I was in a drug store recently, and the following conversation (almost) ensued:
Clerk: Can I help you?
Me: Yes, I’m looking for the hot packs.
Clerk: Aisle 4, look in the back.
Me: Yeah, I was just there.
Clerk: You couldn’t find them?
Me: I saw the metal ones, but I was hoping for the kind where you crack open the inner pouch and the whole bag just gets hot. No rubbing required.
Clerk: Does thermodynamics work differently for you than everybody else?
Let me explain…
Cold and Hot packs work, in many cases, on the same general principle. In each case, a chemical reaction occurs – typically when you break the inner pouch – that either releases or absorbs heat. Depending on the direction of heat flow, in or out of the bag, the pack will become cold or hot.
First, cold packs…
Cold packs have two separate compartments. One contains a granular solid, and the other is just a liquid – water, in fact. The solid is an ionic compound called ammonium nitrate. (“Ionic” because it is composed of two different charged chemicals) When you break the inner bag, water mixes with the ammonium nitrate and, in this process, the salt dissolves. When salts dissolve in water, the ammonium ions (NH4+) are completely separated from the nitrate ions (NO3-) and lose all memory of their history as they become surrounded with the water. Think of it like a mosh pit at a rock concert.
Positive charges (here the ammonium ions) and negative charges (the nitrate ions) are attracted to each other in the salt form, and the positive and negative ions are each attracted to the water molecules in the solution, but there is an energy “trade-off” that occurs. It turns out that the attractive forces between the ions aren’t completely offset by the new attractive forces between the water and the ions. The salt dissolves but, in the process, absorbs a little heat from the surrounding water. We call this process endothermic or “heat absorbing.” The salt, in dissolving, absorbs heat from the surroundings (in this case, the water) and the bag becomes cold. Ta-da!
Now hot packs…
Hot packs work by a very similar process with salt (usually calcium chloride, CaCl2) in one compartment and water in the other. However, when you break the inner pouch, the salt dissolves in an exothermic or “heat releasing” reaction. The excess heat is released to the surroundings (in this case, the water) and the bag becomes hot. Same general reaction (dissolution of a salt), but the heat flows out instead of in.
Other kinds of hot packs?
But there is another kind of hot pack you can buy that consists of iron filings, charcoal, salt, and sawdust in a sealed pouch that is sold in a sealed envelope. To activate the hot pouch, you open the envelope and expose the pouch to air. This leads to an exothermic (heat releasing) chemical reaction between iron and oxygen (in the air) to produce iron oxide (Fe2O3), also known as “rust.”
Try this at home!