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Name: Andy
Status: student
Grade: 9-12
Location: CA
Country: N/A
Date: June 2007

Question:
I understand that dissolving salts in water in an endothermic process. I am interested in cooling a small amount of water (about 150ml) to a temperature at or slightly above freezing. I have found that ammonium nitrate is a common salt used in endothermic dissolutions, but it is difficult to obtain considering its use in explosives. What are some other fairly non-toxic salt solutions that can achieve the desired cooling without requiring great quantities of the salt? Is this even a reasonable expectation?



Replies:
There are other substances besides ammonium nitrate that get cold when dissolved in water; one of the most commonly used is magnesium chloride. But there are a few things you should keep in mind:

1. Not all salts dissolve endothermicly. For instance, the dissolution of calcium chloride (not that different, you would think, form magnesium chloride) is quite exothermic.

2. You say you want to cool the water to a temperature near freezing. Do not forget that dissolving lots of salt in water lowers the freezing point.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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