Dec 27, 2006

Salt and the Boiling Point of Water

Posted in Chemistry, Education, Science at 8:33 am by David Bradley -- 124 Comments; add your comment

How does adding salt to water affect its boiling point? You will find several clues and key words below.

But, first check out this great science project you can download now that includes A Pinch of Salt which will help you answer that question.

The fact that dissolving a salt in a liquid, such as water, affects its boiling point comes under the general heading of colligative properties in chemistry. In fact, it’s a generic phenomenon dissolve one substance (the solute) in another (the solvent) and you will raise its boiling point.

Colligative properties determine how a solvent will behave once it becomes a solution, as it were. The degree of change depends on the amount of solute dissolved in the bulk liquid, not the type of solute.

So, here’s a rough explanation of what’s going on. If a substance has a lower vapour pressure than the liquid (it’s relatively non-volatile in other words) then dissolving that substance in the liquid, common salt (NaCl) in water (H₂O), for instance, will lower the overall vapour pressure of the resulting solution compared with the pure liquid. A lower vapour pressure means that the solution has to be heated more than the pure liquid to make its molecules vaporise. It is an effect of the dilution of the solvent in the presence of a solute.

Put another way, if a solute is dissolved in a solvent, then the number of solvent molecules at the surface of the solution is less than for pure solvent. The surface molecules can thus be considered “diluted” by the less volatile particles of solute. 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. The salt dissolves in the water condensing on the road surface and lowers its freezing point so that the temperature has to fall that bit more before ice will form on the roads.

A much more fun use for freezing point depression is to add salt to ice to make icecream. The About site has some instructions on how to do this, although it’s probably not too tasty.

Curiously, at least one Sciencebase reader was searching for the phrase “how does sugar affect the boiling point of water?” and landed on this page. This is essentially the same question as does salt affect the boiling point of water. The nature of the solute, the material being dissolved in the solvent, is pretty much irrelevant at a first estimate. Rather it is the amount of material that is dissolved (which depends on the materials solubility) that influences the boiling and freezing points as described above.