#### Chemistry (NTSE/Olympiad)

# 1. Matter In Our Surrounding

__Latent Heat__

**Latent Heat **

**Defination : ** The heat energy which has to be supplied to change the state of a substance is called its latent heat.

Latent heat does not raise the temperature but latent heat has always to be supplied to change the state of a substance. The word ‘latent’ means ‘hidden’

Every substance has some forces of attraction between its particles which hold them together. Now, if a substance has to change its state, then it is necessary to break these forces of attraction between its particles. The latent heat does not increase the kinetic energy of the particles of the substance, the temperature of a substance does not rise during the change of state.

**Latent heat is of two types**

**Latent heat of fusion : ** The heat required to convert a solid into the liquid state is called latent heat of fusion. In other words ‘The latent heat of fusion of a solid is the quantity of heat in joules required to convert 1 kilogram of the solid to liquid, with out any change in temperature.

**Example.** The latent heat of fusion of ice = 3.34 × 10 ^{5 } J/kg

**Latent heat of vaporisation : ** The heat required to convert a liquid into the vapour state is called latent heat of vaporisation.

The other words ‘The latent heat of vaporisation of a liquid is the quantity of heat in joules required to convert 1 kilogram of the liquid to vapour or gas, without any change in temperature.

**Example.** Latent heat of vaporisation of water = 22.5 × 10^{5 }J/kg