Why Adding Salt to Water Increases the Boiling Point

If you add salt to water, you raise the water's boiling point, or the temperature at which it will boil. The temperature needed to boil will increase about 0.5 C for every 58 grams of dissolved salt per kilogram of water. This is an example of boiling point elevation, and it is not exclusive to water. It occurs any time you add a nonvolatile solute such as salt to a solvent such as water.

Water boils when the molecules are able to overcome the vapor pressure of the surrounding air to move from the liquid phase to the gas phase. When you add a solute that increases the amount of energy (heat) needed for water to make the transition, a few processes occur.

How Does It Work?

When you add salt to water, sodium chloride dissociates into sodium and chlorine ions. These charged particles alter the intermolecular forces between water molecules.

In addition to affecting the hydrogen bonding between water molecules, there is an ion-dipole interaction to consider: Every water molecule is a dipole, which means one side (the oxygen side) is more negative and the other side (the hydrogen side) is more positive. The positively charged sodium ions align with the oxygen side of a water molecule, while the negatively charged chlorine ions align with the hydrogen side. The ion-dipole interaction is stronger than the hydrogen bonding between the water molecules, so more energy is needed to move water away from the ions and into the vapor phase.

Even without a charged solute, adding particles to water raises the boiling point because part of the pressure the solution exerts on the atmosphere now comes from solute particles, not just solvent (water) molecules. The water molecules need more energy to produce enough pressure to escape the boundary of the liquid. The more salt (or any solute) added to water, the more you raise the boiling point. The phenomenon depends on the number of particles formed in the solution.

Freezing point depression is another colligative property that works the same way: If you add salt to water, you lower its freezing point as well as raise its boiling point.

Boiling Point of NaCl

When you dissolve salt in water, it breaks into sodium and chloride ions. If you boiled all the water off, the ions would recombine to form solid salt. However, there is no danger of boiling the NaCl: The boiling point of sodium chloride is 2575 F or 1413 C. Salt, like other ionic solids, has an extremely high boiling point.