Chemical ice packs work kind of like magic, if you think about it: You break something inside and shake it, and suddenly the whole pack gets icy cold.
What’s funny is that what’s inside is pretty basic stuff. You could conceivably make one yourselfall that’s in it is water and ammonium-nitrate fertilizer.
Breaking the bag or tube inside allows the two ingredients to mix, creating an endothermic reaction that absorbs heat, bringing the ice pack’s temperature down to about 35 °F for about fifteen minutes.
The process may seem like something from modern technology, but it isn’t new. Sometime around 1550, Italians discovered that mixing water with saltpeter (potassium nitrate) was a handy way to cool bottles of liquor.