Class 10 · Science · Chapter 2: Acids, Bases and Salts
Bihar Board · NCERT · CBSE
All 114 objective questions (MCQ) of this chapter — solve first, then tap “Show answer”. Each question shows its source label: «BSEB 2025 · board paper» = verified from the official board paper; «Board-style (practice)» = Study With Me's own practice questions.
← Chapter 2: Acids, Bases and Salts — full notes, video, questions
Which is the right remedy for acidity after overeating?
Correct answer: (A) A solution of baking soda
Baking soda is a base — it neutralises the extra acid. The other three contain acids, so they would increase the acidity.
A curry stain on a white cloth turns reddish-brown when soap is scrubbed on it. What does this show?
Correct answer: (B) Turmeric is an indicator and soap is basic
Curry contains turmeric, which turns from yellow to reddish-brown in a base. Soap is basic, so the stain changed colour.
What colour does phenolphthalein give in a base?
Correct answer: (C) Pink
Phenolphthalein turns pink in a base and stays colourless in an acid.
Which colour does methyl orange give in an acidic solution?
Correct answer: (D) Red
Methyl orange is red in an acid and yellow in a base.
Which gas is formed when zinc reacts with dilute hydrochloric acid?
Correct answer: (A) Hydrogen
Zn + 2HCl → ZnCl2 + H2 — hydrogen. The pop sound with a burning splint is its test.
Which salt is formed in the reaction of zinc with sodium hydroxide?
Correct answer: (B) Sodium zincate
2NaOH + Zn → Na2ZnO2 + H2 — the salt is sodium zincate (Na2ZnO2) and the gas is hydrogen.
Which gas turns lime water milky?
Correct answer: (C) Carbon dioxide
Carbon dioxide: Ca(OH)2 + CO2 → CaCO3 + H2O. CO, O₂ and Cl₂ do not form this precipitate.
Why does the milkiness disappear when excess CO₂ is passed through lime water?
Correct answer: (D) Insoluble CaCO₃ changes into soluble Ca(HCO₃)₂
CaCO3 + H2O + CO2 → Ca(HCO3)2 — calcium hydrogencarbonate is soluble in water, so the solution turns clear.
What is formed when an acid reacts with a base?
Correct answer: (A) A salt and water
A salt and water — NaOH + HCl → NaCl + H2O. This is called neutralisation.
8 mL of HCl neutralises 10 mL of NaOH solution. How much HCl does 20 mL of the same NaOH need?
Correct answer: (B) 16 mL
The ratio 10 : 8 = 20 : 16. Double the base of the same solution needs double the acid.
What colour does the solution become when dilute HCl is added to copper oxide?
Correct answer: (C) Blue-green
CuO + 2HCl → CuCl2 + H2O — a solution of copper(II) chloride is blue-green.
In Activity 2.8, in which solution does the bulb NOT glow?
Correct answer: (D) Glucose solution
Glucose (and alcohol) do not give ions in water, so no current flows. Acids and alkalis give ions — the bulb glows.
Which ion is responsible for the acidic properties of all acids?
Correct answer: (A) H⁺(aq)
In solution acids give H⁺(aq) — this common ion makes every acid acidic.
What is the formula of the hydronium ion?
Correct answer: (B) H₃O⁺
H+ + H2O → H3O+ — the hydronium ion is H₃O⁺.
Which of these ions turns red litmus blue?
Correct answer: (B) OH⁻
Bases give OH⁻ (hydroxide) ions in water, and these ions turn red litmus blue. H⁺ does the opposite, turning blue litmus red. Na⁺ and Cl⁻ do not change litmus.
What is the correct way to dilute a concentrated acid?
Correct answer: (C) Add the acid slowly to water while stirring
Add the acid slowly to water with stirring, so that the heat spreads through plenty of water and nothing splashes.
When a solution of an acid is diluted, the concentration of H₃O⁺ ions…
Correct answer: (D) decreases
Adding water lowers the number of H₃O⁺ ions per unit volume — that is dilution.
