etc.
Example: rusting of iron — a reddish-brown layer (Fe2O3·xH2O) forms on iron in moist air. Silver turning black and the green coating on copper are also examples of corrosion.
10-second revision
- Rust = Fe₂O₃·xH₂O · needs O₂ + moisture
- Prevention: paint, oil/grease, galvanisation, chrome plating, alloys
Board tip · BSEBBoard tip
Definition of corrosion + 3 ways to prevent rusting (paint, oil/grease, galvanising) — a sure 2–3 mark question.
Board tip · CBSEBoard tip
A case question may ask: “Why do iron railings in a seaside town rust faster?” → more moisture and salt in the air.
Check your understandingall correct = mastery ★
1
Rusting of iron requires —
Explanation:
Iron does not rust in dry air or in boiled (O₂-free) water — both are needed.
2
Coating iron with zinc is called —
Explanation:
Galvanisation (a zinc coating) protects iron from rusting.
3
The black coating that forms on silver articles is also corrosion.
Explanation:
A black coating forms on silver due to sulphur compounds in the air — this is also corrosion.
Rancidity — why chip packets are flushed with nitrogen
विकृतगंधिता — चिप्स के पैकेट में नाइट्रोजन क्यों? · NCERT 1.3.2 · Exercise 19, 20(b)
Definition
If oily or fatty foods (chips, namkeen, ghee, butter) are left open for a long time, their taste and smell change. This happens because fats and oils get oxidised by oxygen in the air — this is called rancidity.
Ways to prevent itNotes
- Adding antioxidants — they stop oxidation
- Keeping food in air-tight containers — oxidation slows down
- Filling packets with an unreactive gas such as nitrogen — the oxygen is removed
- Keeping food in a refrigerator or a cool, dark place
Remember
Chip packets are filled with nitrogen, not air, so that the chips do not get oxidised.
Worked example — NCERT exercise 19 — why are oily and fatty foods flushed with nitrogen?Example
So that no oxygen stays in the packet. Nitrogen is an unreactive gas — it does not react with oils/fats. So they are not oxidised and rancidity (going stale) is prevented.
10-second revision
- Rancidity = oxidation of oils/fats (stale smell and taste)
- Prevention: N₂ filling, air-tight container, antioxidants, cool place
Board tip · BSEBBoard tip
Definition of rancidity + ways to prevent it: flushing with nitrogen, adding antioxidants, airtight containers, refrigeration.
Board tip · CBSEBoard tip
Answer “Why nitrogen in chip packets?” with reasoning: nitrogen is unreactive, oxygen is removed, so the oil is not oxidised.
Check your understandingall correct = mastery ★
1
The main reason for filling chip packets with nitrogen is —
Explanation:
N₂ removes the oxygen → the oil is not oxidised → the chips do not go stale.
2
Rancidity is caused by —
Explanation:
Oxidation of fats/oils by oxygen in the air.
3
Substances added to foods to prevent oxidation are called ______.
💡 Hint:
They work “against” oxidation.
Explanation:
Antioxidants prevent oxidation.
Board revision — identify the type, corrosion vs rancidity, chapter map
बोर्ड रिवीजन — पूरा अध्याय एक नज़र में · NCERT Exercise 8, 20 · What you have learnt
Identify the type (the pattern of NCERT exercise 8)Notes
| Reaction (balanced) | Type |
|---|---|
| 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s) | double displacement |
| ZnCO3(s) heat→ ZnO(s) + CO2(g) | decomposition (thermal) |
| H2(g) + Cl2(g) → 2HCl(g) | combination |
| Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) | displacement |
(Note: the NCERT question writes (s) after BaBr2, so write it the same way in your answer; in fact BaBr2 is soluble in water.)
Corrosion vs rancidity (2 marks)Notes
| Corrosion | Rancidity |
|---|---|
| Slow oxidation of metals | Slow oxidation of oils/fats |
| Example: rusting of iron | Example: stale chips/ghee |
| Prevention: paint, grease, galvanisation | Prevention: N₂ packing, air-tight container, antioxidants |
The whole chapter in 40 secondsNotes
balanced equations + state symbols → 5 types (one equation each) → exothermic / endothermic → oxidation/reduction (O and H rules) → corrosion and rancidity.
Worked example — How to structure a board answer (5 marks)Example
- Definition (1 mark) — one clear sentence
- Balanced equation with state symbols (2 marks) — write the condition on the arrow
- Observation / explanation (1 mark) — colour, gas, temperature
- Example or a table of differences (1 mark)
Always write equations balanced — an unbalanced equation can lose marks.
10-second revision
- 1 → many = decomposition · many → 1 = combination
- Element + compound = displacement · compound + compound (exchange ions) = double displacement
- Corrosion (metals) and rancidity (oils) — both are slow oxidation
Board tip · BSEBBoard tip
For long answers (5 marks), write the five types of reactions — definition + one balanced equation each — on one page and learn it.
Board tip · CBSEBoard tip
In whole-chapter questions, identifying the type of reaction is the key skill — learn to name it as soon as you see the equation.
Check your understandingall correct = mastery ★
1
2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s)
Main type of reaction
By gain/loss of O / H — is it redox?
💡 Hint:
Two compounds — ions exchanged.
Explanation:
K⁺ and Ba²⁺ exchanged partner ions (Br⁻, I⁻) = double displacement.
2
ZnCO3(s) heat→ ZnO(s) + CO2(g)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
💡 Hint:
One reactant, two products, heating is needed.
Explanation:
Thermal decomposition, endothermic.
3
Which process takes place in both corrosion and rancidity?
Explanation:
Both are examples of slow oxidation by oxygen in the air.
❓ Full question bank — with answers and explanations — 274 questions
Choose a type: tap a card to see only that type of question. The 54 "Check your understanding" questions are separate, at the end of each lesson.
BSEB previous yearsबोर्ड PYQ130 done · 0 rightIn-text + exercisesNCERT प्रश्न340 done · 0 rightMCQबहुविकल्पी560 done · 0 rightTrue / Falseसही / गलत170 done · 0 rightFill in the blanksरिक्त स्थान160 done · 0 rightMatchमिलान करो50 done · 0 rightAssertion–Reasonकथन-कारण140 done · 0 rightBalancingसमीकरण संतुलन170 done · 0 rightClassifyवर्गीकरण140 done · 0 rightShort & long answerलघु / दीर्घ उत्तरीय340 done · 0 right
The answer to every question appears only after you try. For written questions, first write the answer in your notebook, then compare it with the model answer and mark yourself honestly — the wrong ones come back in review automatically.
Labels: “BSEB 2025 · Board question” = a verified annual-exam question · “BSEB model set” = a model/sample paper (not the annual exam) · “NCERT” = the textbook · “Board-style (practice)” = board-style questions made by us · “CBSE-style” = competency-based practice in the CBSE pattern (not a PYQ).
NCERT in-text questions + questions from the activities (all)
0/13All the questions that appear within the NCERT chapter (pages 6, 10, 13) and the questions asked inside the activities. Write your answer first, then compare it with the model answer.
1
Why should a magnesium ribbon be cleaned before burning it in air?
Model answer (board format)
Contact with air forms a white layer of magnesium oxide on the magnesium ribbon. This layer stops Mg from meeting oxygen directly, so the ribbon does not burn properly. Rubbing with sandpaper removes the layer so that clean Mg reacts with oxygen:
2Mg(s) + O2(g) → 2MgO(s)
Check your answer:
2
Write the balanced equation for the following chemical reactions —
(i) hydrogen + chlorine → hydrogen chloride
(ii) barium chloride + aluminium sulphate → barium sulphate + aluminium chloride
(iii) sodium + water → sodium hydroxide + hydrogen
Model answer (board format)
(i) H2(g) + Cl2(g) → 2HCl(g)
(ii) 3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)
(iii) 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Check your answer:
3
Write a balanced chemical equation with state symbols for the following reactions —
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and a solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Model answer (board format)
(i) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Check your answer:
4
In Activity 1.1 a single product forms along with the release of heat. What type of reaction is it?
