How does science keep your hands toasty?
Hand-warming pads use a solution inside a closed environment with a high freezing temperature to provide a warming effect. A hand warmer starts out at room temperature but, when a metal disc is pressed into the solution, an exothermic reaction is triggered, releasing heat.
Basically, hand warmers use a solution of water and another substance, like sodium acetate. The latter raises the freezing temperature of the solution. In this instance, sodium acetate freezes at 54 degrees Celsius (130 degrees Fahrenheit) but can exist as a supercool liquid with water at a much lower temperature until the crystallization (or freezing) process is initiated.
When the metal disc in the hand warmer is pressed into the supersaturated solution, a few of the molecules of the solution jump into a solid frozen state, in turn causing the rest of the liquid to solidify. This makes the liquid leap to sodium acetate’s freezing temperature. By heating the hand warmer further the frozen sodium acetate will melt, allowing it to again be stored as a supercool liquid until its next use.