A solution turns red litmus blue. Its pH is likely to be
Correct answer: (A) 10
The solution is basic, so its pH is above 7 — among the options only 10.
Which word does the “p” in pH come from?
Correct answer: (B) potenz (German, meaning power)
“p” = potenz, a German word meaning power.
Which of these is a weak acid?
Correct answer: (C) CH₃COOH
Acetic acid (CH₃COOH) gives few H⁺ ions at the same concentration — a weak acid. The other three are strong acids.
Solution A has pH 6 and solution B has pH 8. Which has the higher concentration of H⁺ ions?
Correct answer: (A) A
A (pH 6) has more H⁺ — A is acidic and B (pH 8) is basic.
Which of the following is a strong base?
Correct answer: (B) NaOH
A strong base breaks up almost completely in water and gives many OH⁻ ions. NaOH (caustic soda) and KOH are strong bases. NH₄OH is a weak base. Mg(OH)₂ is a mild base that dissolves only a little, which is why it is used as an antacid (milk of magnesia).
When is rain water called acid rain?
Correct answer: (D) When its pH is below 5.6
When the pH of rain water is below 5.6, it is acid rain.
When does a farmer add quicklime to the soil?
Correct answer: (A) When the soil is too acidic
CaO is a base — it neutralises the extra acid of acidic soil.
Below which pH in the mouth does tooth decay start?
Correct answer: (B) 5.5
Below pH 5.5 the enamel (calcium hydroxyapatite) starts to corrode.
What will the pH of an aqueous solution of sodium carbonate be?
Correct answer: (C) More than 7
Strong base (NaOH) + weak acid (H2CO3) → basic, pH more than 7.
To which family of salts do NaCl and KCl belong?
Correct answer: (D) Chloride family
Both have the same negative radical Cl- — the chloride family.
In the chlor-alkali process, which gas is given off at the anode?
Correct answer: (A) Chlorine
Chlorine at the anode, hydrogen at the cathode, and NaOH forms near the cathode.
Over what is chlorine passed to make bleaching powder?
Correct answer: (B) Dry slaked lime (Ca(OH)₂)
Bleaching powder is made by the action of chlorine on dry slaked lime Ca(OH)2.
What is the chemical name of baking soda?
Correct answer: (C) Sodium hydrogencarbonate
Sodium hydrogencarbonate NaHCO3. Sodium carbonate Na2CO3 is the basis of washing soda.
What is the chemical formula of washing soda?
Correct answer: (D) Na₂CO₃·10H₂O
Washing soda = Na2CO3·10H2O. NaHCO₃ is baking soda and CaSO₄·2H₂O is gypsum.
Which sodium compound is used to remove the permanent hardness of water?
Correct answer: (A) Sodium carbonate
Sodium carbonate (washing soda): Na2CO3 + CaCl2 → CaCO3 + 2NaCl.
What colour does blue copper sulphate turn on heating?
Correct answer: (B) White
When the water of crystallisation leaves, anhydrous CuSO4 is left, which is white.
How many molecules of water of crystallisation are there in one formula unit of gypsum?
Correct answer: (C) 2
Gypsum = CaSO4·2H2O — 2 water molecules.
A solution turns red litmus blue, its pH is likely to be
Correct answer: (D) 10
A basic solution has pH > 7. Of the options, only 10 is greater than 7.
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
Correct answer: (B) HCl
Egg-shell is CaCO₃; only an acid (HCl) releases CO₂ from it, and the CO₂ turns lime-water milky. NaCl, LiCl and KCl are neutral salts.
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be
Correct answer: (D) 16 mL
10 mL NaOH ↔ 8 mL HCl. Doubling the NaOH (20 mL) doubles the HCl = 2 × 8 = 16 mL.
Which one of the following types of medicines is used for treating indigestion?
Correct answer: (C) Antacid
Indigestion means excess acid in the stomach. An antacid (such as milk of magnesia, Mg(OH)₂) is a mild base that neutralises the extra acid.
A solution turns red litmus blue. What would be its pH?
