Freezing point is the temperature at which a substance changes from a liquid to a solid. The freezing points of different substances vary greatly. Mercury, for example, freezes at -38.87 °C. However, gold has a freezing point of 1063 °C. The freezing point of any substance depends on the pressure pushing against the substance. All freezing points specified in this article are based on a pressure of 1 atmosphere (14.696 pounds per square inch [101.325 kilopascals]), the pressure of the atmosphere at average sea level.
The freezing point of a pure substance is identical with its melting point (see Melting point ). For example, water freezes at 0 °C, and its solid form, ice, melts at the same temperature. Thus, at the freezing point, the liquid and solid forms of the substance can exist together in equilibrium (a state of balance). Unless heat is added or removed, they will remain in that state indefinitely because for each amount of liquid that freezes, an equal amount of solid melts.
The composition of a substance affects its freezing point. Pure substances, such as a pure element or a simple compound, freeze at one specific temperature. In contrast, mixtures, which consist of several chemically uncombined substances, freeze over a range of temperatures. Bronze, an alloy of copper and tin, solidifies as the temperature falls from 1000 °C to 800 °C.
The freezing point of most liquids can be lowered by adding another substance. This fact is the basis for using antifreeze in automobile radiators during the winter. Antifreeze contains ethylene glycol, which has a freezing point of -13 °C. A mixture of equal parts of ethylene glycol and of water freezes at about -37 °C.
A significant increase in pressure can affect the freezing point. The application of pressure raises the freezing point of gold, mercury, and other substances that contract upon freezing. Pressure promotes this contraction, and so the substances freeze at a temperature above their normal freezing point.
An increase in pressure lowers the freezing point of a few substances, such as antimony, bismuth, and water. These substances expand as they freeze. Added pressure prevents this expansion from occurring at the normal freezing point. As a result, the substances can freeze only at a lower temperature.
