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Q & A: Freezing Point of Saltwater

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Q:
At what temperature does saltwater freeze?
- Anonymous (age 11)
Bangor, ME
A:

Saltwater has a much lower freezing point (the freezing point is the temperature where something freezes) than freshwater does. And the more salt there is in it, the lower the freezing point gets. So in order to know the exact temperature that it’s going to freeze, you have to know just how salty it is. For saltwater that’s as saturated as it can possibly get (i.e. there’s no way to dissolve any more salt in it no matter how hard you tried), the freezing point is -21.1 degrees Celsius. This is when the saltwater is 23.3% salt (by weight).

-Tamara

p.s. As the water starts to freeze, the salt gets left in the liquid. So if you start out with water that isn't saturated with salt, as it freezes the leftover water will get saturated. So if the water starts to freeze at, for example, -10°C, more will freeze as it's cooled further until finally the last bit will freeze at -21.1°C. Thus for un-saturated saltwater the freezing happens over a range of temperatures, not all at one exact temperature, unlike pure water.    Mike W.


(republished on 07/25/06)

Follow-Up #1: freezing saltwater quantitatively

Q:
at what temperature does salt water freeze if you put 2 table spoons in a little container about 2in by 3in
- ???? (age 13)
A:
This takes a little guess work, because you give just two dimensions of the container, and I need three to calculate the volume. Lets say theyre each about 2.5", giving about 15 cubic inches, or roughly 250 cc. I just weighed 2 tablespoons salt, and got about 30 grams. The mass of  NaCl is about 60gm/mole, so you have about a half mole. When the Na and Cl separate, that makes about one mole total dissolved ions in 250 cc, or about 4 moles per liter.
You get almost 2C reduction for each mole/liter. That would give a little less than 8C reduction in the freezing temperature, to around -8C.  Thats about 18F.

Mike W.

Lee H

(published on 03/04/07)

Follow-up on this answer.