Which of the following are exothermic processes?

I) Reaction of water with quick lime

II) Dilution of an acid

III) Evaporation of water

IV) Sublimation of camphor (crystals)

I and II

1. Reaction of water with quick lime

When water is added to quick lime (CaO), it reacts vigorously and releases a significant amount of heat. This process is highly exothermic. The chemical reaction involved is:

CaO + H₂O → Ca(OH)₂ + heat

In this reaction, calcium oxide reacts with water to form calcium hydroxide (slaked lime), and energy in the form of heat is released, making this an exothermic reaction. The release of heat indicates that the system is giving out energy to the surroundings.

2. Dilution of an acid

Diluting a concentrated acid with water is another exothermic process. When water is added to an acid, especially strong acids like sulfuric acid (H₂SO₄), it releases heat. The heat is produced because the water molecules break the ionic bonds between the acid molecules, and this results in the release of energy. This is why we are advised to always add acid to water, not the other way around, to prevent a sudden, dangerous release of heat.

3. Evaporation of water

Unlike the processes mentioned above, evaporation of water is an endothermic process. In this process, water molecules absorb heat from their surroundings to overcome the intermolecular forces and transition into the gaseous phase. The absorption of heat from the environment means the surrounding area gets cooler, which is why sweat evaporation cools our skin.

4. Sublimation of camphor

Sublimation is the phase transition from a solid directly to a gas, bypassing the liquid phase. When camphor (C₁₀H₁₆O) is heated, it absorbs heat energy to break the bonds between its molecules and transform directly into vapor. Since heat is absorbed in the process, sublimation of camphor is an endothermic reaction.