Class 10 Science Chapter 1: Important & Previous Year Questions – Bihar Board

Correct answer: (C) Hydrogen

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).

Correct answer: (C) Carbon is being oxidised

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.

Correct answer: (B) H₂

Zinc is more reactive than hydrogen, so it removes hydrogen from the acid (displacement):

Test: a burning splinter brought near it gives a ‘pop’ sound — that is H₂ gas.

Correct answer: (C) Ca(OH)₂

Quicklime = CaO. Adding water to it forms slaked lime Ca(OH)₂ and gives out a lot of heat:

CaCO₃ = limestone / marble.

Correct answer: (B) Calcium hydroxide

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.

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:

Here Ba²⁺ and SO₄^2- ions combine to form insoluble BaSO₄; NaCl stays dissolved in the solution.

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:

(Another example: burning of coal C(s) + O₂(g) → CO₂(g))

Correct answer: (D) AgNO₃

Copper is more reactive than silver, so it removes silver from AgNO₃:

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.

Correct answer: (B) Exothermic

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.

Correct answer: (D) Precipitation

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.

Correct answer: (D) Pb(NO₃)₂ → PbO + NO₂ + O₂

In (D) there are 2 N and 6 O on the left but 1 N and 5 O on the right — unbalanced. The balanced form:

In the other three equations the atoms of each element are equal on both sides.

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 —

Another example: CaO(s) + H₂O(l) → Ca(OH)₂(aq) (formation of slaked lime)

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.

Correct answer: (A) Fe₃O₄

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.)