Why salt raises boiling point of 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.

A very similar argument explains why solutes also lower the freezing point. Since the solutes are almost completely excluded from the solid like from the gas they stabilize the liquid. A search of this site will turn up some answers about freezing salt water.

Mike W. But if you want your water to boil faster, just leave it as is. BY Sean Hutchinson. Big Questions Food. The rate of exchange between solvent in the solution and in the air above the solution is lower vapour pressure of the solvent is reduced. A lower vapour pressure means that a higher temperature is necessary to boil the water in the solution, hence boiling-point elevation.

Conversely, adding common salt to water will lower its freezing point. This effect is exploited in cold weather when adding grit rock salt to the roads. You may spit it out after tasting. When we cook sugar syrup, the water starts to evaporate at degrees. As it cooks, more water evaporates, giving a steadily higher concentration of sugar.

As the solution becomes more concentrated, the temperature continues to rise. Adding sugar to water creates a true solution. This results in decreased vapor pressure, raising the boiling point of water. The temperature needed to boil will increase about 0. So yes, salt increases the boiling temperature, but not by very much.