Correct answer: (B) 11
Red litmus turning blue = a basic solution, so pH above 7. Only 11 fits. (NCERT Exercise Q1 asks the same with options 1, 4, 5, 10 → 10)
Which of the following gases turns lime water milky?
Correct answer: (D) CO2
CO2 → a white precipitate of CaCO3: Ca(OH)2 + CO2 → CaCO3 + H2O
Which of the following acids is present in a bee sting?
Correct answer: (A) Methanoic acid
The acid in a sting is methanoic (formic) acid. Rubbing a mild base such as baking soda on it gives relief.
The chemical formula of 'Plaster of Paris' is —
Correct answer: (B) CaSO4·½H2O
Plaster of Paris = calcium sulphate hemihydrate CaSO4·½H2O. CaSO4·2H2O is gypsum.
Acid and base react with each other to form —
Correct answer: (C) Salt
Neutralisation: acid + base → salt + water. NaOH + HCl → NaCl + H2O
Which of the following gases is formed as a result of the reaction of zinc and dilute hydrochloric acid?
Correct answer: (B) H2
Acid + metal → salt + hydrogen. A burning splint makes the gas burn with a pop.
Which of the following gases turns lime water milky?
Correct answer: (B) CO2
CO2 reacts with lime water to give a white precipitate of calcium carbonate, so the water looks milky.
Which among the following is slaked lime?
Correct answer: (C) Ca(OH)2
Slaked lime = calcium hydroxide Ca(OH)2. Quicklime is CaO, and CaCO3 is limestone. Chlorine on dry slaked lime gives bleaching powder.
Toothpaste is generally —
Correct answer: (C) Alkaline
Tooth decay starts when the mouth pH falls below 5.5. Toothpastes are generally alkaline and neutralise the extra acid in the mouth.
Which acid is present in lemon?
Correct answer: (A) Citric acid
Lemons and oranges contain citric acid (NCERT Table 2.3). Lactic acid is in curd, acetic acid in vinegar, formic (methanoic) acid in an ant sting.
The pH value of milk of magnesia is —
Correct answer: (D) 10.5
Milk of magnesia (Mg(OH)2) is a mild base, so its pH is well above 7. NCERT Fig. 2.7 shows it at about 10; the closest option is 10.5.
The chemical name of lime water is —
Correct answer: (B) Calcium hydroxide
Lime water = an aqueous solution of calcium hydroxide Ca(OH)2. «Slaked lime» is its common name, not its chemical name; quicklime/calcium oxide CaO is a different substance.
What is an olfactory indicator?
Correct answer: (D) One whose smell changes in an acidic or a basic medium
A substance whose smell changes in an acidic or a basic medium is an olfactory indicator — like onion, vanilla and clove.
In Activity 2.2, which can work as an olfactory indicator?
Correct answer: (A) Onion, vanilla and clove — all three
The smell of all three behaves differently in acid and in base, so all three are olfactory indicators.
Why should curd not be kept in a copper vessel?
Correct answer: (B) The acid in curd reacts with copper and forms harmful compounds
Curd contains lactic acid. The acid reacts with copper/brass and forms poisonous compounds that make the food unsafe.
What is the heart of a neutralisation reaction in terms of ions?
Correct answer: (C) H⁺ + OH⁻ → H₂O
The acid gives H⁺(aq) and the base gives OH⁻(aq) — together they make water: H+(aq) + OH-(aq) → H2O(l).
What is mixed with baking soda in baking powder?
Correct answer: (D) Tartaric acid
Tartaric acid — weak and edible. It makes NaHCO₃ give CO₂ and leaves no bitter Na₂CO₃ behind.
What is the chemical formula of Plaster of Paris?
Correct answer: (A) CaSO₄·½H₂O
Plaster of Paris = CaSO4·½H2O (calcium sulphate hemihydrate). CaSO₄·2H₂O is gypsum.
At what temperature is gypsum heated to make Plaster of Paris?
Correct answer: (B) 373 K
At 373 K (100 °C). If heated much more, all the water leaves and the plaster will not set.
What colour does phenolphthalein give in NaOH solution?
