The Chemical Reactions That Make Hand Warmers Heat Up

Bryan Edwards

IRON POWDER

Oxygen in the air reacts with this powder to yield iron oxide—rust—and heat. These hotties can reach up to 163 degrees Fahrenheit; military-grade warmers (for heating MREs) can get to upwards of 200 degrees. Manufacturers produce iron powder by either crushing iron or spraying a molten stream with water. Fun fact: We eat about 2 million pounds of iron powder each year in our favorite (fortified) breakfast cereals. Yum!

MICROPOROUS POUCH

Open the plastic packaging and air seeps in through the fabric pouch, setting off the chemistry that heats things up. More pores mean more air, so the pouch for toe warmers has more holes than the one for hands (since, you know, tight stinky snow boots have less circulation).

ACTIVATED CHARCOAL

This porous material holds the water necessary for the oxidizing reaction to occur. It's also thermally conductive, so it helps spread heat evenly. To make it, heat charcoal in the presence of an oxidizer. It expands to create millions of tiny pores between its carbon atoms, increasing the surface area to as much as 2,000 square meters per gram. A pound of activated charcoal has the same surface area as six football fields—that's a lot of crannies for storing water.

SODIUM CHLORIDE

Like in your kitchen, salt amplifies things. As a de-icer in snowy cities, salt can cause car underbellies to corrode. Here too NaCl (with an assist from H₂O) is a handy catalyst that kicks the rust reaction into gear.

VERMICULITE

With a name derived from the Latin word vermiculus, little worm, this hydrated magnesium aluminum silicate expands when heated and looks like, well, worms. The result is a light, highly absorbent, chemically inert, odorless, and fire-resistant supermaterial. It's a great insulator, both here and in some building materials. Along with the activated charcoal, it helps diffuse the iron powder so the filings don't burn too quickly (and sear your skin). Herpetologists use this stuff to keep incubating reptile eggs cozy—your warm-blooded hands should be no problem.