What is the liquid inside a hand warmer?
I. What is the liquid inside a hand warmer?
The liquid inside a hand warmer is a supersaturated solution of sodium acetate (CH₃COONa), a type of supersaturated solution.
II. Overview
Hand warmers, also known as warming packs, come in two types: disposable ones contain powders like iron filings, carbon powder, or salt inside a plastic pouch. Upon tearing the pouch, the powder reacts with oxygen in the air to generate heat. These are single-use only, lasting 8 to 14 hours.The other type is reusable. A sealed plastic bag (PVC) contains liquid and a small circular metal disc. Bending this disc triggers a phase change in the substance from liquid to crystalline state, generating heat through this physical process. After use, it can be boiled in water to revert to liquid form for subsequent use. Depending on size, heating duration ranges from 30 minutes to over an hour, making it suitable for heat therapy.
III. Heating Principle
Appearance:
A sealed plastic pouch filled with clear liquid containing a metal disc.
1. Use——Gently bend the metal disc inside the pouch. The liquid will crystallize (blue translucent) and harden into a solid. It generates heat at approximately 50°C (about 1 hour).
2. Reactivation — After use, boil the pack in water for 10 minutes until crystals fully dissolve back into a clear liquid. Allow to cool before reuse.
3. The product can be reused multiple times following this method.
4. During hot summers, it can be converted into a cooling pack. Use like an ice pack for heatstroke prevention and cooling, but freeze it in the refrigerator beforehand.
To understand how hand warmers work, we must first grasp the concept of supersaturated solutions. At a given temperature, the solubility of a solute in a solvent remains constant—for example, table salt (sodium chloride) dissolves at 36g per 100mL of water at room temperature.When the concentration in a solution equals its solubility, it is called a saturated solution. When the concentration exceeds solubility, it becomes a supersaturated solution. The liquid inside hand warmers is a supersaturated solution of sodium acetate (CH₃COONa).
IV. Advantages
One advantage of hand warmers is their reusability. Simply place the warmer in boiling water for about 10 minutes, and the solidified solute will dissolve again. This occurs because solubility increases at high temperatures. During dissolution, the solute undergoes an endothermic reaction, once more forming a supersaturated solution.
Reportedly, the packaging is made of PVB material. While non-toxic at room temperature, this material releases toxic substances when heated. Since the National Quality Supervision Bureau has not tested this product, its safety remains questionable.