Why is antifreeze put into car radiators?

So that the water used as coolant in most car radiators does not freeze overnight. If it does then, because ice expands as it freezes and melts, it can crack the engine block. The antifreeze lowers the freezing point of the coolant and reduces the risk. The more antifreeze, the less risk of freezing. But too much and it will boil when the car is running.

Additives in antifreeze also reduces corrosion and raises the boiling point.