How does the boiling point of a liquid evolve if a non-volatile solute is added?

1 Answer
May 26, 2016

Answer:

Lowering the vapor pressure (e.g., by dissolving a non-volatile solute) raises the normal boiling point of any liquid.

Explanation:

The boiling point of a liquid are the conditions of temperature and pressure when the vapour pressure of the liquid becomes equal to the surrounding pressure, and bubbles of vapour form directly in the liquid. The normal boiling point is specified when the ambient pressure is #1# #"atmosphere"#.

For example, the normal boiling point of water is #100# #""^@C#, of ethanol, #78.5# #""^@C#, of hexane #68.5# #""^@C#. If a non-volatile material such as sodium chloride is dissolved in water, this reduces the vapor pressure. You can think of this (approximately) as dissolved ions partially blocking and reducing the rate of vaporization of water molecules, whereas the rate of condensation remains constant.

The reduction in vapor pressure means that the water (or any solvent) must be heated to a higher temperature to reach the point where the vapor pressure is once again equal to 1 atmosphere. Thus, the boiling point (temperature) is increased.