Model answer (board format)
It is a combination reaction (two reactants → one product), and since heat is given out it is also exothermic.
2Mg(s) + O2(g) → 2MgO(s)
Check your answer:
5
Which form of energy is used in the decomposition reactions that take place in black-and-white photography?
Model answer (board format)
Light energy (sunlight) — this is decomposition by light:
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
2AgBr(s) sunlight→ 2Ag(s) + Br2(g)
Check your answer:
6
Mix about 2 g of barium hydroxide and 1 g of ammonium chloride in a test tube and touch the bottom of the test tube with your palm. What do you feel? Is it exothermic or endothermic?
Model answer (board format)
The test tube feels cold — the reaction takes heat from the surroundings (your palm). So it is an endothermic reaction.
Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH3 + 2H2O
Check your answer:
7
A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ with water.
Model answer (board format)
(i) X = calcium oxide (quicklime), formula CaO.
(ii) CaO(s) + H2O(l) → Ca(OH)2(aq) + heat
This forms slaked lime Ca(OH)2, whose solution is used for whitewashing. On the wall it slowly reacts with CO2 in the air to form a shiny layer of CaCO3: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
(Note: some guide-books write X = Ca(OH)₂, because the whitewash solution is slaked lime; but part (ii) asks for “the reaction of X with water”, so according to NCERT X = CaO.)
Check your answer:
8
Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.
Model answer (board format)
In the electrolysis of water:
2H2O(l) electricity→ 2H2(g) + O2(g)
In a water molecule, hydrogen and oxygen are in the ratio 2 : 1, so 1 molecule of oxygen forms along with 2 molecules of hydrogen. So the gas with double the volume is hydrogen (it collects at the cathode, the negative electrode).
Check your answer:
9
Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Model answer (board format)
Iron is more reactive than copper. It displaces copper from CuSO4 solution and forms ferrous sulphate, which is pale green. So the blue colour fades to pale green (brown copper deposits on the nail).
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Check your answer:
10
Give an example of a double displacement reaction other than the one given in Activity 1.10.
Model answer (board format)
A white precipitate of silver chloride when solutions of silver nitrate and sodium chloride are mixed:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
(Another example: Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) — a yellow precipitate)
Check your answer:
11
Identify the substances that are oxidised and the substances that are reduced in the following reactions —
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Model answer (board format)
(i) 4Na(s) + O2(g) → 2Na2O(s) — Na is oxidised (gains oxygen), O₂ is reduced.
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l) — H₂ is oxidised (gains oxygen), CuO is reduced (loses oxygen).
Check your answer:
12
Recall Activity 1.2 (lead nitrate + potassium iodide). (i) What was the colour of the precipitate? Name the compound precipitated. (ii) Write the balanced equation for this reaction. (iii) Is this a double displacement reaction?
Model answer (board format)
(i) A yellow precipitate — lead iodide (PbI2).
(ii) Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(iii) Yes — the ions of the two reactants (NO3- and I-) were exchanged.
Check your answer:
13
A magnesium ribbon burns in air with a dazzling flame and forms white magnesium oxide. Is magnesium being oxidised or reduced in this reaction?
Model answer (board format)
Mg is gaining oxygen, so magnesium is being oxidised.
2Mg(s) + O2(g) → 2MgO(s)
Check your answer:
NCERT exercises — all 20 questions + the group activity
0/21All 20 questions at the end of the chapter — Q1–3 are multiple choice (instant check), the rest come with model answers in board format.
1
Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.
💡 Hint:
The substances that are products (Pb, CO₂) are not being oxidised or reduced in this reaction.
Explanation:
PbO loses O → PbO is reduced (d is true); C gains O → C is oxidised (c is true). (a) and (b) are incorrect → answer (i).
2
Fe2O3 + 2Al → Al2O3 + 2Fe — the above reaction is an example of a —
Explanation:
The more reactive Al displaces Fe from Fe₂O₃ = displacement (d).
3
What happens when dilute hydrochloric acid is added to iron filings? Choose the correct answer.
Explanation:
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) — metal + dilute acid → salt + H₂.
4
What is a balanced chemical equation? Why should chemical equations be balanced?
Model answer (board format)
Balanced chemical equation: an equation in which the number of atoms of each element is equal on both sides — reactants and products. For example:
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Why needed: according to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction — the total mass of the products = the total mass of the reactants. Atoms are neither created nor destroyed, so a chemical equation must be balanced.
Check your answer:
5
Translate the following statements into chemical equations and then balance them —
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Model answer (board format)
(a) N2(g) + 3H2(g) → 2NH3(g)
(b) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
(c) 3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(s)
(d) 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Check your answer:
6
Balance the following chemical equations —
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Model answer (board format)
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3 (already balanced)
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
Check your answer:
7
Write the balanced chemical equations for the following reactions —
(a) calcium hydroxide + carbon dioxide → calcium carbonate + water
(b) zinc + silver nitrate → zinc nitrate + silver
(c) aluminium + copper chloride → aluminium chloride + copper
(d) barium chloride + potassium sulphate → barium sulphate + potassium chloride
Model answer (board format)
(a) Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
(b) Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)
(d) BaCl2(aq) + K2SO4(aq) → BaSO4(s) + 2KCl(aq)
Check your answer:
8
Write the balanced chemical equation for the following and identify the type of reaction in each case —
(a) potassium bromide(aq) + barium iodide(aq) → potassium iodide(aq) + barium bromide(s)
(b) zinc carbonate(s) → zinc oxide(s) + carbon dioxide(g)
(c) hydrogen(g) + chlorine(g) → hydrogen chloride(g)
(d) magnesium(s) + hydrochloric acid(aq) → magnesium chloride(aq) + hydrogen(g)
Model answer (board format)
(a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s) — double displacement
(b) ZnCO3(s) heat→ ZnO(s) + CO2(g) — decomposition (thermal)
(c) H2(g) + Cl2(g) → 2HCl(g) — combination
(d) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) — displacement
(Note: the state symbols follow the question; in fact BaBr2 is soluble in water.)
Check your answer:
9
What does one mean by exothermic and endothermic reactions? Give examples.
Model answer (board format)
Exothermic reaction: a reaction in which heat is given out along with the formation of products. Example — burning of natural gas:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
(Others: the reaction of quicklime with water, respiration, formation of compost)
Endothermic reaction: a reaction in which energy (heat, light or electricity) is absorbed. Example — thermal decomposition of calcium carbonate:
CaCO3(s) heat→ CaO(s) + CO2(g)
(Another: 2AgCl(s) sunlight→ 2Ag(s) + Cl2(g))
Check your answer:
10
Why is respiration considered an exothermic reaction? Explain.
Model answer (board format)
When food is digested, carbohydrates break down into glucose. In the cells, glucose combines with oxygen and provides energy — this energy keeps us alive and lets us work. Since energy is given out, respiration is an exothermic reaction.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
Check your answer:
11
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Model answer (board format)
In combination, two or more reactants join to form one product; in decomposition, one reactant breaks down into two or more simpler products. The two processes are opposites of each other. Combination usually gives out energy; decomposition needs energy to be supplied.
| Combination | Decomposition |
|---|---|
| CaO(s) + H2O(l) → Ca(OH)2(aq) | CaCO3(s) heat→ CaO(s) + CO2(g) |
| 2H2(g) + O2(g) → 2H2O(l) | 2H2O(l) electricity→ 2H2(g) + O2(g) |
Check your answer:
12
Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.
