Do not mistake hot ice for the ice cube that used to be and melted into a puddle of water when you left it in the kitchen rather than the freezer. Infact, hot ice is a reference to sodium acetate trihydrate. You can already see things getting complicated with that kind of name, huh? This substance exhibits the postulates of a solid at room temperature and melts at 58 degrees Celsius. If you wish you can supercool the liquid to reduce it below the freezing point without any need for crystallization. A nucleus forms around as the remaining liquid actively solidifies.
The sodium acetate as well as water are the two ingredients that you find in most hand warmers and heat pads. Crystallization commences once you bend the metal disk. This results in the expulsion of heat while the substance solidifies. If you feel the need to reuse it simply dissolve the crystals in hot, boiling water. The catch is that each of those crystals must be dissolved or they just solidify again. Below is a video showing how sodium acetate coated on the hands can lead to hot ice crystallizing while it is trapped inside a jar. Notice the consistency there!