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Antifreeze is a water-based liquid coolant used in gasoline and diesel engines. Compounds are added to the water to reduce the freezing point of the mixture below the lowest temperature that the engine is likely to be exposed, and to inhibit corrosion in cooling systems which often contain a range of electrochemically different metals (aluminum, cast iron, copper, lead solder, etc.). The term 'coolant' is to be preferred as, in warm climates, the effect of these compounds is to increase the boiling point of the coolant, which should then be more properly referred to as 'anti-boil'. Even in cold weather, however, once the engine is warmed up it will never get anywhere near the freezing point of water again. Thus its primary purpose is to prevent corrosion.
Until the late 1930s, methanol was the most widely used antifreeze. While effective in preventing the coolant from freezing, its low boiling point and low specific heat capacity led to considerably less cooling than water alone. Also, the concentration of methanol would tend to be reduced over time due to its greater tendency to evaporate than the water with which it was mixed.
Ethylene glycol became available in 1937 and was marketed as "permanent antifreeze", since its higher boiling point provided advantages for summertime use as well as during cold weather. It is still used today. Ethylene glycol antifreeze is poisonous and should be kept away from any person or creature (children and especially dogs) that might be tempted by its sweetish taste. (It forms calcium oxalate crystals in the kidneys and can cause acute renal failure and death.) For this reason bittering agent (denatonium benzoate - trade name Bitrex) is usually added to engine coolant to make it taste disgusting. All spills should be cleaned out, or else an area in which it may be present should be kept inaccessible to those who might lap it up or lick it off their fur.
Propylene glycol, on the other hand, is non-toxic and may be labelled as "non-toxic antifreeze". It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems, or in pipes in homes (and these are only a few places where it is used). It is also used in food, medicines, and cosmetics. Propylene glycol is "generally recognized as safe" by the FDA for use in food.
Most commercial antifreeze formulations include corrosion inhibiting compounds, and a green or blue fluorescent dye to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point of -40°. In warmer areas weaker dilutions are used.
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