Model answer (board format)
Heat (thermal decomposition): 2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
Light (decomposition by light): 2AgBr(s) sunlight→ 2Ag(s) + Br2(g)
Electricity (electrolysis): 2H2O(l) electricity→ 2H2(g) + O2(g)
Check your answer:
13
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Model answer (board format)
| Displacement reaction | Double displacement reaction |
|---|---|
| A more reactive element removes a less reactive element from its compound | Two compounds exchange ions |
| A + BC → AC + B | AB + CD → AD + CB |
| Usually a colour change / a metal deposits | Usually a precipitate forms |
Displacement: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Double displacement: Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Check your answer:
14
In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.
Model answer (board format)
Copper is more reactive than silver, so it displaces silver:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
Check your answer:
15
What do you mean by a precipitation reaction? Explain by giving examples.
Model answer (board format)
A reaction in which an insoluble solid (a precipitate) forms is called a precipitation reaction. These are usually double displacement reactions.
Example 1: sodium sulphate + barium chloride → a white precipitate of barium sulphate:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Example 2: lead nitrate + potassium iodide → a yellow precipitate of lead iodide:
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
Check your answer:
16
Explain the following in terms of gain or loss of oxygen with two examples each — (a) oxidation (b) reduction
Model answer (board format)
(a) Oxidation: a substance gains oxygen in a reaction.
2Cu(s) + O2(g) heat→ 2CuO(s) — Cu is oxidised
2Mg(s) + O2(g) → 2MgO(s) — Mg is oxidised
(b) Reduction: a substance loses oxygen in a reaction.
CuO(s) + H2(g) heat→ Cu(s) + H2O(l) — CuO is reduced
ZnO(s) + C(s) → Zn(s) + CO(g) — ZnO is reduced
Check your answer:
17
A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed.
Model answer (board format)
Element X = copper (Cu); the black compound = copper(II) oxide (CuO).
2Cu(s) + O2(g) heat→ 2CuO(s)
Check your answer:
18
Why do we apply paint on iron articles?
Model answer (board format)
Iron rusts in moist air (corrosion), which needs both oxygen and moisture. Paint forms a layer on the iron surface, so the iron does not come in contact with air and moisture and does not rust.
Check your answer:
19
Oil and fat containing food items are flushed with nitrogen. Why?
Model answer (board format)
Oils and fats get oxidised by oxygen in the air and become rancid — their smell and taste change. Filling the packet with nitrogen removes the oxygen; nitrogen is unreactive, so no oxidation takes place and rancidity is prevented.
Check your answer:
20
Explain the following terms with one example each — (a) corrosion (b) rancidity
Model answer (board format)
(a) Corrosion: when a metal is slowly eaten away by substances around it — moisture, air, acids, etc. Example: reddish-brown rust (Fe2O3·xH2O) on iron in moist air; a black coating on silver and a green coating on copper.
(b) Rancidity: the change in smell and taste of fats and oils due to their oxidation. Example: chips or ghee left open for many days smell stale. Prevention: air-tight containers, antioxidants, flushing with nitrogen.
Check your answer:
21
Take four beakers A, B, C, D. Put 25 mL of water in A, B, C and copper sulphate solution in D. Measure their temperatures. Then add potassium sulphate to A, ammonium nitrate to B, anhydrous copper sulphate to C and fine iron filings to D (two spatulas each), stir, and measure the temperatures again. Which reactions are exothermic and which are endothermic?
Model answer (board format)
| Beaker | What was added | Change in temperature (usually) | Conclusion |
|---|---|---|---|
| A | K2SO4 in water | falls slightly | endothermic |
| B | NH4NO3 in water | falls a lot (beaker cold) | endothermic |
| C | anhydrous CuSO4 in water | rises | exothermic |
| D | Fe powder + CuSO4 solution | rises | exothermic |
Reaction in D: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(Note: in A, B, C, dissolving/hydration of a salt is mainly a physical process, but heat is still given out or taken in; D is a true chemical (displacement) reaction.)
Check your answer:
🏛️ BSEB Model Question Paper 2026 (Science, 112) — not an annual-exam question
0/6Only questions that we have seen and checked ourselves on an official Bihar Board question paper are here. The answers are written by Study With Me in simple language (not the board's official answer key).
Source: BSEB official Model Question Paper 2026, Science (112) — from the board's official website; a paper released for practice, not a question asked in any exam
1
Which of the following pairs represents single displacement reaction?
Explanation:
Copper is more reactive than silver, so it removes silver from AgNO3:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
In (A) and (B) copper is less reactive than Na and Mg; in (C) silver is less reactive than iron — so no reaction takes place there.
2
The decomposition of vegetables and their conversion into compost is an example of which reaction?
Explanation:
When vegetables rot into compost, heat is given out, so it is an exothermic reaction (NCERT Chapter 1 gives this same example). A compost heap feels warm to touch.
3
Which of the following is not an oxidation reaction?
Explanation:
In combustion and respiration, substances combine with oxygen — these are oxidation. Precipitation (such as the formation of BaSO4) is a double displacement reaction — oxygen is neither added nor removed. So the answer is (D).
Note: digestion of food is mainly the breaking down of large molecules; the digested food (glucose) is oxidised later, in respiration. Among the options, the clearest “not oxidation” answer is precipitation.
4
Which of the following equations is not balanced?
Explanation:
In (D) there are 2 N and 6 O on the left but 1 N and 4 O on the right — unbalanced. The balanced form:
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
In the other three equations the atoms of each element are equal on both sides.
5
What is a combination reaction?
Model answer (board format)
A reaction in which two or more reactants combine to form a single product is called a combination reaction.
Example: burning of a magnesium ribbon in air —
2Mg(s) + O2(g) → 2MgO(s)
Another example: CaO(s) + H2O(l) → Ca(OH)2(aq) (formation of slaked lime)
Check your answer:
6
What is rancidity? Explain with examples.
Model answer (board format)
Rancidity: when food containing oil or fat is left open for many days it gets oxidised, so its taste and smell change (go bad). This is called rancidity.
Examples: chips or namkeen left open, or old butter, giving off a strange smell.
Prevention: filling chip packets with nitrogen gas instead of oxygen, adding antioxidants, keeping food in air-tight containers or in the fridge.
Check your answer:
🏛️ Annual Secondary Exam 2025 (Science, subject code 112)
0/7Source: BSEB Annual Secondary Exam 2025, Science (112) — question paper released on the board's official website; no set is printed on the paper
1
Which gas is liberated at the Cathode during electrolysis in water?
Explanation:
In the electrolysis of water, hydrogen forms at the cathode (negative electrode) and oxygen at the anode (positive electrode). The volume of hydrogen is double that of oxygen (2 : 1).
2H2O(l) electric current→ 2H2(g) + O2(g)
2
Which statement is true regarding the reaction given below?
ZnO + C → Zn + CO
Explanation:
Carbon gained oxygen (C → CO), so carbon is oxidised. ZnO lost oxygen (ZnO → Zn), so ZnO is reduced.
Oxidation and reduction are both happening in the same reaction — it is a redox reaction.
3
Which of the following gases is formed as a result of the reaction of Zinc and dilute hydrochloric acid?
Explanation:
Zinc is more reactive than hydrogen, so it removes hydrogen from the acid (displacement):
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Test: a burning splinter brought near it gives a ‘pop’ sound — that is H2 gas.
4
Which among the following is slaked lime?
Explanation:
Quicklime = CaO. Adding water to it forms slaked lime Ca(OH)2 and gives out a lot of heat:
CaO(s) + H2O(l) → Ca(OH)2(aq) (+ heat)
CaCO3 = limestone / marble.
5
The chemical name of lime water is-
Explanation:
Lime water = a clear solution of Ca(OH)2 in water. ‘Slaked lime’ is its common name; its chemical name is calcium hydroxide — so (B).
Quicklime and calcium oxide = CaO — that is not lime water.
