Unlike disposable hand warmers, these hand warmers are reusable. They are small plastic bags with clear liquid and a small metal disc inside. To release the heat, bend the metal disc. Hey, presto! The liquid turns into crystals that give off heat. The heat lasts about 45 minutes or longer if you have it in your pocket. To reset the hand warmer, boil it in water for about 10 minutes until the crystals become liquid again. You can cook several at the same time.
How does the hand warmer work? The liquid in the bag consists of water and sodium acetate. When you bend the small disk, tiny pieces of metal are released. These move the sodium acetate, an amazing chemical, and cause a chain reaction. The sodium acetate turns into crystals , which release the stored heat energy. It gets up to 54°C! You can reverse the process by boiling the bag until the liquid runs clear again.