By interfering with water’s molecules, dissolved substances not only lower its freezing point, they also raise its boiling point by making it harder for the water molecules to fly off into the air.
With ethylene glycol dissolved in it, your coolant water has to get to a higher temperature than usual before it will boil. A mixture of 50 percent ethylene glycol in water won’t boil until 226 degrees Fahrenheit (108 degrees Celsius).
That’s less of an advantage than it used to be, however, because today’s cooling systems are pressurized, and at elevated pressures the boiling points of both water and ethylene glycol are already higher than they would be at atmospheric pressure.
In a car’s cooling system, straight antifreeze will freeze sooner than a mixture of antifreeze and water. Water keeps antifreeze from freezing.