Correct answer: (C) Pink
Phenolphthalein is colourless in acid and pink in a base. NaOH is a base.
In which solution is the smell of onion lost?
Correct answer: (D) Dilute NaOH
Onion, vanilla and clove oil are olfactory indicators — their smell disappears in a base (NaOH) and stays in an acid.
Which gas is evolved when zinc reacts with dilute sulphuric acid?
Correct answer: (A) Hydrogen
Zn + H₂SO₄ → ZnSO₄ + H₂
— acid + metal → salt + hydrogen.
Equal Mg ribbons are treated with HCl (tube A) and CH₃COOH (tube B) — where is the fizzing more vigorous?
Correct answer: (A) In A, because HCl is a strong acid
HCl ionises completely and gives more H⁺, so H₂ forms faster in A.
A solution reacts with crushed egg-shells to give a gas that turns lime water milky. The solution contains —
Correct answer: (B) HCl
Egg-shell is CaCO₃; only HCl (an acid) releases CO₂ from it. The other three are salts. (NCERT Exercise 2)
10 mL of NaOH is completely neutralised by 8 mL of HCl. How much of the same HCl is needed for 20 mL of the NaOH?
Correct answer: (C) 16 mL
In neutralisation acid and base react in a fixed ratio — double the NaOH, double the HCl: 2 × 8 = 16 mL. (NCERT Exercise 3)
Glucose and alcohol contain hydrogen — why are they not classed as acids?
Correct answer: (D) They do not give H⁺ ions in water
Acidic character comes from forming H⁺(aq) in water. Glucose/alcohol do not ionise — no ions, no current, no acidity. (Exercise 6)
When water is added to an acid solution, the concentration of hydronium ions (H₃O⁺) —
Correct answer: (A) decreases
On dilution the number of ions stays the same but the volume increases — concentration per unit volume decreases (it does not become zero).
Which solution has the most H⁺ ions?
Correct answer: (B) pH 2
pH 2 is the smallest, so it has the most H⁺. pH 8 is basic.
What colour does universal indicator give in a roughly neutral solution?
Correct answer: (C) Green
Around pH 7 universal indicator is green. Red is a strong acid, violet a strong base.
Tooth enamel starts to decay below which pH?
Correct answer: (D) 5.5
When the mouth pH falls below 5.5, the calcium phosphate of enamel decays.
What is suitable to rub on a bee sting?
Correct answer: (A) Baking soda
A mild base (baking soda) neutralises the acidic sting. Vinegar is for a wasp’s basic sting.
Which salt gives an acidic aqueous solution?
Correct answer: (B) NH₄Cl
NH₄Cl comes from strong HCl and weak NH₄OH, so the solution is acidic. Na₂CO₃ and CH₃COONa are basic.
Where is chlorine released during the electrolysis of brine?
Correct answer: (C) At the anode
Cl₂ is released at the anode and H₂ at the cathode. NaOH stays in the solution.
Bleaching powder is formed by the action of which pair?
Correct answer: (D) Ca(OH)₂ + Cl₂
Bleaching powder is made by passing chlorine over dry slaked lime.
Which gas makes a cake rise?
Correct answer: (A) CO₂
Decomposition of NaHCO₃ releases CO₂, which makes bubbles in the dough.
Which is the correct formula of washing soda?
Correct answer: (B) Na₂CO₃·10H₂O
Washing soda is the decahydrate of sodium carbonate — Na₂CO₃·10H₂O.
Which pair is correct?
Correct answer: (C) Bleaching powder — Ca(ClO)₂
Ca(ClO)₂ is bleaching powder. Baking soda is NaHCO₃, washing soda is Na₂CO₃·10H₂O and gypsum is CaSO₄·2H₂O.
Which of these is a base?
Correct answer: (D) Ca(OH)₂
Ca(OH)₂ is a base. HCl is an acid, NaCl is a salt, CO₂ is an acidic oxide.
What colour does phenolphthalein give in a base?
Correct answer: (A) Pink
Phenolphthalein is colourless in acid and pink in a base.
