Any substance that dissolves in water has an upper limit of solubility in water. This condition is called saturation. When the temperature is constant, and we add a substance that exceeds the saturation state, it needs to be heated in order to dissolve it in water. For most substances, once the temperature falls back to room temperature, the saturated mass will change back to crystals and will no longer dissolve in water. However, for some substances, even if they fall back to room temperature, the substance will still dissolve in water. Exceeded saturation, we call oversaturation.
This state is unstable. Once triggered by some mechanism, the material will begin to crystallize, and at the same time release the heat that was previously absorbed, so that there will be a warm package effect.
Take sodium acetate as an example. Take an Erlenmeyer flask filled with distilled water. Add sodium acetate to dissolve it in water until it is insoluble. Then add one more amount of sodium acetate to the Erlenmeyer flask and put it on the hot plate. Heat it to completely dissolve it. Place the dissolved sodium acetate solution at room temperature and slowly cool it. If no other factors exist, it should be a supersaturated solution. Finally, make a supersaturated sodium acetate solution. In addition, adding some sodium acetate crystals will make the solution crystallize rapidly and will release a lot of heat.
The reaction of this reaction is:
This supersaturated solution of sodium acetate can be reused. When it is crystallized, it is slowly cooled after reheating, and it can be restored to the original supersaturated solution, because the heating can make the reaction proceed to the right and the crystals dissolve in water. It is an ionized state, so it can be reused continuously.
Note: Put the supersaturated solution of sodium acetate in the sealed plastic bag and the solution weight of 0.05-0.1% potassium permanganate and the appropriate amount of colored gold powder, and then put it into the starter consisting of metal clips with torsion spring. Just light Lightly gripping a metal clip with a torsion spring in a sealed plastic bag, the supersaturated solution of sodium acetate in the bag can be quickly activated to produce a crystalline core, and the supersaturated solution of sodium acetate crystallizes to release heat for heating, and the temperature thereof can reach About 50 ° C. After cooling and then placed in hot water about 80 ° C soaked or cooked 3-5 minutes, but also reheat.