Antifreeze prevents water in a car’s cooling system from freezing by lowering its freezing point. Ethylene glycol is the most commonly used substance as it meets all necessary criteria.
Antifreeze is a liquid added to an automobile’s cooling system to ensure that the water in it does not solidify. The reason this works is that the freezing point of a liquid is lowered when something melts in it. This something can be 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 of freezing point depression is linearly related to the number of molecules dissolved in the liquid.
The freezing point decrease in dilute solutions can be explained as follows. As the temperature of the liquid decreases, the molecules that make it up move more slowly and experience an attractive force between them. In pure water, at 32°F (0°C), this attractive force is strong enough to arrange the water molecules into a regular crystalline pattern, greatly decreasing their mobility and causing ice to form.
In theory, anything that dissolves in water can be used as an antifreeze. In practice, there are several limiting constraints. The first is that the substance should mix with water in any ratio. Some liquids are difficult to dissolve or crystallize at lower temperatures. The second is that antifreeze should be inert, i.e. not react chemically with anything it contacts in the cooling system. Third, it should be cheap; and fourth, it shouldn’t cause unwanted pressure to build up within the cooling system – this means that the antifreeze should have a high boiling point.
The almost universally used substance that matches all of these specifications is ethylene glycol, which has a boiling point of 387°F (197°C). A cooling system that has a 1:1 ratio of glycol to water has a freezing point of approximately -40°F (-40°C), which is ideal for the normal range of applications.
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