Which one of the following reactions is an example of decomposition reaction?
- (a)CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
- (b)2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)
- (c)CuO + H₂ —Heat→ Cu + H₂O
- (d)Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Answer
Why
Correct — B, 2AgCl(s) → 2Ag(s) + Cl₂(g). A decomposition reaction is one in which a single compound breaks down into two or more simpler substances. Here silver chloride, driven by sunlight (photo-decomposition), splits into silver metal and chlorine gas — one reactant becoming two products. The other options are not decompositions: (a) is the combustion of methane, while (c) and (d) are displacement/redox reactions in which one element replaces another.
Why the others are wrong
- (a)CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) — This is the combustion (oxidation) of methane — two reactants combine and burn; no single compound breaks into simpler parts, so it is not a decomposition.
- (c)CuO + H₂ —Heat→ Cu + H₂O — Hydrogen reduces copper oxide (a displacement/redox reaction); two reactants are involved and one element displaces another, so it is not a decomposition.
- (d)Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — Iron displaces copper from copper sulphate — a single displacement reaction with two reactants, not the breakdown of one compound.
Concept
In a decomposition reaction a single compound is broken down into two or more products, usually by supplying energy as heat, light or electricity. It is the reverse of a combination reaction and is often endothermic. Identifying it is a matter of counting: a decomposition has exactly one reactant.
Three options have two reactants (combustion, and two displacement reactions), so they are ruled out at once. Only silver chloride sits alone on the left, breaking into silver and chlorine under sunlight — the hallmark of decomposition.
Key facts
- In a decomposition, one compound splits into two or more products, e.g., 2AgCl → 2Ag + Cl₂ under light.
- It can be thermal (by heat), electrolytic (by electricity) or photochemical (by light).
- Silver chloride and silver bromide darken in light — the basis of old photographic film.
- Combination/combustion, displacement and double-displacement are the other main reaction types.
A single reactant splitting into two or more products marks a decomposition — option (b).
Study next
Common traps
- Confusing displacement (Fe + CuSO₄) with decomposition — displacement has two reactants.
- Missing that sunlight, heat or electricity is often the trigger for a decomposition.
Classify a given equation; a decomposition is a single reactant breaking into two or more products.
Related PYQs
At nearly 70 °C, sodium bicarbonate shows the property of gradually decomposing, which makes it usable in bakery products. The product of decomposition responsible for this use of sodium bicarbonate is
- (a) Carbon dioxide
- (b) Hydrogen
- (c) Water vapour
- (d) Oxygen
Answer(a) Carbon dioxide
Same concept — thermal decomposition. Baking soda (NaHCO₃) decomposes on heating to release carbon dioxide, which raises the dough; both items rest on a single compound breaking down on heating.
Practice
- practice — not a real PYQ
Heating calcium carbonate (CaCO₃ → CaO + CO₂) is an example of a
- (a)combination reaction
- (b)decomposition reaction
- (c)displacement reaction
- (d)neutralisation reaction
Answer(b) decomposition reaction — a single compound breaks down on heating. - practice — not a real PYQ
Electrolysis of water (2H₂O → 2H₂ + O₂) is which type of reaction?
- (a)combination
- (b)displacement
- (c)decomposition
- (d)double displacement
Answer(c) decomposition — an electrolytic decomposition of one compound.