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Cars get hot when they are running, so they must be cooled. Water is a good coolant, but because it expands when it freezes, it could harm the metal parts of the car. Antifreeze has a much lower freezing point. It is mixes with water to lower the freezing point of the water so it will not harm the car when it is cooling the engine.
What is why a car requires a substance called antifreeze in its radiator?