Hand warmer chemistry leads to toasty fun chemistry demonstrations. Here are four safe and easy ways to make chemical hand warmers or hot packs and a look at their science. Also, learn how commercial chemical hand warmers work.
Calcium Chloride and Water Hand Warmer
Dissolving calcium chloride (CaCl2) in water releases heat. Breaking the chemical bonds in the crystals releases more energy than the solution absorbs, making it an exothermic process. Calcium chloride is readily available as a desiccant, de-icing agent, and road salt from stores or you can find it online. It’s non-toxic and inexpensive.
- Calcium chloride
- Plastic bag
- Pour a bit of calcium chloride into a small plastic bag and seal the bag.
- Pour some water into a larger bag. Place the small bag containing calcium chloride inside this bag. Seal the larger bag.
- Activate the hand warmer by breaking open the inner bag.
Calcium chloride is highly hygroscopic. It readily absorbs water and forms a hydrate [CaCl2·(H2O)x)]. The hydration chemical reaction is highly exothermic, so if you have the option, use anhydrous calcium chloride for your chemical hand warmer rather than its hydrate. Either works, though, and the salt absorbs water so readily that finding the hydrate is easier.
Sodium Acetate or Hot Ice Hand Warmer
Sodium acetate (CH3COONa) also goes by the name “hot ice” because crystallization of the chemical releases heat and forms crystals that look a bit like chunks of ice. This non-toxic chemical is easily made from vinegar and baking soda or you can purchase it online.
- Sodium acetate
- Coin or other small piece of metal
- Plastic bags
- Seal the coin inside a small bag.
- Place this bag inside a larger bag containing supersaturated sodium acetate. Get this by dissolving as much sodium acetate as possible in hot water.
This chemical hand warmer is reusable. After the sodium acetate crystallizes, simply melt it to start the process over.
Sodium acetate acts as a chemical hand warmer by releasing heat as it crystallizes from a supersaturated solution. Basically, forming the chemical bonds that make crystals releases heat. Hand warmers you buy in a store often contain a solution of sodium acetate in water that is kept separate from a piece of metal. Activating the pack exposes the sodium acetate to the metal. The metal surface acts as a nucleation site for immediate crystallization.
Note: Just about anything activates crystallization of sodium acetate! So, for demonstration purposes, it’s far easier leaving the bag and coin out of the situation entirely. Make a supersaturated sodium acetate solution. Then, poke it or pour it and watch it instantly crystallize. Feel the heat of the reaction.
Magnesium Sulfate and Water Chemical Hand Warmer
Dissolving magnesium sulfate (MgSO4) in water is an exothermic reaction. But, be careful, because the most widely encountered form of magnesium sulfate is its heptahydrate (MgSO4 · 7H2O), which goes by the common name of Epsom salt. Dissolving Epsom salt is an endothermic reaction that acts as a cold pack! Order magnesium sulfate online and make sure it is the anhydrous chemical.
- Magnesium sulfate
- Plastic bags
- Pour a bit of magnesium sulfate into a small plastic bag and seal the bag.
- Pour water into a larger bag. Add the sealed bag of magnesium sulfate and then seal the larger bag.
- Activate the chemical hand warmer by breaking the seal on the inner bag.
How Chemical Hand Warmers Work
Chemical hand warmers you buy in a store typically include either sodium acetate and a metal disc or else iron powder and a filler (vermiculite, sawdust, or activated charcoal) soaked with a bit of salty water. The iron-based hand warmer releases heat via an oxidation reaction. Exposing the iron to oxygen in air makes iron oxide (Fe2O3) or rust. The salty water speeds the reaction, while the filler moderates it so it does not get too hot or end too quickly.
Make an Iron Chemical Hand Warmer
Make your own iron-based chemical hand warmer:
- Iron powder or filings (the smaller the particles, the better)
- Salt (sodium chloride, NaCl)
- Warm water
- Filler (sand, sawdust, vermiculite, or sodium polyacrylate gel)
- Plastic or cloth bag
- In the bag, mix together 1-1/2 tablespoons iron filings, 1-1/2 tablespoons salt, 1-1/2 tablespoons filler, and 1-1/2 tablespoons warm water. You don’t have to use a tablespoon, of course, but do use small equal measures of each ingredient. The reason for using warm water instead of cold water is so the salt dissolve.
- Shake or squeeze the bag. This mixes the chemicals. If the bag gets too hot, set it down or wrap it in a towel.
If you want to make a chemical handwarmer that gets activated at a later time, keep the salt and water separate from the iron and filler. When you’re ready, mix the chemicals and start the reaction.
- ChemEd Xchange (2019). “Crystallization of Supersaturated Sodium Acetate – Demonstration.”
- 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.
- Kemp, Robert; Keegan, Suzanne E. (2000). “Calcium Chloride” in Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a04_547
- Odochian, Lucia (1995). “Study of the nature of the crystallization water in some magnesium hydrates by thermal methods”. Journal of Thermal Analysis and Calorimetry. 45 (6): 1437–1448. doi:10.1007/BF02547437