Which of the following are exothermic processes ?
(i) Reaction of water with quick lime
(ii) Melting of ice
(iii) Evaporation of water
(iv) Dilution of an acid

Question

Which of the following are exothermic processes ? 
(i) Reaction of water with quick lime 
(ii) Melting of ice 
(iii) Evaporation of water 
(iv) Dilution of an acid

Accepted Answer

Option 3 is the correct answer because it includes the processes that are exothermic, meaning they release heat.

1. **Explanation for Option 3: (i) Reaction of water with quick lime and (iv) Dilution of an acid**

- **(i) Reaction of water with quick lime**: When water is added to quick lime (calcium oxide), it reacts to form calcium hydroxide. This reaction is highly exothermic, releasing a significant amount of heat. The chemical equation for this reaction is:
     $$
     \text{CaO (s) + H}_2\text{O (l) → Ca(OH)}_2\text{ (aq) + heat}
     $$
     The release of heat is evident as the mixture becomes hot.

- **(iv) Dilution of an acid**: When an acid is diluted with water, the process is exothermic. The ions in the acid interact with water molecules, releasing energy in the form of heat. This is why it is always advised to add acid to water slowly to prevent splattering due to the heat generated.

2. **Explanation for why other options are incorrect:**

- **Option 1: (i) and (ii)**
     - **(ii) Melting of ice**: This is an endothermic process. Melting requires the absorption of heat to change the state from solid (ice) to liquid (water). Therefore, it does not release heat and is not exothermic.

- **Option 2: (ii) and (iii)**
     - **(iii) Evaporation of water**: Evaporation is also an endothermic process. It requires heat to convert water from a liquid to a gaseous state. This process absorbs heat from the surroundings, which is why it feels cool.

- **Option 4: (iii) and (iv)**
     - As explained, (iii) Evaporation of water is endothermic, not exothermic. Therefore, this option is incorrect.

In conclusion, Option 3 correctly identifies the exothermic processes: the reaction of water with quick lime and the dilution of an acid. Both processes release heat, aligning with the definition of exothermic reactions.