The principle is as follows:
Under the action of catalyst, high-purity iron powder reacts with oxygen in the air to convert chemical energy into heat energy. . Vermiculite, a mineral material, is used for thermal insulation, and oxygen permeability is controlled by non-woven packaging, so as to control the oxidation reaction speed and adjust the heating temperature and time. In order to make the temperature last longer, the material of the bag should be very special, consisting of raw material layer, gelatin layer and non-woven bag. The non-woven bag is made of microporous breathable film. It must also have a traditional sealed outer bag gelatin layer. When in use, the outer bag is removed, so that the inner bag (non-woven bag) is exposed to the air, and oxygen in the air enters the interior through the breathable film. The time and temperature of heat release are controlled by the oxygen permeability of the gas permeable membrane. If oxygen permeates too quickly, heat will be released at once, which may burn the skin. If oxygen permeates too slowly, there will be no temperature. It is a dark brown solid after use, containing carbon powder, NaCl solid, Fe2O3 solid and magnesium aluminum salt.
Negative electrode: Fe-2e-=Fe2+
Positive electrode: O2+2H2O+4e-=4OH-
Total reaction: 2Fe+O2+2H2O=2Fe(OH)2.
4Fe(OH)2+2H2O+O2 = 4Fe(OH)3↓2Fe(OH)3 = = = = fe2o 3+3H2O
So, it has nothing to do with water. Putting water in is like throwing a dry battery into the water. Redox reaction is difficult to occur, so it will not generate heat.