UPSC NDA I 2017 General Ability Solved Paper

Concept:The heat needed to change a liquid into a gas without changing its temperature is called latent heat of vaporization.Explanation:When a liquid absorbs heat, its temperature normally rises.But during vaporization, the added heat is used to overcome intermolecular forces, not to increase molecular kinetic energy.So the temperature stays constant even as the liquid turns to gas.This absorbed heat is called "latent" (hidden) because it does not cause a temperature rise.It is measured per unit mass, for example water at $100^\circ\text{C}$ requires about $2.26 \times 10^6\ \text{J/kg}$.This is different from specific heat capacity, which causes a temperature change.Mechanical equivalent of heat ( $4.1868\ \text{J/cal}$ ) converts work into heat, unrelated to phase change.Quenching is rapid cooling for metal heat treatment, not a heating process.Thus, the correct term for heat needed for liquid-to-gas transition at constant temperature is latent heat of vaporization.Answer:C. Latent heat of vaporization