Question

Why does sodium chloride solution freeze at lower temperature than water, but boils at higher temperature than water?

Answer 1

Sodium chloride solution freezes at a lower temperature and boils at a higher temperature than pure water due to colligative properties.

1. Freezing Point Depression: Adding NaCl lowers the vapour pressure of water, making it harder for water molecules to form a solid structure. Hence, the solution freezes at a lower temperature than pure water.
2. Boiling Point Elevation: The same decrease in vapour pressure means more heat is required for the solution’s vapour pressure to equal atmospheric pressure, so the solution boils at a higher temperature than pure water.

These effects occur because the number of solute particles (Na⁺ and Cl⁻) disrupts the phase changes of the solvent.