Gel hand warmers work by storing sodium acetate in a liquid state, and then allowing it to rapidly crystallize. Because sodium acetate has a freezing point of 130 degrees Fahrenheit, it rapidly increases to this temperature when it crystallizes.
The process of freezing causes a discontinuity in temperature changes in materials. As water approaches 32 degrees Fahrenheit, it begins to freeze; it then remains close to 32 degrees Fahrenheit until the remainder of the water has frozen. Crystallization occurs at the exact freezing point of the material. If the water is prevented from freezing as it fell below 32 degrees Fahrenheit, the temperature would increase to 32 degrees Fahrenheit as soon as it began to crystallize.
Sodium acetate has a high freezing point of 130 degrees Fahrenheit, but it is easy to keep it from crystallizing at lower temperatures. Gel hand warmers maintain a mixture of sodium acetate and water at room temperature without crystallizing. Once the disk in the center of the heat pack is clicked, it initializes crystallization. The sodium acetate then rapidly coalesces to its crystallized state, raising the entire substance to the freezing point of 130 degrees Fahrenheit. After use, the sodium acetate can be melted, bringing it back to its uncrystallized state. This allows repeated uses of the heat pack.