Vapor Pressure Change

Another property of a liquid that changes when a solute is added to it, is the vapor pressure.

First let's review what vapor pressure is. As you know, liquids will evaporate. The rate and extent to which it evaporates depends on the temperature. If you put a liquid at a certain temperature into a closed, evacuated container it will evaporate until the vapor exerts a certain amount of pressure. That pressure is called the vapor pressure.

An evacuated container is not necessary. If the liquid evaporates into air, the vapor is mixed with air and the pressure that it exerts cannot be measured directly. However, the vapor does still exert pressure and it is still called the vapor pressure of the liquid.

The amount of evaporation increases when the temperature increases. When the temperature is such that the vapor pressure is just as high as the atmospheric pressure, the liquid boils. That temperature is called the boiling point. (...)

This graph shows the vapor pressure of pure water increasing as the temperature increases. When the temperature reaches 100°C the vapor pressure reaches 1 atm and the water boils.

Graph of vapor pressure vs. temperature.

When a solute is added to a liquid, it will decrease the vapor pressure of the liquid, as long as the solute itself is not volatile.

In fact this is the way in which the boiling point is affected. Remember, the boiling point is the temperature at which the vapor pressure equals the atmospheric pressure. So if you add something to the solution that decreases the vapor pressure - then you will have to heat the liquid to even higher temperatures before the vapor pressure reaches atmospheric pressure. That's how the boiling point is increased.

Graph showing the vapor pressure of water and a solution vs. temperature.

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