If a radiator is completely filled to the brim with water while the engine is cool, a problem develops as the engine gets hot. As the engine heats up, so does the water in the radiator, and it starts to expand. As it expands, the water will overflow out of the radiator, just like water overflows out of a cooking pot when it boils over.
This problem is avoided by adding a antifreeze to the water in the radiator. Antifreeze increases the boiling point (the point at which the water boils) of the water, making it so the water must get hotter before it boils. It also lowers the freezing point (the temperature at which the water freezes) so the water must get colder before the water freezes. It is also important not to overfill your car radiator to prevent overflow.
Radiator
What is the purpose of using antifreeze in a radiator?
Antifreeze increases the boiling point of the water in the radiator.
