Mixture with water provides the Freeze protection and boil over protection. Water by itself is very corrosive and does not have the freeze and boil over protection that an engine requires. Water by itself is corrosive to metals. The most familiar corrosion is rust. Rust develops as metals come in contact with air and water.
Corrosion occurs with the other metals in a cooling system, copper, iron, aluminum etc.
A good antifreeze coolant has an inhibitor package to protect against corrosion. The easiest way to test antifreeze for corrosion protection is with test strips. Immersion momentarily into a water and antifreeze solution, of a chemically treated pad on the end of an inert plastic strip. The pad caries a pH indicator, a buffering agent, and a color reagent correlated especially to the various concentrations and freeze points of antifreeze containing solutions. The Antifreeze should be less than 120 degrees Fahrenheit.