A popular, inexpensive, product sold in northern climes is a single-use packet that warms up upon “activation” and stays warm for a few hours. They come in different sizes, most are small enough to fit in an adult palm, and they’re advertised as “hand warmers,” “feet warmers,” or generically as heat packs. The label of these heat packs tells the user to simply open the outer package before use, and the packet will warm gradually to about 38°C. It will remain warm for up to six hours.
The exothermic reaction that most of these hand warmers use is the simple rusting of iron. The package’s label hints at this in the list of ingredients: iron, water, activated carbon, and salt. Some heat packs also list vermiculite as an ingredient. Those of us who have owned certain models of cars or trucks appreciate the psychological heat generated by our vehicles rusting away before our eyes, and now this vile chemical reaction has been packaged to do good.
There are two common heat pack products—reusable and single use. In this experiment, we tested the single-use heat packs.
We placed a heat pack in one of our new BioChamber 2000 units and measured the temperature ~1 cm above the heat pack. Because another word for rusting is oxidation, and oxidation sometimes involves oxygen, we used an Oxygen Gas Sensor to measure the change in O2 concentration in the sealed BioChamber.
The Logger Pro graph here shows some typical data for our experiment. We see this simple setup as a jumping-off point for several investigations, including
Maximum temperature reached by a single-use heat pack
Total time a heat pack stays warm
Comparing a commercial heat pack vs. “homemade” heat pack
Comparing a single-use heat pack vs. a reusable heat pack
Determining the amount of iron in a heat pack
Kinetic study of the redox reaction in a heat pack
This is a fun and easy experiment. Try it today and encourage your students to conduct their own research to discover how heat packs work.