A common hand warmer contains iron powder that reacts with oxygen in a chemical reaction to emit heat. This type of chemical reaction is called an exothermic reaction because more energy is released when the products are formed than is used to break apart the reactants.
Disposable hand warmers generate heat by a simple chemical reaction that, in essence, creates rust. Each pouch contains iron powder, salt, water, and an absorbent material that is usually pulverized wood or a polymer such as polyacrylate, a silicon-based mineral known as vermiculite, or a caustic substance such as slaked lime. When the pouch is opened, oxygen drifts across the permeable cover and into the inside of the bag where it recombines with the iron to form a reddish brown substance called iron oxide (Fe2O3) and release heat. The slaked lime helps to prevent the oxidation from taking place in the presence of air and the other additives speed up the process and help evenly distribute the heat produced, which can reach temperatures as high as 135 degF.
Ask students to open a disposable hand warmer and observe the temperature change as it warms up. Have them gently feel the hand warmer to see what it feels like and what they can tell about the ingredients. Ask them to consider how this chemical reaction is categorized: Is it exothermic or endothermic? The answer depends on comparing the amount of energy required to break bonds in the reactants with the energy released when the new bonds are formed in the products. This is why a scientific term, the enthalpy change, is used to describe the total energy used and released in chemical reactions.