Q & A: Salt and the boiling point of water

Q:
Why does adding salt make the boiling temperature of water rise???
- soeun lee
Auckland Girls Grammar, Auckland, New Zealand
A:
Souen- That's a good question. It turns out to be easy to give an answer to someone who's studied a little Statistical Mechanics, but I'll try to give an answer that doesn't assume that sort of background.

Salt (or other solutes, like sugar) can easily dissolve in liquid water. However, taking the solute out of the water and putting it in the gas phase (air) requires a lot of energy. At temperatures around the water boiling point, these solutes stay in the liquid.

Now the 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.

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. (and Tom J.)

(published on 10/22/2007)

Follow-Up #1: Why does salt water have a higher boiling point than distilled water?

Q:
Why does salt water have a higher boiling point than distiller water? Can you please explain in terms of the positive and negative charges of the particles and the rubbing off of electrons please? Thank you. My teacher taught it to us but I missed the lesson.
- Ash (age 14)
Australia
A:
Ash- I've marked your question as a follow-up to a similar question, which has a version of the answer.

Your teacher may have given an answer "in terms of the positive and negative charges of the particles and the rubbing off of electrons", but that sounds fundamentally wrong. Any solute in water raises the boiling point, so long as the solute stays in the liquid water. It's true that part of why salt dissolves well in water is that it falls apart into charged particles, but some uncharged molecules also dissolve in water and also raise the boiling point.

Another way of putting the answer is to say that if the solute stays in the liquid, there is less room for it to find various different states as the liquid boils away. When not so many states are available, we say that the "entropy" is reduced. The basic rule that tells us what will happen is that nature always heads toward an increase in entropy. So having solutes in there goes against boiling, which doesn't then occur until the temperature is higher. At the higher temperature it turns out that boiling still increases net entropy, thanks to the water molecules getting more space to run around.

Mike W.

(published on 02/23/2013)

Follow-Up #2: different salts and boiling

Q:
Would different types of salt waters (sea salt vs table salt) have different boiling points or would it all be the same? And which would have a higher density?