6
Describe a precipitation reaction with an example.
Model answer (board format)
Precipitation reaction: when two solutions are mixed and an insoluble solid (a precipitate) forms and settles down, the reaction is called a precipitation reaction. These are usually double displacement reactions.
Example: mixing barium chloride solution with sodium sulphate solution gives a white precipitate of barium sulphate:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Here Ba2+ and SO42- ions combine to form insoluble BaSO4; NaCl stays dissolved in the solution.
Check your answer:
7
Define a combination reaction. Give one example of an exothermic combination reaction.
Model answer (board format)
Combination reaction: a reaction in which two or more reactants combine to form a single product is called a combination reaction.
Example of an exothermic combination — pouring water on quicklime (CaO); a lot of heat is given out:
CaO(s) + H2O(l) → Ca(OH)2(aq) (+ heat)
(Another example: burning of coal C(s) + O2(g) → CO2(g))
Check your answer:
Multiple-choice questions (MCQ) — 1-mark board type
0/56Tap an option — you will know at once if it is right or wrong. If you are wrong you first get a hint, then one more try. Read the labels carefully: the BSEB label is only on verified questions; everything else is board-style practice (in the board pattern, but not taken from any exam).
1
Which of the following is a chemical change?
Explanation:
When coal burns a new substance (CO₂) forms and heat is given out — a chemical change. The others only change state/form.
2
Rusting of iron is a —
Explanation:
A new substance Fe2O3·xH2O forms — a chemical change.
3
In a chemical equation, the symbol (aq) means —
Explanation:
(aq) = aqueous = dissolved in water.
4
Which oxide of iron is formed when iron reacts with steam?
Explanation:
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) — steam forms the magnetic oxide of iron Fe3O4. (This is a BSEB model-set question, not from the annual exam.)
5
What type of reaction is 2Zn + O2 → 2ZnO?
Explanation:
Two reactants → one product = combination.
6
What type of reaction is CaCO3(s) heat→ CaO(s) + CO2(g)?
Explanation:
One reactant breaks into two products on heating = thermal decomposition (used to make lime).
7
When a magnesium ribbon burns in air, it forms —
Explanation:
It burns with a dazzling white flame and forms a white powder, magnesium oxide.
8
Why is a magnesium ribbon rubbed with sandpaper before burning?
Explanation:
The MgO layer hinders burning (NCERT in-text question 1).
9
Which gas is given off when dilute sulphuric acid is poured on zinc granules?
Explanation:
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
10
The colour of the precipitate formed on mixing solutions of lead nitrate and potassium iodide is —
Explanation:
Yellow lead iodide (PbI2) forms.
11
Balancing a chemical equation is based on which law?
Explanation:
Mass is neither created nor destroyed in a chemical reaction.
12
On balancing Fe + H2O → Fe3O4 + H2, the coefficients of Fe, H₂O, Fe₃O₄ and H₂ respectively will be —
Explanation:
3Fe + 4H2O → Fe3O4 + 4H2 — Fe: 3 = 3, O: 4 = 4, H: 8 = 8.
13
The chemical formula of quicklime is —
Explanation:
CaO = quicklime; Ca(OH)2 = slaked lime.
14
The chemical name of slaked lime is —
Explanation:
CaO(s) + H2O(l) → Ca(OH)2(aq) — calcium hydroxide.
15
The reaction of adding water to quicklime is —
Explanation:
Two substances → one product, and a lot of heat is given out (the beaker becomes hot).
16
2–3 days after whitewashing, a shiny layer of which substance forms on the wall?
Explanation:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
17
What type of reaction is respiration?
Explanation:
Energy is given out when glucose combines with oxygen.
18
Which products are formed when crystals of ferrous sulphate are heated?
Explanation:
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g) — there is a smell of burning sulphur.
19
The brown fumes given off on heating lead nitrate are of which gas?
Explanation:
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g) — the brown gas is NO2.
20
The ratio of the volumes of hydrogen and oxygen released in the electrolysis of water is —
Explanation:
2H2O(l) electricity→ 2H2(g) + O2(g) — H₂ : O₂ = 2 : 1.
21
When white silver chloride is kept in sunlight, its colour becomes —
Explanation:
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g) — grey because silver forms.
22
Which type of reaction is used in black-and-white photography?
Explanation:
Decomposition of AgBr / AgCl by light.
23
When barium hydroxide and ammonium chloride are mixed, the test tube —
Explanation:
This is an endothermic reaction — it takes heat from your palm.
24
When an iron nail is put in copper sulphate solution, the colour of the solution —
Explanation:
FeSO4 (pale green) forms.
25
Which metal can not displace copper from copper sulphate solution?
Explanation:
Ag is less reactive than Cu.
26
Dilute HCl added to iron filings forms —
Explanation:
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
27
The white precipitate formed on mixing solutions of sodium sulphate and barium chloride is —
Explanation:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
28
Precipitation reactions produce —
Explanation:
NCERT summary: precipitation reactions produce insoluble salts.
29
What type of reaction is NaOH + HCl → NaCl + H2O?
Explanation:
Acid + base → salt + water — exchange of ions.
30
In terms of oxygen, oxidation means —
Explanation:
Oxidation = gain of oxygen or loss of hydrogen.
31
Reduction is —
Explanation:
Reduction = loss of oxygen or gain of hydrogen.
32
In CuO + H2 → Cu + H2O the substance reduced is —
Explanation:
CuO lost oxygen.
33
In ZnO + C → Zn + CO the substance oxidised is —
Explanation:
C gained oxygen (CO formed).
34
In MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 the substance oxidised is —
Explanation:
HCl lost hydrogen to form Cl₂ = oxidation; MnO₂ lost O = reduction.
35
When copper powder is heated in air, the colour of its surface becomes —
Explanation:
2Cu(s) + O2(g) heat→ 2CuO(s) — black CuO.
36
The chemical formula of rust is —
Explanation:
Hydrated iron(III) oxide.
37
Silver articles turning black after some time is called —
Explanation:
Silver turning black is an example of corrosion.
38
The colour of the layer that forms on copper vessels is —
Explanation:
A green layer on copper — corrosion.
39
In galvanisation, iron is coated with which metal?
Explanation:
Galvanisation = coating with zinc.
40
Rancidity is caused by —
Explanation:
Oxidation of fats/oils by oxygen in the air.
41
Which gas is filled in chip packets?
Explanation:
N₂ is unreactive — it prevents oxidation (going stale).
42
What do antioxidants prevent in foods?
Explanation:
They prevent the oxidation of fats/oils.
43
Which of the following is a decomposition reaction?
Explanation:
One reactant (H₂O) → two products.
44
Which of the following is a displacement reaction?
Explanation:
The element Zn removed Cu from a compound.
45
Which of the following is a double displacement reaction?
Explanation:
The ions of two compounds were exchanged.
46
Which of the following is an endothermic reaction?
Explanation:
Heat has to be supplied continuously to break down CaCO₃.
47
Which of the following is not a sign of a chemical reaction?
Explanation:
Folding paper does not form any new substance.
48
Burning of natural gas (methane) produces —
Explanation:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) — exothermic.
49
On balancing C3H8 + O2 → CO2 + H2O, the coefficient of O₂ will be —
Explanation:
C3H8 + 5O2 → 3CO2 + 4H2O — O on the right = 6 + 4 = 10.
50
Hydrogen and chlorine combine to form HCl — this reaction is —
Explanation:
H2(g) + Cl2(g) → 2HCl(g)
51
Which of the following is an oxidation–reduction (redox) reaction?
Explanation:
CuO is reduced (loses O) and H₂ is oxidised (gains O) at the same time.
52
The correct state symbol for water in the electrolysis of water is —
Explanation:
Liquid water = (l).
53
Which of the following is the most reactive metal?
Explanation:
Reactivity: Zn > Fe > Cu > Ag.
