Hot ice is made from sodium acetate. You can buy hot ice or it is easily made from baking soda and vinegar. Hot ice is used in many textile industries, as an agent in synthetic rubber production, in the food industry as a food enhancer and in many brands of heating pads and hand warmers.
Cooling sodium-acetate below its melting point causes the liquid to crystallize, releasing huge amounts of heat as it is cooled. When a process releases energy into the surrounding areas, it is known as “exothermic” (from the Green word “thermo” meaning heat and “exo” meaning outside). This cooling of the sodium-acetate is an exothermic process and the ice ends up being hot. It’s solidifies so fast that you can actually form sculptures with it, as you pour the hot ice.
To make the sodium-acetate you need one liter of clear vinegar or white vinegar and three to four tablespoons of baking soda (sodium bicarbonate).
In a saucepan, add the baking soda to the vinegar a small amount at a time and stir with each additional amount of soda that is added. The vinegar and soda will react to form sodium-acetate and carbon dioxide gas. The lower the temperature of the boiling, the less discoloration you will have.
Caution: Adding the baking soda too quickly will cause it to foam and run over the pan, so add a half of a teaspoon at a time and sprinkle very slowly into the vinegar.
Boil the mixture so that it concentrates the sodium-acetate until a crystal skin or a film begins to form on the surface. Remove the solution from the heat when the skin has formed or when you have about 100-140ml of the solution remaining in the pan. It can take up to one hour and you will need to use low heat. It is okay if the solution is a bit discolored.
After removing from the heat, cover it to prevent evaporation. You can put a lid on the pan or add it to another container and cover with plastic wrap. There should not be any crystals in the solution, but if you have some, stir in a small amount of water or vinegar to dissolve the crystals.
Put it in the refrigerator so that it can cool. Afterwards you can use it in a poultice or heat pack on any injuries without being burned or further injured.
PBS.org has a great recipe for making hand warmers with hot ice. You can also make hot poultices or heat packs.