The cold pack contains ammonium nitrate crystals and a thin, breakable pouch of water. When the pack is squeezed, the water pouch breaks and, with a little shaking, the ammonium nitrate dissolves in the water.
When any chemical dissolves in water, it may either absorb heat, get cold, or release heat, get hot. Ammonium nitrate is one of those that absorb heat. It takes the heat right out of the water, thereby cooling it. And the amount of cooling isn’t trivial. That cold pack can actually get down close to freezing.
Because doctors keep blowing hot and cold about when to apply heat to an injury and when to apply cold, there are almost as many hot packs on the market as there are cold packs. The hot packs contain one of those chemicals that give off heat when they dissolve in water, usually crystals of calcium chloride or magnesium sulfate.
But why should a chemical absorb or release heat during the simple process of dissolving in water? After all, at home we dissolve crystals of two common chemicals, salt and sugar, in water time after time, yet we never see the sugar, for example, cooling off our hot coffee or heating up our iced tea. The fact is that salt and sugar are exceptions.
When a chemical substance dissolves in water, it is a two-step process: first, the chemical’s solid, crystalline structure is broken down, and then a reaction takes place between the water and the broken-down chemical parts. The first step invariably has a cooling effect, while the second step has a heating effect.
If step one cools more than step two heats, as in the case of ammonium nitrate, the overall effect is cooling. If it’s the other way around, as it is with calcium chloride and magnesium sulfate, the overall effect is heating. In the cases of salt and sugar, the two steps happen to be just about equal, so they cancel each other out and there is very little change in temperature.
Here’s what’s going on during the two-step process of solid crystals dissolving in water:
A crystal is a rigid, three-dimensional, geometric arrangement of particles. The particles may be atoms, ions (charged atoms), or molecules, depending on the substance we’re talking about. We’ll just call them particles.
Step 1: The particles must first be released from their rigid positions in the crystal in order to be able to float about freely in the water. To break down any rigid structure requires the expenditure of energy; somebody or something has to supply the sledgehammer blows that knock the structure apart. During the breakdown of the crystal’s structure, therefore, some heat energy must be borrowed from the water, and the water cools down accordingly.
Step 2: The liberated particles don’t just swim around in splendid isolation. They have a strong mutual attraction water molecules (see p. 48). If they didn’t, they wouldn’t have been interested in dissolving in the first place. So soon as they are in the drink, they are literally attacked by water molecules, which rush to cluster around them like floating magnets around a submarine. When magnets (or molecules) are attracted to something, they expend energy in the rush toward their targets. This energy heats up the water.
Now it’s just a matter of which effect is bigger: the cooling effect from the breakdown of the solid or the warming effect from the particles’ attraction for water molecules. If the cooling is bigger, the net effect will be that the water gets colder when the solid dissolves. That’s how it is with ammonium nitrate. If the warming is bigger, the net effect is that the water gets warmer when the solid dissolves; that’s how it is with calcium chloride and magnesium sulfate.
Salt and sugar? It’s just an accident that the two effects are approximately equal and cancel each other out. So there is practically no net cooling or heating when salt or sugar dissolves in water. (Actually, salt, sodium chloride, does cool the water very slightly when it dissolves.)
Ammonium nitrate is a common fertilizer and calcium chloride is a common drying agent, sold for drying out damp closets and basements. You may have these chemicals around the house or farm. Stir ammonium nitrate into water and the water will get very cold. Stir calcium chloride into water and it will get quite hot.
Don’t cover and shake; the heat can make the liquid splatter. A couple of tablespoons of the solid in a glass of water will do.