Antifreeze is a liquid added to the cooling system of an automobile to ensure that the water within it does not freeze solid. The reason it works is that the freezing temperature of a liquid is lowered when something is dissolved in it. This something can be either a solid or a liquid. This phenomenon was originally discovered by the French scientist Francois Raoult in the late 19th century. Raoult also discovered that the degree to which the freezing point is lowered is linearly related to the number of molecules dissolved in the liquid.
The decrease of freezing point in diluted solutions can be explained as follows. As the temperature of the liquid decreases, the molecules making it up move more slowly and experience an attractive force between each other. In pure water, at 32°F (0°C), this attractive force is powerful enough to arrange the water molecules in a regular crystal pattern, greatly decreasing their mobility and causing the formation of ice.
In theory, anything that dissolves in water can be used as an antifreeze. In practice, there are several limiting constraints. First is that the substance should mix together with water in any ratio. Some liquids are difficult to dissolve, or crystallize at lower temperatures. Second is that the antifreeze should be inert, that is, not react chemically with anything it comes into contact with in the cooling system. Third, it should be cheap; and fourth, it should not cause the buildup of unwanted pressure within the cooling system — this means the antifreeze should have a high boiling point.
The almost universally-used substance that matches all these specifications is ethylene glycol, which has a boiling point of 387°F (197°C). A cooling system that has a 1:1 ratio between glycol and water has a freezing point of about -40°F(-40°C), ideal for the normal range of applications.