54
Which gas does sodium metal give when it reacts with water?
Explanation:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
55
Decomposition of vegetable matter into compost is an example of which type of reaction?
Explanation:
NCERT: formation of compost is an example of an exothermic reaction.
56
Why is silver chloride stored in dark-coloured bottles?
Explanation:
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g) — a dark bottle protects it from light.
True or false
0/171
A magnesium ribbon burns in air with a dazzling white flame.
Explanation:
And a white powder, MgO, forms.
2
Chemical equations must be balanced because of the law of conservation of mass.
Explanation:
Atoms are neither created nor destroyed.
3
Most decomposition reactions are exothermic.
Explanation:
Decomposition needs energy (heat/light/electricity) to be supplied — such reactions are usually endothermic.
4
Respiration is an endothermic reaction.
Explanation:
Energy is released in respiration — exothermic.
5
In the electrolysis of water, the volume of oxygen is double that of hydrogen.
Explanation:
The volume of hydrogen is double (2 : 1).
6
Copper can displace silver from silver nitrate solution.
Explanation:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
7
Precipitation reactions produce insoluble salts.
Explanation:
Such as BaSO₄, AgCl, PbI₂.
8
Loss of hydrogen is called oxidation.
Explanation:
Oxidation = gain of O or loss of H.
9
Corrosion and rancidity are both effects of oxidation.
Explanation:
Oxidation of a metal = corrosion; oxidation of oils/fats = rancidity.
10
Painting iron prevents it from rusting.
Explanation:
Paint keeps iron away from air and moisture.
11
The chemical formula of rust is Fe₃O₄.
Explanation:
Rust = Fe2O3·xH2O; Fe3O4 forms from the reaction of iron with steam.
12
Silver chloride turns grey in sunlight.
Explanation:
Decomposition by light forms silver.
13
The test tube becomes hot when barium hydroxide and ammonium chloride are mixed.
Explanation:
The test tube becomes cold — endothermic.
14
A lot of heat is given out when water is added to quicklime.
Explanation:
CaO(s) + H2O(l) → Ca(OH)2(aq) — exothermic.
15
No reaction takes place when zinc is put in copper sulphate solution.
Explanation:
Zn is more reactive than Cu — it displaces Cu.
16
The reaction of sodium hydroxide with hydrochloric acid is a displacement reaction.
Explanation:
It is neutralisation = double displacement.
17
On heating crystals of ferrous sulphate, their green colour changes and there is a smell of burning sulphur.
Explanation:
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
Fill in the blanks
0/16You can write the answer in English, Hindi or as a formula (e.g. “nitrogen” or “N2”). Then press Check.
1
A magnesium ribbon burns in air to form a white powder called ______.
Explanation:
2Mg(s) + O2(g) → 2MgO(s)
2
The state symbol (g) means ______.
3
The chemical formula of quicklime is ______.
4
In the electrolysis of water, the ratio of the volumes of hydrogen and oxygen is ______.
💡 Hint:
H₂ : O₂ — which one has double the volume?
5
Reactions in which energy is absorbed are called ______ reactions.
6
The colour of the lead iodide precipitate is ______.
7
When an iron nail is put in copper sulphate solution, ______ forms, so the solution turns pale green.
Explanation:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
8
A reaction in which a precipitate forms is called a ______ reaction.
9
Loss of oxygen from a substance is called ______.
10
Both oxygen and ______ are needed for iron to rust.
11
The method of coating iron with zinc is called ______.
12
______ gas is filled in chip packets to remove oxygen.
13
Thermal decomposition of ferrous sulphate gives Fe₂O₃, SO₂ and ______.
14
The brown gas given off on heating lead nitrate is ______.
15
In black-and-white photography, silver bromide undergoes ______ decomposition.
16
The reaction of zinc with dilute sulphuric acid gives zinc sulphate and ______ gas.
Match the columns (column A ↔ column B)
0/5For each row choose the correct letter from column B in the drop-down, then press Check.
1
Match each reaction with its type.
Column B: A. Combination · B. Double displacement (precipitation) · C. Thermal decomposition · D. Decomposition by light · E. Displacement
1. 2H2 + O2 → 2H2O
2. CaCO3 heat→ CaO + CO2
3. 2AgBr sunlight→ 2Ag + Br2
4. Zn + CuSO4 → ZnSO4 + Cu
5. AgNO3 + NaCl → AgCl + NaNO3
2
Match each substance with its common name / identity.
Column B: A. White precipitate · B. Slaked lime · C. Black compound (on heated copper) · D. Quicklime · E. Yellow precipitate
1. CaO
2. Ca(OH)2
3. PbI2
4. BaSO4
5. CuO
3
Match each observation / method with its reason.
Column B: A. Protection from rust · B. Preventing rancidity · C. Endothermic reaction · D. Removing the oxide layer · E. Displacement of copper
1. Brown coating on an iron nail in CuSO₄
2. Nitrogen in chip packets
3. Paint on an iron gate
4. Rubbing a Mg ribbon with sandpaper
5. Test tube turns cold with Ba(OH)₂ + NH₄Cl
4
Match each term with its meaning.
Column B: A. An insoluble solid formed in a solution · B. Oxidation of oils and fats (going stale) · C. Loss of oxygen / gain of hydrogen · D. A metal slowly getting eaten away · E. Gain of oxygen / loss of hydrogen
1. Oxidation
2. Reduction
3. Corrosion
4. Rancidity
5. Precipitate
5
Match each state symbol with its meaning.
Column B: A. Liquid · B. Gas · C. Solid · D. Aqueous solution
1. (s)
2. (l)
3. (g)
4. (aq)
Assertion–Reason
0/14First check whether A is true, then whether R is true, and finally — does R give the reason for A?
1
Assertion (A): A magnesium ribbon is cleaned before burning it in air.
Reason (R): The oxide layer on magnesium stops it from burning properly.
2
Assertion (A): It is necessary to balance a chemical equation.
Reason (R): Mass is neither created nor destroyed in a chemical reaction.
3
Assertion (A): The reaction of quicklime with water is exothermic.
Reason (R): A large amount of heat is given out in this reaction.
4
Assertion (A): Decomposition of calcium carbonate is an endothermic reaction.
Reason (R): In decomposition reactions, energy has to be supplied to break the reactants.
5
Assertion (A): Iron displaces copper from copper sulphate solution.
Reason (R): Iron is more reactive than copper.
6
Assertion (A): Copper displaces iron from ferrous sulphate solution.
Reason (R): Copper is less reactive than iron.
Explanation:
A is false (the less reactive Cu cannot remove Fe); R is true.
7
Assertion (A): The reaction of silver nitrate with sodium chloride is a precipitation reaction.
Reason (R): The silver chloride formed is a white precipitate that is insoluble in water.
8
Assertion (A): In CuO + H2 → Cu + H2O, CuO is oxidised.
Reason (R): In this reaction CuO loses oxygen.
Explanation:
Losing oxygen = reduction; so A is false and R is true.
9
Assertion (A): Respiration is an exothermic reaction.
Reason (R): In the cells, glucose combines with oxygen and releases energy.
10
Assertion (A): Chip packets are filled with nitrogen gas.
Reason (R): Nitrogen is a very reactive gas that quickly combines with oil.
Explanation:
A is true; but nitrogen is unreactive, so R is false.
11
Assertion (A): Rusting of iron is corrosion.
Reason (R): Rusting happens only in dry oxygen; no moisture is needed.
Explanation:
Rusting needs both oxygen and moisture — R is false.
12
Assertion (A): In the electrolysis of water, one test tube collects twice as much gas as the other.
Reason (R): Hydrogen is a lighter gas than oxygen.
Explanation:
Both statements are true, but the reason for double the volume is that H : O = 2 : 1 in H₂O, not that hydrogen is lighter.
