How can salt change the boiling point of water?

1 Answer
Jun 11, 2018

Adding salt to water increases its boiling point because of a colligative property, known as " Elevation in boiling point".


The boiling point of any liquid is defined as the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure.

When salt is added 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.

Water is a polar molecule and hence has 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 of a water molecule. 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 vapour 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 raised is the boiling point. The phenomenon depends on the number of particles formed in the solution. Hence, it is called a colligative property.

The temperature needs to be increased about a one-half degree Celsius for every 58 grams of dissolved salt per kilogram of water.