Hand warmer principle
Hand warmers, hand warmers can be called generically, but there are two types: one is disposable, the plastic bag contains some powder such as iron powder, carbon powder, salt, etc., when you use it, just tear the bag, and the powder inside will remain Oxygen in the air will generate heat when it comes into contact with it. It can only be used once, and the use time ranges from 8 to 14 hours. The other is reusable. The sealed plastic bag (PVC) contains some liquid and a small round metal. As long as you pull this metal piece, the substance inside will change from liquid state to crystalline state. This physical process heats up. After you release the water and boil it back to liquid state, you can prepare it for the next use. It depends on its size. The time can vary from 30 minutes to more than an hour, and can be used as a hot compress.
　　The heating principle of hand warmers
　　Appearance: A sealed plastic bag filled with a transparent liquid with a metal piece in the liquid.
　　1. Use-gently bend the metal sheet in the bag, the liquid in the bag will appear crystalline (blue translucent) and harden into a solid. The fever temperature is about 50 degrees (about 1 hour).
　　2. Standby-After use, put it in boiling water and boil for 10 minutes until the crystal is completely restored to a transparent liquid. After cooling, it can be used again.
　　3. The product can be used repeatedly according to the above method.
　　4. In the hot summer, you can change it to cold treasure, use it like an ice pack, and prevent it from cooling down, but you need to freeze it in the refrigerator before use.

　　 To understand the principle of "Hand Warmer" warming hands, we must first know what a super saturated solution is. At a certain temperature, the solubility of the solute in the solvent is unchanged. For example, the solubility of salt (sodium chloride) in water is 36g/100mL of water at room temperature. When the concentration in a solution is equal to its solubility, the solution is called saturated (saturated); and when the solubility of the solution is higher than its solubility, the solution is called a supersaturated solution. The liquid in the hand warmer is a supersaturated solution of sodium acetate (CH3COONa).
　　 Because the concentration of the supersaturated solution is too high, it is not stable. When the metal is twisted, the slight vibration generated is enough to crystallize the solute and become a more stable solid. However, this process is exothermic, so the warming bag will begin to warm up.

One advantage of 　　 warm handbags is that they can be recycled. As long as the hand warmer is put into boiling water and heated for about 10 minutes, the condensed solute will be dissolved again; this is due to the increase in solubility of the solute at high temperatures. In the process of dissolution, the solute undergoes an endothermic reaction and becomes a supersaturated solution again.