Hot, Cold and Reusable: Which Pack Makes a New Substance?

Three Packs in the Trainer's Kit

1On cold November nights, the Ridgeview Middle School marching band plays in the stands at high school football games. The athletic trainer, Mr. Kaveh Jafari, keeps a kit with three kinds of packs: hand warmers for cold fingers, a reusable gel warmer and instant cold packs for sprains. After one game, band members Mirela Popescu and Tobiah Greenleaf wondered about them. All three packs change temperature, but do they all work by a chemical reaction?

2Their science teacher, Ms. Yvette Mbatha, reminded them how scientists decide. Every pure substance has characteristic properties, such as its density, its melting point and whether it dissolves in water. The mass or shape of a sample can change, but these properties stay the same for one substance. If the substances after a change have different characteristic properties from the substances before it, a new substance formed, and a chemical reaction happened.

3The hand warmer is a cloth pouch of dark gray powder. It starts to warm when its outer wrapper is torn open and air reaches the powder, which is mostly iron with a little salt, water and charcoal. The gel warmer is a sealed pouch of clear liquid with a small metal disc inside. When the disc is clicked, white crystals spread through the pouch and it warms up. After an adult boils the used pouch in a pot of water for about 10 minutes, it is clear liquid again and can be used once more. The cold pack is a bag of white solid called urea with a small pouch of water inside. Squeezing the bag breaks the water pouch, and the urea disappears into the water as the pack gets cold.

4First, the students started one of each pack in a 21 °C classroom and taped a thermometer to it (Table 1). Then Ms. Mbatha opened the packs. From an unused hand warmer, she pulled out gray grains with a magnet. From a used one, she collected the red-brown grains that made up most of the powder. She let the liquid from a used cold pack evaporate in a dish for a week, which left white crystals. From the gel warmer, she kept crystals from its first use and from its fifth use, after four resets. She measured each melting point with a lab melting-point tool (Table 2).