Understanding the Effect of Salt on Water's Boiling Point

When a little common salt (which is sodium chloride, NaCl) is added to water, it dissolves and forms a solution. Water is the solvent and salt is the solute. Pure water boils at 373K (or $100^\circ\text{C}$) at standard atmospheric pressure.

Boiling Point Elevation Explained

Adding a non-volatile solute, like salt, to a solvent, like water, changes certain physical properties of the solvent. These properties depend only on the number of solute particles in the solution, not on their identity. Such properties are called colligative properties.

One important colligative property is the elevation of the boiling point. Here's why it happens:

• The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure.
• When a non-volatile solute (salt) is added to water, the vapor pressure of the solution becomes lower than the vapor pressure of pure water at the same temperature.
• This is because the solute particles occupy some space at the surface, reducing the number of solvent molecules that can escape into the gas phase.
• To reach the required atmospheric pressure and boil, the solution needs to be heated to a higher temperature than pure water.

Why Salt Elevates Boiling Point Above 373K

Since adding salt lowers the vapor pressure of water, the solution must be heated to a temperature higher than the boiling point of pure water ($373\text{K}$) for its vapor pressure to become equal to the atmospheric pressure. This increase in boiling point is known as boiling point elevation ($\Delta T_b$).

The magnitude of the boiling point elevation depends on the concentration of the solute particles. The more solute particles added, the greater the elevation of the boiling point.

Therefore, adding salt to water will cause its boiling point to increase above $373\text{K}$.

Conclusion

Based on the principles of colligative properties, specifically boiling point elevation, adding a non-volatile solute like common salt to water raises its boiling point above the boiling point of pure water ($373\text{K}$).