Boiling Point Elevation

Boiling Point Elevation is the simple idea that the more concentrated a solution is, the higher the boiling point will be.

So, if pure water boils at 100oC, then adding a little salt might bring the boiling point up by a few tenths of a degree. Adding a LOT of salt might bring it up a degree or two.

The reasons behind this are essentially the same as the reasons behind the decrease in vapor pressure. That shouldn't be surprising, since boiling point can be defined as the "temperature at which the vapor pressure is equal to the atmospheric pressure."

When a solute is added to a solvent the interactions between the particles changes.

First since we can assume that there is an attraction between the solvent and the solute that is similar in strength (or stronger) than the solvent's attraction for itself, we know that these additional attractions will make it more difficult for molecules of the solvent to break away and boil.

This means that molecules will need to be moving faster in order to break away. This is due to the South Street Effect. Therefore, in order to boil, the solution will need to be at a higher temperature.

The Math:

Given that the concentration of the solution is related to size of the change in the boiling point, we can do the following math:

where Δ T is the change in the boiling point, k_b is the boiling point elevation constant (which depends on the identity of the solvent), m is the molality of the solution, and i is the vant Hoff factor.