CAPF AC 2022 Q39: Science & Technology – Chemistry
- (a) combination reaction
- (b) decomposition reaction
- (c) oxidation reaction
- (d) addition reaction
Answer: (a) combination reaction
Explanation
Quicklime and water
How to think: count the substances on each side.
Two substances join to make one.
That is a combination reaction.
The reaction: CaO + H2O → Ca(OH)2 + heat.
| Substance | Common name |
|---|---|
| CaO | Quicklime |
| Ca(OH)2 | Slaked lime |
| CaCO3 | Limestone, marble, chalk |
The three basic patterns:
| Type of reaction | Pattern |
|---|---|
| Combination | A + B → AB |
| Decomposition | AB → A + B |
| Displacement | A + BC → AC + B |
Also note: the reaction gives out a lot of heat, so it is exothermic.
Slaked lime is used for whitewashing; on the wall it slowly turns into calcium carbonate.
Answer: (a) combination reaction.
Why each option is right or wrong
- (a) CORRECT. Two reactants form a single product, so it is a combination reaction.
- (b) Incorrect. In decomposition one substance breaks into two or more.
- (c) Incorrect. No element gains oxygen or loses electrons here, so it is not oxidation.
- (d) Incorrect. Addition reaction is the term for adding atoms across a double bond in organic compounds.
Key point
CaO + H2O → Ca(OH)2 + heat: combination and exothermic. Whitewash: Ca(OH)2 + CO2 → CaCO3 + H2O. Heating limestone: CaCO3 → CaO + CO2 (decomposition, endothermic).
Remember
CaO + H2O → Ca(OH)2: combination, exothermic.
Common mistakes
- Mixing up quicklime (CaO) with slaked lime (calcium hydroxide).
- Marking addition reaction, a term kept for unsaturated carbon compounds.
- Calling it decomposition because heat is involved.
Exam tip
Educated guess: the word combination matches the picture exactly: two things combine into one. This is the first example of the type in NCERT.
Sources
- NCERT Class X Science — Chemical Reactions and Equations (combination reaction)
- NCERT Class XI Chemistry — The s-Block Elements (compounds of calcium)