Question

Why is the freezing point of a solution, LOWER than that of the pure solvent?

Answer 1

Because freezing point depression operates.....

Explanation:

The addition of a salt, any salt, to a solvent decreases the freezing point of that solvent by an amount proportional to the concentration of that salt in solution. This is a well-known colligative property. And thus addition of salt to water decreases the melting point below `0` `""^@C`. This is also observed when (in a cold North American climate), they salt the roads in winter to encourage the ice and frost to melt (with disastrous effects on the auto chassis!).

You should look up colligative properties in your text: these include (i) freezing point depression; (ii) boiling point elevation; and (iii) osmotic potential.