Question

What will be the effect on the pressure of water if salt is added to it?

Answer 1

Pressure will decrease.

Explanation with how this effects boiling point

total pressure in the liquid and the air at the boundary are the same- otherwise one would push the other into a smaller space. Part of the pressure in the liquid comes from the solutes, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapor pressure, meaning the pressure of water vapor that would stay in equilibrium with the liquid, is reduced by the same amount because of the solutes. (I've simplified and approximated a little here, since the pressure doesn't quite break up into separate parts due to the salt and the water.)

Water boils when the vapor pressure of the water gets to be as big as the pressure of the atmosphere. At that point, vapor bubbles in the water can grow. You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmospheric pressure, so it has a higher boiling point.