The freezing point of a liquid is lowered when there is an additive, such as salt or sugar. The higher the salt or sugar content in a liquid, the lower its freezing point will be.
Saltwater does not freeze at the same degree that regular, undiluted water does because of all of the salt that the oceans contain. The more salt added to water, the colder it can get before it reaches its freezing point. The same goes for sugar. This is known as freezing point depression and is determined by the amount of solute particles that dissolve in the solvent.