How does methyl orange look in an acid?
Correct answer: (B) Red
Methyl orange turns red in an acidic medium.
Litmus is obtained from which source?
Correct answer: (C) Lichen
Litmus is a purple dye extracted from lichen.
In which medium is the onion smell lost?
Correct answer: (D) Dilute NaOH
NaOH is a base — the smell of onion, vanilla and clove is lost in it.
Which gas is formed from zinc and dilute H₂SO₄?
Correct answer: (A) H₂
Acid + metal → salt + hydrogen.
Which gas turns lime water milky?
Correct answer: (B) CO₂
CO₂ forms a white precipitate of CaCO₃ with lime water, Ca(OH)₂.
Black CuO dissolves in dilute HCl to give a solution of which colour?
Correct answer: (C) Blue-green
CuO + 2HCl → CuCl₂ + H₂O. CuCl₂ is blue-green.
Why does the bulb not glow in aqueous glucose?
Correct answer: (D) Ions are not formed
Glucose and alcohol do not give H⁺, so no current flows.
Why does dry HCl gas not affect dry litmus?
Correct answer: (A) Water is needed to form H⁺
Without water HCl cannot give H⁺, so dry litmus does not change.
How is concentrated sulphuric acid diluted?
Correct answer: (B) Acid into water, slowly
Add the acid slowly to water and swirl. The other way round, water can boil and splash.
Which is the strongest acid?
Correct answer: (B) pH 1
At pH 1 the amount of H⁺ is the greatest.
What is the nature of an antacid?
Correct answer: (C) Mild base
A base such as Mg(OH)₂ makes a salt and water with the extra acid.
What is an aqueous solution of Na₂CO₃ like?
Correct answer: (D) Basic
A solution of sodium carbonate is basic, with pH above 7.
What is the formula of bleaching powder?
Correct answer: (A) Ca(ClO)₂
Bleaching powder is Ca(ClO)₂. CaO is quicklime.
What is the formula of washing soda?
Correct answer: (B) Na₂CO₃·10H₂O
Washing soda is sodium carbonate decahydrate.
What remains when blue vitriol is heated?
Correct answer: (C) White CuSO₄
The colour turns white and drops of water collect on the tube.
At what temperature is plaster of Paris formed?
Correct answer: (D) 373 K
Heating gypsum at 373 K gives CaSO₄·½H₂O.
A universal indicator is a mixture of —
Correct answer: (A) several indicators
A universal indicator is a mixture of several indicators, so it shows a different colour at each H⁺ concentration.
Why does nitric acid not give hydrogen with most metals?
Correct answer: (B) it is a strong oxidising agent
HNO₃ is a strong oxidising agent — it oxidises the H₂ formed to water, so H₂ is usually not obtained.
Which is an ingredient of antacids?
Correct answer: (C) NaHCO₃
Baking soda NaHCO₃ is mildly basic and neutralises excess stomach acid.
In the chlor-alkali process, where does sodium hydroxide form?
Correct answer: (D) near the cathode
Cl₂ forms at the anode, H₂ at the cathode, and NaOH solution near the cathode.
Which acid is injected by the stinging hair of nettle leaves?
Correct answer: (A) methanoic acid
Nettle and ant stings inject methanoic acid; rubbing a dock leaf (basic) gives relief.
How many molecules of water of crystallisation are in one formula unit of washing soda?
Correct answer: (B) 10
Washing soda Na₂CO₃·10H₂O — 10 water molecules. Blue vitriol 5, gypsum 2, POP ½.
Rain is called acid rain when its pH is —
Correct answer: (C) below 5.6
Rain water with pH below 5.6 is acid rain; it lowers the pH of rivers and harms aquatic life.
Dissolving which substance in water is highly exothermic?
Correct answer: (D) concentrated H₂SO₄
Dissolving a concentrated acid (and NaOH pellets) in water gives out a lot of heat, so acid is added slowly to water.
