Latent heat is the quantity of heat absorbed or released by a substance undergoing a change of state, such as ice changing to water or water to steam, at constant temperature and pressure
. It is associated with phase changes, including solid to liquid, liquid to gas, and vice versa
. There are three types of latent heat, corresponding to the three phases of a substance: solid, liquid, and gas
- Latent heat of fusion : The amount of heat required to melt a substance from its solid to liquid state at a constant temperature and pressure
- Latent heat of vaporization : The amount of heat required by the unit mass of a substance to vaporize from its liquid to gaseous state at a constant temperature and pressure
- Latent heat of sublimation : The amount of heat required by the unit mass of a substance to convert directly from solid to gaseous state at a constant temperature and pressure
Latent heat is related to the concept of enthalpy, which is the energy content of a substance in a specific state
. During a phase change, the substance's enthalpy changes, and the latent heat is the energy required to overcome the attractive forces holding the molecules together in their new state
. For example, when a solid substance turns into a liquid, it needs to absorb energy to push the molecules into a wider, more fluid volume. Conversely, when a substance changes from a gas phase to a liquid, its density levels need to increase, and the substance then releases or loses energy so that the molecules come closer together
