Thermal Physics

Changes of State (Phase Changes)

The particles of a solid or liquid possess kinetic energy and potential energy. The potential energy is due to the electro-static attraction between the particles. To change the state of a substance, energy must be supplied or removed. For example, energy must be supplied to melt ice; energy must be removed from steam to change it into liquid water. If energy is supplied to a piece of ice which is at zero degrees Celsius it will cause the piece of ice to melt without causing it to change temperature. During the change of state, the energy supplied goes into increasing the total potential energy stored in the bonds between the molecules.

Similarly, during any change of state, it is the potential energy of the particles which changes; the average kinetic energy (and therefore the temperature) remains constant.

"Latent heat" is the term used to describe energy which causes a change of state without change of temperature.

This is the quantity of energy needed to change the state of 1kg of a substance without a change in temperature.

units Jkg^-1

This is the quantity of energy needed to change the state of 1mol of a substance without a change in temperature.

units Jmol^-1

The energy needed, DQ, to change the state of mkg (or nmols) of a substance (without a change in temperature) is given by

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