13
Assertion (A): NaOH + HCl → NaCl + H2O is a double displacement reaction.
Reason (R): In it, two compounds exchange ions.
14
Assertion (A): Silver articles turn black after some time in air.
Reason (R): Silver is less reactive than copper.
Explanation:
Both are true, but silver turns black because of corrosion (reaction with sulphur compounds in the air).
Balance the equations (interactive)
0/17Fill in a coefficient in front of each formula (blank = 1). The atom count below updates live — when both sides are equal, press Check. Only the smallest whole numbers count as correct.
1
Balance the equation by filling in the coefficients (blank = 1).
Mg + O2 → MgO
2
Balance the equation by filling in the coefficients (blank = 1).
H2 + Cl2 → HCl
3
Balance the equation by filling in the coefficients (blank = 1).
Na + H2O → NaOH + H2
4
Balance the equation by filling in the coefficients (blank = 1).
Fe + H2O → Fe3O4 + H2
💡 Hint:
First equalise O (4 in Fe₃O₄), then Fe and H.
5
Balance the equation by filling in the coefficients (blank = 1).
N2 + H2 → NH3
6
Balance the equation by filling in the coefficients (blank = 1).
H2S + O2 → H2O + SO2
💡 Hint:
O on the right = H₂O + 2×SO₂; fill in the coefficient of O₂ last.
7
Balance the equation by filling in the coefficients (blank = 1).
BaCl2 + Al2(SO4)3 → BaSO4 + AlCl3
💡 Hint:
Treat the SO₄ group as one unit.
8
Balance the equation by filling in the coefficients (blank = 1).
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
9
Balance the equation by filling in the coefficients (blank = 1).
NaOH + H2SO4 → Na2SO4 + H2O
10
Balance the equation by filling in the coefficients (blank = 1).
Al + CuCl2 → AlCl3 + Cu
💡 Hint:
Cl: multiples of 2 on the left, multiples of 3 on the right — LCM 6.
11
Balance the equation by filling in the coefficients (blank = 1).
Pb(NO3)2 → PbO + NO2 + O2
💡 Hint:
First count N, then O.
12
Balance the equation by filling in the coefficients (blank = 1).
C3H8 + O2 → CO2 + H2O
💡 Hint:
Order: C → H → O (O last).
13
Balance the equation by filling in the coefficients (blank = 1).
Al + Fe2O3 → Al2O3 + Fe
14
Balance the equation by filling in the coefficients (blank = 1).
KClO3 → KCl + O2
15
Balance the equation by filling in the coefficients (blank = 1).
CH4 + O2 → CO2 + H2O
16
Balance the equation by filling in the coefficients (blank = 1).
FeSO4 → Fe2O3 + SO2 + SO3
17
Balance the equation by filling in the coefficients (blank = 1).
Fe + O2 → Fe2O3
Identify the type of reaction (interactive)
0/14Choose one option in each row, then press Check. (The redox row uses the Class 10 definition — gain/loss of oxygen or hydrogen; where that is not clear, the row is not asked.)
1
CaO(s) + H2O(l) → Ca(OH)2(aq)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
Two → one; a lot of heat; no oxidation/reduction by O/H.
2
2H2O(l) electricity→ 2H2(g) + O2(g)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
Electrolysis; energy is taken in; oxygen is removed from H₂O (reduction) — hydrogen and oxygen are released.
3
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
Main type of reaction
In terms of heat
Explanation:
Decomposition by light — light energy is absorbed.
4
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Main type of reaction
In terms of heat
Explanation:
The more reactive Zn removed Cu; the solution gets slightly warm.
5
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
Main type of reaction
By gain/loss of O / H — is it redox?
Explanation:
Exchange of ions; a yellow precipitate.
6
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
Burning — heat is given out; methane is oxidised (C gains O, H is lost).
7
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
Main type of reaction
In terms of heat
Explanation:
Thermal decomposition; brown NO₂ gas.
8
CuO(s) + H2(g) heat→ Cu(s) + H2O(l)
By gain/loss of O / H — is it redox?
Explanation:
CuO is reduced, H₂ is oxidised.
9
BaCl2(aq) + H2SO4(aq) → BaSO4(s) + 2HCl(aq)
Main type of reaction
By gain/loss of O / H — is it redox?
Explanation:
White BaSO₄ precipitate — double displacement.
10
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Main type of reaction
In terms of heat
Explanation:
Mg displaced hydrogen from the acid; the test tube gets warm.
11
C(s) + O2(g) → CO2(g)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
Burning of coal — combination, exothermic, C is oxidised.
12
2HNO3(aq) + Ca(OH)2(aq) → Ca(NO3)2(aq) + 2H2O(l)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
Neutralisation = double displacement; heat is given out.
13
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
Main type of reaction
In terms of heat
Explanation:
Thermal decomposition — the green colour changes, smell of burning sulphur.
14
2H2(g) + O2(g) → 2H2O(l)
Main type of reaction
In terms of heat
By gain/loss of O / H — is it redox?
Explanation:
H₂ is oxidised (gains O).
Very short answer questions (1 mark)
0/161
What is a chemical equation?
Model answer (board format)
Writing a chemical reaction briefly using chemical formulae and symbols instead of words is called a chemical equation. For example 2Mg + O2 → 2MgO.
Check your answer:
2
What does (aq) mean in a chemical equation?
Model answer (board format)
(aq) = aqueous solution — the substance is dissolved in water.
Check your answer:
3
Write the chemical formula of slaked lime.
Model answer (board format)
Ca(OH)2 (calcium hydroxide).
Check your answer:
4
Which gases are given off when crystals of ferrous sulphate are heated?
Model answer (board format)
Sulphur dioxide (SO2) and sulphur trioxide (SO3) — 2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
Check your answer:
5
Name the brown fumes given off on heating lead nitrate.
Model answer (board format)
Nitrogen dioxide (NO2).
Check your answer:
6
Give one example (with an equation) of an exothermic reaction.
Model answer (board format)
CaO(s) + H2O(l) → Ca(OH)2(aq) — the reaction of quicklime with water.
Check your answer:
7
In the electrolysis of water, which gas is released in double the volume?
Model answer (board format)
Hydrogen (H₂ : O₂ = 2 : 1).
Check your answer:
8
Which is more reactive, iron or copper?
Model answer (board format)
Iron — that is why it displaces copper from CuSO4.
Check your answer:
9
What is a precipitate?
Model answer (board format)
An insoluble solid formed in a reaction that separates out of the solution is called a precipitate (e.g. BaSO4).
Check your answer:
10
Define oxidation in terms of hydrogen.
Model answer (board format)
Loss of hydrogen from a substance is oxidation (e.g. HCl → Cl₂).
Check your answer:
11
Write the chemical formula of rust.
Model answer (board format)
Fe2O3·xH2O (hydrated iron(III) oxide).
Check your answer:
12
Write one way to prevent rancidity.
Model answer (board format)
Keeping food in an air-tight container / filling the packet with nitrogen / adding antioxidants (any one).
Check your answer:
13
What is galvanisation?
Model answer (board format)
Coating iron with a layer of zinc to protect it from rusting is called galvanisation.
Check your answer:
14
What is a redox reaction? Give an example.
Model answer (board format)
A reaction in which one substance is oxidised and another is reduced at the same time. Example: CuO + H2 → Cu + H2O
Check your answer:
15
What does the colour of silver chloride become when it is kept in sunlight?
Model answer (board format)
From white to grey — because silver forms.
Check your answer:
16
Write the general form of a displacement reaction.
Model answer (board format)
A + BC → AC + B (A is more reactive than B).
Check your answer:
Short answer questions (2–3 marks)
0/12Both verified short-answer questions from the 2025 annual exam (combination reaction, precipitation reaction) are now in the 🏛️ BSEB 2025 part above — they are not repeated here. All questions below are board-style practice questions.
