Also referred to as antifreeze, "coolant" is usually a 50/50 mixture of water and an alcohol like ethylene glycol. This mixture circulates inside special channels or "galleys" inside your engine then back through your car's radiator, where air flowing through the vanes in the radiator cools the mixture before it is once again circulated through the engine. Special compounds inside the coolant help it prevent corrosion and the formation of bubbles, which can actually damage the inside of components like the radiator or water pump. The alcohol part of the coolant mixture is there to lower the mixture's freeze point, preventing the coolant from freezeing in very cold temperatures and damaging components like the radiator (remember that water expands when it freezes).
Even though modern coolant can last for several years between changes, since the anti-corrosion and other compounds inside the coolant do eventually break down automakers recommend changing the fluid at dedicated intervals.
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The above was written by Garrett McKinnon and was published in the Fall 2011 edition of Vehicle MD