From: firstname.lastname@example.org (Gerald L. Hurst)
Subject: Re: Science Fair Cold Pack Salts
Date: 18 Feb 1996 23:36:29 GMT
In article <email@example.com>, firstname.lastname@example.org says:
>I'm a 7th grader working on a science fair project. I have several questions
>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.