Choose the correct order of increasing pH: gastric juice, pure water, lemon juice, milk of magnesia
Correct answer: (A) gastric juice < lemon juice < pure water < milk of magnesia
pH: gastric juice ≈ 1.2 < lemon juice ≈ 2.2 < pure water 7 < milk of magnesia ≈ 10 (NCERT Fig. 2.7).
Five solutions have pH 4, 1, 11, 7 and 9. Which is the correct decreasing order of H⁺ ion concentration?
Correct answer: (B) 1 > 4 > 7 > 9 > 11
More H⁺ means lower pH, so decreasing H⁺ = increasing pH: 1 > 4 > 7 > 9 > 11 (second part of NCERT Exercise Q9).
Which of these has the lowest pH?
Correct answer: (C) vinegar
Vinegar (acetic acid) ≈ 3; water 7; blood ≈ 7.4; baking soda ≈ 8.3. Most acidic = lowest pH = vinegar.
Solution P has pH 3 and Q has pH 5. Compared with Q, the H⁺ concentration in P is about —
Correct answer: (D) 100 times more
A difference of 2 pH units = 10 × 10 = 100 times. The lower pH (P) has more H⁺.
A white solid X fizzes in dilute HCl. Gas Y turns lime water milky and puts out a burning candle. One product is CaCl₂. What is X?
Correct answer: (A) CaCO₃
Y = CO₂ (milky, candle out), and CaCl₂ formed, so X contains both Ca and carbonate: X = CaCO₃.
Metal M in dilute H₂SO₄ gives a gas that pops with a burning splint. Heated with NaOH solution, M gives the same gas and the salt Na₂MO₂. What is M?
Correct answer: (B) Zn
Pop = H₂. The metal that gives H₂ with both an acid and NaOH, forming Na₂MO₂, is zinc (Na₂ZnO₂, sodium zincate). Cu, Ag and Au do not give H₂ with dilute acids.
A white powder P mixed with water sets into a hard solid Q in a few minutes. Heating Q at 373 K gives P back. P and Q are —
Correct answer: (C) P = Plaster of Paris, Q = gypsum
P = CaSO₄·½H₂O (Plaster of Paris) takes up water to form Q = CaSO₄·2H₂O (gypsum); heating gypsum at 373 K gives POP again.
Which of the following is an alkali?
Correct answer: (D) NaOH
NaOH dissolves in water, so it is an alkali. CuO is a base but not an alkali.
Case: A farmer tested the soil of his field and got a pH of 4.8. His crop needs a pH of about 6.5–7. What should he add to the soil, and why?
Correct answer: (A) Quicklime (CaO) — it is a base and will neutralise the extra acid
pH 4.8 = acidic soil. Adding a base (quicklime CaO, slaked lime Ca(OH)2 or chalk CaCO3) neutralises it and the pH rises. Vinegar and lemon are acids — they would lower the pH further; salt is neutral — it would not change the pH.
Data-based: Four solutions have pH values P = 2, Q = 6, R = 9, S = 13. What is the correct increasing order of H⁺ ion concentration?
Correct answer: (A) S < R < Q < P
Lower pH = more H⁺. So H⁺ is lowest in S (pH 13) and highest in P (pH 2): S < R < Q < P.
Situation: After eating sweets, the pH in the mouth fell to about 5. Which is the most scientific step to protect the teeth?
Correct answer: (A) Rinse the mouth at once and brush with a basic toothpaste
Mouth bacteria make acids from leftover sugar; below pH 5.5 the enamel (calcium hydroxyapatite) starts to corrode. Rinsing removes acid and sugar, and a basic toothpaste neutralises the remaining acid. Enamel does not dissolve in water, but it is corroded by acid.
A student read four pH values: A = 2, B = 6, C = 7, D = 12. Which is most harmful to tooth enamel?
Correct answer: (A) A
The lowest pH gives the most H⁺. A (pH 2) is the most harmful to enamel.
Two beakers have the same amount of Zn. Dilute HCl is added to A and vinegar to B. Where is hydrogen formed faster?
Correct answer: (A) A
HCl is a strong acid and acetic acid (vinegar) is weak. The reaction is faster in A.
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