Freezing point depression

When a solute such as sugar is dissolved in water, the solution freezes at a lower temperature than the pure water does. Chemists describe it as the amount that the freezing point of a solution decreases from the freezing point of the pure solvent.

In ice cream and sorbet, sugar and other dissolved solids do this job. Because the mix does not all freeze at one temperature, some of the water stays liquid even at freezer temperatures, which keeps the dessert softer and scoopable. Too little sugar gives a hard, icy product, and too much gives one that stays slushy.

This is also why salt is added to ice around an ice cream maker: it lowers the freezing point of the ice bath so the mix can chill below freezing.