Read · 1 min
when something is dissolved in a solvent, the solute particles disrupt the formation of solid crystals of the solvent that would normally form at the freezing temperature of the pure solvent. The solute particles want to solidify in a different crystal form than the solvent particles, so more energy must be taken out of the system in order to get the two types of particles to agree and solidify. This is why salt water freezes at a lower temperature than pure water (water freezes in hexagonal crystals while NaCl forms simple cubic crystals).
The very same reason causes boiling point of salt water to be higher in than the boiling point of pure water.