The amount of heat absorbed or released to change the state of a unit mass of a substance, without changing its temperature, is called the latent heat of that substance of that state.
The unit of latent heat in the CGS system is calories/gram. The SI unit of latent heat is joule/kg.
Difference Between Latent heat and Sensible Heat
| Latent heat | Sensible heat |
| The state of matter changes, but the temperature remains the same. | Sensible heat does not change the state of matter, but the temperature does. |
| It cannot be detected by the thermometer. | Sensible heat can be detected with the help of a thermometer. |
The latent heat of fusion is the amount of heat required to convert a unit mass of the substance from the solid state to the liquid state without changing the temperature.
The latent heat of melting of ice is 40 kcal/g, meaning that 40 kcal of heat is required to change 1 g of ice at 0°C to 1 g of water at 0°C at standard pressure.
The heat required to completely vaporize a unit mass of a liquid at its boiling point at constant pressure is called the latent heat of vapourization of that liquid.
The latent heat of the vaporization of water is 537 kcal/g, meaning that 537 kcal of heat is required to convert 1 g of water at 100°C to 1 g of steam at 100°C at standard pressure.
The latent heat of vaporization is greater than the latent heat of melting Reason: The intermolecular spacing of molecules must increase much more to change a solid from liquid to vapour than to convert it to liquid by applying heat. As a result, a lot of work has to be done. So more heat energy is required. This heat provides absorption. So the latent heat of vapourization is greater than the latent heat of melting.