Latent heat is the quantity of heat released or absorbed by a substance undergoing a change of state that occurs without changing its temperature, such as ice changing to water or water to steam, at constant temperature and pressure. It is also called heat of transformation.

Two of the more common forms of latent heat are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: solid → liquid → gas.

When a substance is heated at a uniform rate, the temperature of the solid rises uniformly until the melting point is reached. At the melting point heat is absorbed and used to melt the solid without any temperature change (latent heat of fusion). All the energy at this point is used to weaken the intermolecular forces between the particles in the solid.

The temperature starts to increase uniformly again until the boiling point is reached. At the boiling point heat is absorbed without any change in temperature (latent heat of vaporization), all the energy absorbed is being used to overcome the intermolecular forces between the particles in the liquid. When all the liquid has been vaporized to gas the temperature will once again increase.

Latent heat released during condensation is an important source of energy to drive atmospheric systems like hurricanes and cumulus clouds.

## Units and Measures

Latent heat (L) is the quantity of heat absorbed or released when a substance changes its physical phase from solid to liquid at the melting point or from liquid to gas at the boiling point at constant temperature and pressure. For example, the latent heat of vaporization is the energy a substance absorbs from its surroundings in order to overcome the attractive forces between its molecules as it changes from a liquid to a gas and in order to do work against the external atmosphere as it expands. In thermodynamic terms the latent heat is the enthalpy of evaporation (ΔH), i.e. L=ΔH=ΔU+pΔV, where ΔU is the change in the internal energy, p is the pressure, and ΔV is the change in volume.

Following are the units for measuring latent heat:

• SI Units: Kilo Joule per Kilogram (kJ/kg)
• Imperial Units: British thermal unit per Pound (Btu/lb)

### Example Values

• Water
• Latent heat of fusion: 334 KJ/Kg
• Latent heat of vaporization: 2260KJ/Kg