What Is the Chemistry Behind Hand Warmers?

Written by ronald kimmons | 13/05/2017
What Is the Chemistry Behind Hand Warmers?
Hand warmers are a cheap and easy way to warm your extremities in cold weather. (Hand of the hand image by Roman Rvachov from Fotolia.com)

A chemical reaction results when the molecular bonds between atoms are forged or broken. Hand warmers are cheap packages that campers, hikers and adventurers buy to give themselves a cheap, easy method of getting warm in a cold environment by means of a safe and simple chemical reaction.

Types of Reactions

There are two types of chemical reactions: endothermic and exothermic. Endothermic reactions absorb heat from their environment, while exothermic reactions release heat into their environment. What occurs in an activated hand warmer is an exothermic reaction.


Some people seem to think that there is a big difference between what occurs in a hand warmer and what occurs in a campfire, but they are not so different. Both work by a solid substance forging a molecular bond with oxygen from our atmosphere.


Most hand warmers include iron, salt, carbon, cellulose and vermiculite, which are all surrounded by a polypropylene bag.


When you activate a hand warmer, the salt inside acts as a catalyst between the atmospheric oxygen and the iron in the warmer. A catalyst is a substance that facilitates a chemical reaction without actually being a part of it. This combination of iron and oxygen creates iron oxide, which is commonly known as rust. The carbon helps to disperse the heat evenly through the hand warmer, while the vermiculite acts as an insulator that keeps the heat from escaping too quickly.


Because the reaction between the iron and oxygen cannot be easily reversed, each hand warmer can only be used once. For this reason, if you are taking hand warmers on a trip, remember that each hand warmer lasts about eight hours.

Some environmentalists are concerned because, due to their disposable nature, hand warmers often end up as litter on the ground. Though unsightly when left on the ground, hand warmers are not toxic.

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