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From: (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: Science Fair Cold Pack Salts
Date: 18 Feb 1996 23:36:29 GMT

In article <4g849e$>, says:

>I'm a 7th grader working on a science fair project. I have several questions 
>reguarding salts. 
>1) What kinds of salts are used in cold packs? 
>2) How do they make the water get colder? 
>3) Why doesn't table salt make the water get colder? 
>4) Are there any salts that make the water get hotter? 

More freebee homework problems, eh? 

1) Most often ammonium nitrate, NH4NO3
2) This particular salt and numerous others require more
   energy to break up the crystal lattice into ions than
   is gained from the energy of hydration of those ions
   as they attach themselves to water molecules. The driving
   force which causes dissolution in the face of having to
   absorb energy is the increase in entropy.
3) The hydration energy is roughly equal to the crystal 
   energy in this particular case. This also explains why the 
   solubility of salt, unlike that of AN, does not increase 
   much with temperature.
4) Yes, there are numerous salts which undergo exothermic
   dissolution. In some instances they can raise the 
   temperature enough to scald your hand. Some materials
   have such great hydration energy that they form stable
   hydrates like CaCl2:4H20 (calcium chloride). Some salts 
   will get hot by hydration without dissolving. Calcium 
   sulfate (plaster of Paris) is an example.

If you need an exothermic salt for your project, you might
consider anhydrous magnesium sulfate, which can be made by
drying Epsom salts in a hot (400 deg F) oven in a disposable 
open pan which you can cover loosely when you turn the oven
off to keep out moisture while the material cools. Once cool, 
you'll have to break up the solid fairly quickly and store it 
in a jar with a tight lid to  protect it from moisture until 
you are ready for the demonstration. This dry salt will 
happily absorb water from the air (or your hands - wear gloves) 
until it is  once again the original Epsom salts, MgSO4:7H2O. 

Do not use your Mom's good cookware for this project. The
pan must be free of holes, because the salt is going to melt
before it dries. Let a parent help you with the project, 
because they are going to be really mad if you spill hot, 
molten Epsom salts in the oven or worse, burn yourself.


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