VAPOR PRESSURE LOWERING
Adding non-volatile solute to volatile solvent lowers the solvent’s vapor pressure. For example, water normally boils at 100ºC (212ºF) but if you add a substance like salt to the solution, it decreases the vapor pressure of the solution.
First, recall what the vapor pressure of a liquid is. For a liquid inside a closed container at a given temperature, some amount will evaporate and bounce around, hitting the walls of the container. The pressure exerted by the liquid’s vapor is the liquid’s vapor pressure.
How quickly a liquid will evaporate is dependent on the amount of exposed surface area. In a cup in of pure water, the exposed surface is entirely made up of water molecules. However, if a non-volatile solute like sugar were mixed with the water, some of the molecules at the surface will be sugar. This effectively lowers the exposed surface area of water, making it less able to evaporate and in turn lowering the vapor pressure.
- The water will only boil if its vapor pressure is equal to atmospheric pressure. Thus you would need to heat the water to a higher temperature.
- The vapor pressure lowering is a colligative property because it depends on the ratio of solute particles to solvent and not the identity (type) of solute. The more sugar you add to your cup of water the less surface area that is exposed.