Question

How does sodium chloride affect the boiling point of water\[?\]

Answer 1

Hint: Here sodium chloride is the non-volatile solute and water is the solvent. Also sodium chloride is an ionic compound and hence undergoes dissociation in aqueous medium to form the corresponding ions. These ions interfere with the intermolecular forces and cause an elevation of boiling point of water.

Complete step-by-step answer:
Water boils when the water molecules are able to overcome the vapour pressure of the surrounding air to move from the liquid phase to the gas phase. When we add salt or sodium chloride to water, it dissociates into sodium and chloride ions. These charged particles alter the intermolecular forces between water molecules. In addition to the hydrogen bonding present between water molecules, we need to consider the ion-dipole interaction.

Water is a polar molecule and hence acts as a dipole, which means one side i.e. the oxygen side is more negative and the other side i.e. the hydrogen side is more positive. The positively charged sodium ions align with the oxygen side of a water molecule, while the negatively charged chloride ions align with the hydrogen side. The ion-dipole interaction is much stronger than the hydrogen bonding between the water molecules, so more energy is needed to move the water molecules away from the ions into the vapour phase resulting in an increase in boiling point of water.

Note: This is an example of a colligative property- boiling point elevation, and it is not exclusive to water. It occurs any time when you add a non-volatile solute such as salt to a solvent such as water as addition of salt to pure water makes it impure hence resulting in an increase in boiling point of the solvent.