Physics Theory : Change of State & Latent Heat

Change of state occurs when the heat gained by (or lost from) a body is not accompanied by a rise (or drop) in its temperature.

Latent heat is defined as the heat gained or lost by a body during a change of state when it does not result in any change in temperature.
It is also referred to as hidden heat.
It is given by the product of the mass of that body and its specific latent heat.
∴ Latent heat = Mass × Specific latent heat

Specific latent heat of fusion is defined as the amount of heat required to change a unit mass of a solid at its melting point into a liquid, without changing its temperature.

Specific latent heat of vaporization is defined as the amount of heat required to change a unit mass of a liquid at its boiling point into a gas (vapor), without changing its temperature.

Fusion or melting is the process by which a solid changes into the liquid state at a certain fixed temperature by the absorption of heat energy.
The fusion point or melting point is the fixed temperature at which a solid starts changing into the liquid state.

Solidification or freezing is the process by which a liquid changes into the solid state at a certain fixed temperature by the liberation of heat energy.
The solidification point or freezing point is the fixed temperature at which a liquid starts changing into the solid state.

Melting point and freezing point are the same temperature and have the same numerical value.
For instance, the melting point of ice is 0^oC and the freezing point of water is also 0^oC.

Vaporization or boiling is the process by which a liquid changes into the gaseous state at a certain fixed temperature by the absorption of heat energy.
The vaporization point or boiling point is the fixed temperature at which a liquid changes into the gaseous state.

Liquefaction or condensing is the process by which a gas changes into the liquid state at a certain fixed temperature by the liberation of heat energy.
The liquefaction point or condensing point is the fixed temperature at which a gas changes into the liquid state.

Boiling point and condensing point are the same temperature and have the same numerical value.
For instance, the boiling point of water is 100^oC and the condensing point of steam is also 100^oC.

Increase in pressure results in a decrease in the freezing point of a substance.
For example, the freezing point of water drops below 0^oC with an increase in pressure.
Increase in pressure results in an increase in the boiling point of a substance.
For example, water boils above 100^oC at high pressures.

Addition of solutes results in a decrease in the freezing point of a substance.
For example, when salt [a solute] is added to water to form salt water, its freezing point falls below 0^oC.
Addition of solutes results in an increase in the boiling point of a substance.
For example, when salt [a solute] is added to water to form salt water, its boiling point becomes greater than 100^oC.