Why use Anti freeze solutions in engine radiator?

In cold climates, there is always a danger that the water in the cooling system may get frozen. This may result in damage to the entire system including in extreme cases the bursting of the radiator core and the cylinder jackets. 

To avoid this some additives are used, which when with water in suitable proportions, lower the freezing point of water. Such an additive is called antifreeze and the solution thus formed becomes an antifreeze solution. Even for hot climates the use of antifreeze has the incidental advantage that the boiling point of the cooling water is also raised.

The requirement of the anti-freeze may be enumerated as;

They should be thoroughly miscible with water.

They should not have any corrosive action on system components.

Their boiling point should be high so that there is minimum loss due to evaporation.

The anti-freezes most commonly used are wood alcohol (methyl alcohol), denatured alcohol (ethyl alcohol), glycerin, ethylene glycerol etc. Each of these has its own advantages and disadvantages. Alcohol is quite effective, but it is very volatile and due to this reason evaporation losses are high. The glycerin is less volatile, but it is comparatively costly and also it attacks rubber hose pipes.

          The amount, by which the freezing point of cooling water is lowered, depends upon the proportion of the anti-freezing in the cooling water. For example, 50% concentrations of ethylene glycerol lower the freezing point of water to about -37^oC