1
Is milk turning into curd a physical or a chemical change? Give a reason.
Model answer (board format)
A chemical change. New substances with new properties (lactic acid, etc.) form in curd — the taste becomes sour and the texture changes, and curd cannot be turned back into milk.
Check your answer:
2
Which observations tell us that a chemical reaction has taken place? Write any four.
Model answer (board format)
- Change in state (e.g. a solid/gas forms)
- Change in colour (Fe + CuSO₄: blue → pale green)
- Evolution of a gas (bubbles of H₂ from Zn + H₂SO₄)
- Change in temperature (heat from CaO + H₂O)
Check your answer:
3
Why is ice melting into water not a chemical reaction?
Model answer (board format)
Ice and water are both H2O — no new substance forms, only the state changes (solid → liquid), and the water can be frozen back into ice. So it is a physical change.
Check your answer:
4
Write the following word equation as a balanced chemical equation with state symbols — zinc + dilute sulphuric acid → zinc sulphate + hydrogen
Model answer (board format)
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
(It is already balanced: Zn 1 = 1, H 2 = 2, S 1 = 1, O 4 = 4.)
Check your answer:
5
What is the law of conservation of mass? Give an example.
Model answer (board format)
Law: mass can neither be created nor destroyed in a chemical reaction — total mass of products = total mass of reactants.
Example: 2H2 + O2 → 2H2O — 4 g of hydrogen and 32 g of oxygen combine to form 36 g of water.
Check your answer:
6
What do you observe when crystals of ferrous sulphate are heated? Write the equation and name the type of reaction.
Model answer (board format)
Observations: the green crystals first lose their water and change colour; on further heating, gases with a smell of burning sulphur are given off and a reddish-brown solid (Fe2O3) is left.
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g)
Type: thermal decomposition (endothermic).
Check your answer:
7
What happens when lead nitrate is heated? Write the balanced equation.
Model answer (board format)
Lead nitrate undergoes thermal decomposition — brown fumes (NO2) are given off along with oxygen gas, and lead oxide is left behind.
2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
Check your answer:
8
What happens when carbon dioxide is passed through lime water? Write the equation.
Model answer (board format)
The lime water turns milky, because insoluble calcium carbonate forms:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Check your answer:
9
Write three differences between exothermic and endothermic reactions.
Model answer (board format)
| Exothermic | Endothermic |
|---|---|
| Heat is given out | Energy is absorbed |
| The surroundings get warm | The surroundings get cold |
| Example: CH4 + 2O2 → CO2 + 2H2O | Example: CaCO3 heat→ CaO + CO2 |
Check your answer:
10
What conditions are needed for iron to rust? Write three ways to prevent rusting.
Model answer (board format)
Conditions needed: oxygen (air) and moisture (water) — both.
4Fe(s) + 3O2(g) + 2xH2O(l) → 2Fe2O3·xH2O(s)
Prevention:
- Painting, or applying oil/grease
- Galvanisation (a coating of zinc)
- Chrome plating / making alloys (such as stainless steel)
Check your answer:
11
Why is silver chloride kept in dark-coloured bottles? Give the equation.
Model answer (board format)
In sunlight, silver chloride undergoes decomposition by light (white → grey). To prevent this it is kept in dark-coloured bottles:
2AgCl(s) sunlight→ 2Ag(s) + Cl2(g)
Check your answer:
12
Identify the oxidising agent and the reducing agent in CuO + H2 → Cu + H2O. (Beyond NCERT — guide-book level)
Model answer (board format)
- CuO — is itself reduced and oxidises H₂ → oxidising agent
- H₂ — is itself oxidised and reduces CuO → reducing agent
Check your answer:
Long answer questions (5 marks) — board format
0/6Remember the structure: definition → balanced equation (with state symbols) → observation → example/table.
1
Briefly describe the main types of chemical reactions (combination, decomposition, displacement, double displacement, oxidation–reduction) with one example each.
Model answer (board format)
1. Combination — two or more reactants → one product:
CaO(s) + H2O(l) → Ca(OH)2(aq)
2. Decomposition — one reactant → two or more simpler products (by heat/light/electricity):
CaCO3(s) heat→ CaO(s) + CO2(g)
3. Displacement — a more reactive element removes a less reactive one from its compound:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
4. Double displacement — exchange of ions between two compounds (a precipitate often forms):
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
5. Oxidation–reduction (redox) — one substance is oxidised (gain of O / loss of H) and another is reduced (loss of O / gain of H) at the same time:
CuO(s) + H2(g) heat→ Cu(s) + H2O(l)
Check your answer:
2
Why must a chemical equation be balanced? Explain on the basis of the law of conservation of mass and balance Fe + H2O → Fe3O4 + H2 step by step.
Model answer (board format)
Why: according to the law of conservation of mass, mass is neither created nor destroyed in a reaction; atoms only join up in new ways. So the atoms of every element must be equal on both sides.
Step 1 — skeletal equation: Fe + H2O → Fe3O4 + H2
Step 2 — count the atoms:
| Element | Left | Right |
|---|---|---|
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
Step 3 — start with the compound with the most atoms (Fe₃O₄): to equalise O, put 4 in front of H₂O → Fe + 4H2O → Fe3O4 + H2
Step 4 — equalise H: H = 8 on the left, so 4 in front of H₂ → Fe + 4H2O → Fe3O4 + 4H2
Step 5 — equalise Fe: 3 in front of Fe →
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Check: Fe 3 = 3, H 8 = 8, O 4 = 4 ✓ (state symbols added as well.)
Check your answer:
3
Describe the electrolysis of water (Activity 1.7). Write the observations, the equation and the reason for the volume ratio of the gases.
Model answer (board format)
Method: fix two carbon electrodes in a plastic mug and connect them to a 6 V battery. Fill the mug with water and add a few drops of dilute sulphuric acid (so that electricity flows easily). Invert test tubes filled with water over both electrodes and pass the current.
Observations:
- Gas bubbles form at both electrodes and the water in the test tubes is pushed down
- One test tube collects twice as much gas as the other
- A burning splinter near the larger volume of gas (at the cathode) gives a pop = hydrogen; the other gas makes the splinter burn more brightly = oxygen
2H2O(l) electricity→ 2H2(g) + O2(g)
Reason for the ratio: in a water molecule H and O are in the ratio 2 : 1, so the volumes of H₂ and O₂ are also 2 : 1.
Type: electrolytic decomposition — endothermic (electrical energy is taken in).
Check your answer:
4
Define oxidation and reduction in terms of oxygen and hydrogen. Explain a redox reaction with an example and write two effects of oxidation in daily life.
Model answer (board format)
| In terms of oxygen | In terms of hydrogen | |
|---|---|---|
| Oxidation | gain of oxygen | loss of hydrogen |
| Reduction | loss of oxygen | gain of hydrogen |
Redox reaction: one in which one substance is oxidised and another is reduced at the same time.
CuO(s) + H2(g) heat→ Cu(s) + H2O(l)
Here CuO loses oxygen → reduction; H₂ gains oxygen → oxidation.
Another example: MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 — HCl is oxidised (loss of H), MnO₂ is reduced (loss of O).
Effects in daily life:
- Corrosion — rust on iron (Fe2O3·xH2O), silver turning black; prevention: paint, galvanisation
- Rancidity — oils/fats getting oxidised and going stale; prevention: filling with nitrogen, air-tight containers, antioxidants
Check your answer:
5
What is a decomposition reaction? Give one example each, with equations, of thermal, electrolytic and photolytic decomposition. Why are decomposition reactions usually endothermic?
Model answer (board format)
Decomposition: a reaction in which one reactant breaks down into two or more simpler products.
