Hand Warmers and Heat Packets
In the first two, a Bramble heat packet is displayed, which contains iron, water, salt, active carbon, super absorbent polymer, and Vermiculite. When the hand warmer is exposed to air by removing the outer packaging, the oxygen reacts with the iron powder inside to form iron oxide. This exothermic reaction lasts up to 8 hours, maintaining an average temperature of 40ºC. The amount of iron in the packet varies by brand, affecting the reaction time [5]. When the packet is cut open to be exposed to more air (not recommended) , the reaction intensifies, reaching temperatures of about 80ºC. This reaction is irreversible, so once the reaction is complete, the hand warmer is disposed. The video shows the packet after it has been shaken. In the center of the packet, the temperature rises over a course of 10 minutes.
In the last two, a reusable hand warmer is displayed. Unlike the one of the top, this hand warmer operates using a phase change process. It contains supersaturated sodium acetate and a small piece of metal. When boiled, the contents are in liquid phase. Pressing the metal piece triggers crystallization, creating a nucleation site where crystals form and release stored heat [6]. The video shows the pressing of the metal piece. Once this happens, the heat spreads at the 10 second mark.