When a liquid is cooled, the average energy of the molecules decreases.

At some point, the amount of heat removed is great enough that the attractive forces between molecules draw the molecules close together, and the liquid freezes to a solid.

Microscopic view of a liquid.	Microscopic view of a solid.

The temperature of a freezing liquid remains constant, even when more heat is removed.

The freezing point of a liquid or the melting point of a solid is the temperature at which the solid and liquid phases are in equilibrium.

The rate of freezing of the liquid is equal to the rate of melting of the solid and the quantities of solid and liquid remain constant.

Factors That Affect Freezing Point

• Types of Molecules: the types of molecules that make up a liquid determine its freezing point. If the intermolecular forces between molecules are:
• relatively strong, the freezing point will be relatively high.
• relatively weak, the freezing point will be relatively low.

		= C = O = H
methyl ether (C2H6O) The freezing point of methyl ether is -138.5oC. The relatively weak dipole-dipole forces and London dispersion forces between molecules results in a lower freezing point compared to ethyl alcohol.	ethyl alcohol (C2H6O) The freezing point of ethyl alcohol is -117.3oC. Although dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the higher freezing point compared to methyl ether.