How Chemical Hand Warmers Work

Chemical hand warmers can keep your hands toasty for several hours.
Chemical hand warmers can keep your fingers toasty for several hours. Jamie Grill Photography, Getty Images

If your fingers are cold or your muscles ache, you can use chemical hand warmers to heat them up. There are two types of chemical hand warmer products, both using exothermic (heat-producing) chemical reactions. Here is how they work.

Key Takeaways: Chemical Hand Warmers

  • Chemical hand warmers rely on exothermic chemical reactions to release heat.
  • There are two main types of chemical hand warmers. One type releases heat by air activation. The other type releases heat when a supersaturated solution crystallizes.
  • Air-activated hand warmers are single use products. Chemical solution hand warmers are re-usable.

How Air Activated Hand Warmers Work

Air-activated hand warmers are long-lasting chemical hand warmers that start working as soon as you unseal the packaging, exposing it to oxygen in​ the air. The packets of chemicals produce heat from oxidizing iron into iron oxide (Fe2O3) or rust. Each packet contains iron, cellulose (or sawdust -- to bulk up the product), water, vermiculite (serves as a water reservoir), activated carbon (distributes heat uniformly), and salt (acts as a catalyst). This type of hand warmer produces heat anywhere from 1 to 10 hours. It's common to shake the packets to improve circulation, which speeds the reaction and increases the heat. It's possible to get a burn from direct contact between the hand warmer and skin, so the packaging warns users to put the product on the outside a sock or glove and to keep the packets away from children, who could get burned more easily. Air-activated hand warmers cannot be re-used once they have stopped heating.

How Chemical Solution Hand Warmers Work

The other type of chemical hand warmer relies on crystallization of a supersaturated solution. The crystallization process releases heat. These hand warmers don't last as long (usually 20 minutes to 2 hours), but they are re-usable. The most common chemical inside this product is a supersaturated solution of sodium acetate in water. The product is activated by flexing a small metal disk or strip, which acts as a nucleation surface for crystal growth. Usually, the metal is stainless steel. As the sodium acetate crystallizes, heat is released (up to 130 degrees Fahrenheit). The product can be recharged by heating the pad in boiling water, which dissolves the crystals back into the small amount of water. Once the package cools, it is ready to use again.

Sodium acetate is a food-grade, non-toxic chemical, but other chemicals can be used. Some chemical hand warmers use supersaturated calcium nitrate, which is also safe.

Other Types of Hand Warmers

In addition to chemical hand warmers, you can get battery-operated hand warmers and also products that work by burning lighter fluid or charcoal inside special cases. All of the products are effective. Which you choose depends on the temperature you want, how long you need the heat to last, and whether you need to be able to re-charge the product.

How to Make a Chemical Hand Warmer

It's easy to make a DIY hand warmer using iron, salt, and water in a plastic bag.

Materials

  • Iron filings
  • Salt (sodium chloride)
  • Warm (not hot) water
  • Sand, sawdust, vermiculite, or sodium polyacrylate gel
  • Zip-top plastic bags

Procedure

  1. In a small zip-top bag, mix 1-1/2 tablespoons iron filings, 1-1/1 tablespoons salt, 1-1/2 tablespoons sand (or other absorbent material), and 1-1/2 tablespoons warm water.
  2. Squeeze the air out of the plastic bag and seal it.
  3. It's a good idea to place the bag of chemical inside another bag, remove the excess air, and seal it.
  4. Shake or squeeze the content of the bag for about 30 seconds to mix the contents and form a slush. The bag will get hot and will remain hot as long as the chemical reaction proceeds. If the bag gets too hot to hold, set it down. Don't get burned! Another option is to wrap the bag in a sock or towel.

This is an air-activated hand warmer. Even though most of the air is squeezed out, enough remains in the bag for the oxidation reaction. If you closely examine the contents of the bag after the reaction is complete, you'll see the iron has changed into iron oxide or rust. This type of reaction cannot be reversed unless energy is added, so the hand warmer cannot be re-used. If you want to prepare a homemade hand warmer for later use, keep the salt and water separate from the iron and filler until you're ready for the reaction to occur.

Sources

  • Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0.
  • Dinçer, Ibrahim; Rosen, Marc (2002). "Thermal Energy Storage (TES) Methods." Thermal Energy Storage: Systems and Applications (1st ed.). John Wiley & Sons. ISBN 0-471-49573-5.
  • Hakkin Warmers Co. Ltd. "History." www.hakukin.co.jp