(i) Thermal decomposition (by heat) — in making lime:
CaCO3(s) heat→ CaO(s) + CO2(g)
(ii) Electrolytic decomposition (by electricity):
2H2O(l) electricity→ 2H2(g) + O2(g)
(iii) Photolytic decomposition (by light) — in black-and-white photography:
2AgBr(s) sunlight→ 2Ag(s) + Br2(g)
Why endothermic: energy (heat, light or electricity) has to be supplied to break the bonds of the reactant — energy is absorbed, so decompositions are usually endothermic.
Check your answer:
6
Explain displacement and double displacement reactions on the basis of Activities 1.9 and 1.10, with observations and equations. Write two differences between them.
Model answer (board format)
Displacement (Activity 1.9): when clean iron nails are kept in CuSO4 solution for 20 minutes, a brown coating (copper) forms on the nails and the blue colour of the solution fades (pale green).
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Reason: Fe is more reactive than Cu.
Double displacement (Activity 1.10): mixing BaCl2 solution with Na2SO4 solution gives a white precipitate.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
Reason: Ba2+ and SO42- ions combine to form insoluble BaSO4 — an exchange of ions.
| Displacement | Double displacement |
|---|---|
| One element and one compound take part | Two compounds take part |
| A + BC → AC + B | AB + CD → AD + CB |
| Usually a colour change / a metal deposits | Usually a precipitate forms |
Check your answer:
🏛️ Board exam corner — Bihar Board (BSEB)— CBSE
Switch the board with «BSEB | CBSE» at the top — the NCERT chapter stays the same; board tips and questions change.
Verified BSEB questionsBoard tip
In the Bihar Board (BSEB) annual secondary exam, science questions are objective (1 mark, OMR), short answer (2 marks) and long answer (5 marks). The textbook is the same NCERT book.
🏛️ BSEB Model Question Paper 2026 (Science, 112) — not an annual-exam question 6🏛️ Annual Secondary Exam 2025 (Science, subject code 112) 7
🏛️ All board questions on one page (printable, Hindi) →
More past BSEB questions (PYQs) will be added here only after their source is verified. Every other question is marked “board-style” — it follows the board pattern but does not claim to be from any particular year.
CBSE-style · competency-based questionsCaution
CBSE Class 10 Science puts weight on competency-based questions — case/source-based, assertion–reason and questions that apply ideas to everyday situations. The textbook is the same NCERT book.
Note: the questions below are practice questions we wrote in the CBSE competency-based pattern (case / assertion–reason / data). They are not from any year's CBSE paper (not PYQs).
1
Case: Riya dipped two clean iron nails in copper sulphate solution for 20 minutes. She saw that the blue colour of the solution faded and a brown coating formed on the nails. What is the correct reason for these observations?
💡 Hint:
Which metal has a brown colour? Which ion gives the blue colour?
Explanation:
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) — the more reactive Fe displaces Cu; the brown coating is copper, and blue CuSO₄ changes to pale green FeSO₄.
2
Data-based: In an electrolysis-of-water experiment, 24 mL of gas collected in the test tube over the cathode. At the same time, which gas and how much will be in the test tube over the anode?
💡 Hint:
Look at the ratio of H₂ to O₂ molecules in the equation.
Explanation:
2H2O(l) → 2H2(g) + O2(g) — H₂ forms at the cathode and O₂ at the anode, in the volume ratio 2 : 1. So 24 mL of H₂ comes with 12 mL of O₂.
3
Assertion (A): After burning a magnesium ribbon in air, the mass of all the white ash collected is more than the mass of the ribbon.
Reason (R): During burning, oxygen from the air combines with magnesium to form magnesium oxide.
Explanation:
Both statements are true and R correctly explains A: in 2Mg + O2 → 2MgO the mass of the oxygen is added to the product. Mass is conserved — total (Mg + O₂) = MgO.
4
Assertion (A): Chip packets are filled with nitrogen gas.
Reason (R): Nitrogen is a highly reactive gas that stops oil from getting oxidised quickly.
Explanation:
A is true but R is false: nitrogen is an unreactive gas. It pushes out the oxygen, so the oil does not get oxidised (no rancidity).
5
Experiment: A student heated green ferrous sulphate crystals in a test tube. Which observation proves that this is a chemical decomposition and not just loss of water?
💡 Hint:
Which observation shows that a new substance (a new gas, a new solid) has formed?
Explanation:
2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g) — the smell of SO₂/SO₃ and the reddish-brown Fe₂O₃ are new substances. Drops of water only show that water of crystallisation was lost.
6
Case: When a white solid X is heated strongly, a gas is given off that turns lime water milky, and a white solid Y is left. When Y is added to water, a lot of heat is released. (i) Identify X and Y. (ii) Write balanced equations for both reactions. (iii) Name the type of each reaction.
Model answer (board format)
(i) X = calcium carbonate (limestone) CaCO3; Y = calcium oxide (quicklime) CaO. The gas that turns lime water milky is CO2.
(ii) CaCO3(s) heat→ CaO(s) + CO2(g)
CaO(s) + H2O(l) → Ca(OH)2(aq) (+ heat)
(iii) The first is thermal decomposition (endothermic); the second is combination (exothermic).
Check your answer:
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🧠 What you have learnt + equation sheet
What you have learnt (NCERT summary, in simple words):
- A chemical equation shows a reaction in formulae; it must be balanced (conservation of mass).
- Combination: two or more substances → one new substance.
- Decomposition: one substance → two or more simpler substances (by heat/light/electricity).
- Heat given out = exothermic; energy taken in = endothermic.
- Displacement: a more reactive element removes a less reactive element.
- Double displacement: exchange of ions; precipitation gives insoluble salts.
- Oxidation = gain of O / loss of H; reduction = loss of O / gain of H.
- Corrosion (metals) and rancidity (oils and fats) — both are effects of oxidation.
| What | Balanced equation |
|---|---|
| Burning of Mg (combination) | 2Mg(s) + O2(g) → 2MgO(s) |
| Zn + dilute acid | Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) |
| Yellow precipitate | Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) |
| Iron + steam | 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) |
| Slaked lime (exothermic combination) | CaO(s) + H2O(l) → Ca(OH)2(aq) |
| Shine of whitewash | Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) |
| Burning of coal / hydrogen | C(s) + O2(g) → CO2(g) · 2H2(g) + O2(g) → 2H2O(l) |
| Burning of methane | CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) |
| Respiration (exothermic) | C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy |
| Thermal decomposition of FeSO₄ | 2FeSO4(s) heat→ Fe2O3(s) + SO2(g) + SO3(g) |
| Decomposition of limestone | CaCO3(s) heat→ CaO(s) + CO2(g) |
| Lead nitrate (brown NO₂) | 2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g) |
| Electrolysis of water | 2H2O(l) electricity→ 2H2(g) + O2(g) |
| Decomposition by light | 2AgCl(s) sunlight→ 2Ag(s) + Cl2(g) · 2AgBr(s) sunlight→ 2Ag(s) + Br2(g) |
| Endothermic (test tube turns cold) | Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH3 + 2H2O |
| Displacement | Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) · Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) |
| Refining of silver | Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s) |
| Thermite (displacement) | Fe2O3 + 2Al → Al2O3 + 2Fe |
| Double displacement / precipitation | Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) · AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) |
| Neutralisation | NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) |
| Redox | 2Cu(s) + O2(g) heat→ 2CuO(s) · CuO(s) + H2(g) heat→ Cu(s) + H2O(l) |
| Redox (in terms of H) | MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 · ZnO + C → Zn + CO |
| Rust | 4Fe(s) + 3O2(g) + 2xH2O(l) → 2Fe2O3·xH2O(s) |
Source: NCERT Science Class 10 (2026-27) · the “BSEB 2025” tag is only on verified questions; “BSEB model set” is not the annual exam; everything else is board-style practice. Spotted a mistake? Tell us.