Physical Properties of Liquids
. B. Freezing Point The freezing point of a substance is the temperature at which the liquid form of the substance becomes a solid. The molecules of a liquid arrange into a more ordered structure as the liquid freezes. The freezing point of a substance is essentially the same as its melting point—that is, the point at which a solid becomes a liquid. When a liquid freezes to become a solid, its volume usually shrinks by approximately 10 percent as its molecules move closer together. In solid aluminum, for example, each atom has 12 neighboring atoms, each at a distance of 2.86 x 10-8 cm. In liquid aluminum, each atom has 10 or 11 neighboring atoms at a distance of 2.96 x 10-8 cm. Thus, the atoms are less tightly packed in the liquid, and the liquid must contract as it freezes. The exceptions to this rule are water and the liquid forms of gallium and bismuth. These substances expand upon freezing. The structure of their solid state is less dense than that of their liquid state near the fr