What Is Heat of Vaporization?

Latent heat of vaporization, that is, the amount of heat absorbed by a liquid mass per unit mass during the vaporization process when the temperature is constant.

Latent heat of vaporization

Latent heat of vaporization, that is, the amount of heat absorbed by a liquid mass per unit mass during vaporization when the temperature is constant. The unit of latent heat of vaporization is "kilojoules per kilogram (KJ / kg)".

The process of changing a substance from a liquid state to a vapor state is called vaporization. There are two types of vaporization, evaporation and boiling. Both are endothermic: evaporation occurs only on the surface of the liquid; boiling occurs both inside and on the surface of the liquid. During vaporization, the average distance between molecules increases and the volume increases sharply. It is necessary to overcome the intermolecular gravity and work against atmospheric pressure. It decreases with increasing temperature, because at higher temperatures liquid molecules have greater energy and the difference between liquid and gas phases becomes smaller.

The average distance of liquid molecules of the same substance is much smaller than in gas. During vaporization, the average distance between molecules increases and the volume increases sharply. It is necessary to overcome the intermolecular gravity and work against atmospheric pressure. Therefore, vaporization absorbs heat. The heat absorbed when a unit mass of liquid is transformed into steam of the same temperature is called latent heat of vaporization, or heat of vaporization for short. It decreases with increasing temperature, because at higher temperatures liquid molecules have greater energy and the difference between liquid and gas phases becomes smaller. At the critical temperature, the substance is in a critical state, the difference between the gas phase and the liquid phase disappears, and the heat of vaporization is zero.

Take water as an example: the heat absorbed by heating 1 kg of saturated water, all of which becomes dry saturated steam, is called latent heat of vaporization. Latent heat of vaporization decreases with increasing pressure.

For example, at an absolute pressure of 3.92 MPa, the latent heat of vaporization of water is 1718 x 10 ^ 3 J / kg. At an absolute pressure of 9.81 MPa, the latent heat of vaporization of water becomes 1328 x 10 ^ 3 J / kg.