How Refrigerators Work

Cold Packs

Speaking of refrigeration and coldness, have you ever used one of those "instant cold packs" that looks like a plastic bag filled with liquid. You hit it, shake it up and it gets extremely cold. What's going on here?

The liquid inside the cold pack is water. In the water is another plastic bag or tube containing ammonium-nitrate fertilizer. When you hit the cold pack, it breaks the tube so that the water mixes with the fertilizer. This mixture creates an endothermic reaction -- it absorbs heat. The temperature of the solution falls to about 35 F for 10 to 15 minutes.

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