Winter Warm-Up

You step outside on a winter day to find a fresh layer of snow blanketing the ground. There’s only one thing to do: Make a snowball! You grab a handful of frozen fluff, then another, and another. By now, your fingers should be ice-cold. But you’ve got a trick up your sleeve—or, more accurately, in your gloves. Before you left the house, you tucked handwarmers into your gloves to keep your hands toasty.

Remove a handwarmer packet from its plastic package, give it a shake, and it immediately starts to heat up. Why? It’s thanks to a simple chemical reaction. A handwarmer contains iron (Fe) powder and a small amount of water. When you expose the handwarmer to air, oxygen (O₂) enters through tiny holes in the wrapper. That oxygen interacts with iron atoms—the smallest units of an element—stripping away some of their negatively charged electrons, explains Sally Mitchell. She’s a high school chemistry teacher in Rye, New York, and an education expert for the American Chemical Society.

You step outside on a winter day. A fresh layer of snow covers the ground. There’s only one thing to do. Make a snowball! You grab a handful of frozen fluff, and another, and another. Your fingers should be ice-cold by now. But you’ve got a trick up your sleeve. Actually, the trick is in your gloves. Before you left the house, you put handwarmers in your gloves to keep your hands toasty.

Take a handwarmer packet from its plastic package and shake it. Right away, it starts to heat up. Why? Because of a simple chemical reaction. A handwarmer contains iron (Fe) powder and a small amount of water. Expose the handwarmer to air, and oxygen (O₂) enters through tiny holes in the wrapper. That oxygen reacts with iron atoms (the smallest units of an element). This strips away some of their negatively charged electrons, explains Sally Mitchell. She’s a high school chemistry teacher in Rye, New York, and an education expert for the American Chemical Society. 

The water allows the electrons to move from iron atoms to oxygen atoms. This forms a compound called ferric oxide (Fe2O3), known as rust. During this exothermic reaction, energy is released as heat (see Bringing the Heat).

Handwarmers also contain salt, which acts as a catalyst to speed up the reaction. Without salt, the reaction couldn’t create a significant amount of heat fast enough. Manufacturers also add activated charcoal—a form of the element carbon (C)—and the mineral vermiculite. Both store water, which they release gradually once the handwarmer is exposed to air. Without these compounds, the reaction would happen too fast. Instead of a handwarmer that remains pleasantly warm for a long time, you’d have one that quickly became too hot to handle.

The water allows the electrons to move from iron atoms to oxygen atoms. This forms a compound called ferric oxide (Fe2O3). It’s also known as rust. This exothermic reaction releases energy as heat (see Bringing the Heat).

Handwarmers also contain salt. It acts as a catalyst to speed up the reaction. Without salt, the reaction couldn’t create a large amount of heat fast enough. Manufacturers also add other ingredients. One is activated charcoal, a form of the element carbon (C). The other is a mineral called vermiculite. Both store water, and they release it little by little when the handwarmer is exposed to air. Without these compounds, the reaction would happen too fast. Your handwarmer wouldn’t stay nice and warm for a long time. Instead, it would quickly become too hot to handle.