Have you ever tried one of those gel hand warmers which are activated with a simple click? Just fold a little metal foil inside is the transparent liquid crystallizes visibly, becoming hot in a few moments. This happens thanks to an exothermic chemical reaction that triggers the solidification of the solution sodium acetate present in the hand warmer, which releases heat. These objects are reusable, just immerse them in boiling water to melt the crystals and return them to their liquid state, ready for a new “click”.
But what really happens inside that hand warmer? In this experiment (which you can find in the attached video) let’s find out, dismantling the device, producing the substance at home and explaining the chemistry ofsodium acetate and of supersaturated solutions.
How the reusable gel hand warmer works: sodium acetate
Opening the hand warmer reveals a liquid composed mainly of sodium acetate dissolved in water. It is precisely this salt that makes the “magic” of instant crystallization possible. The great thing is that it can also be easily obtained at home, starting from two very common ingredients: vinegar And baking soda.
When acetic acid (in vinegar) and baking soda react, they neutralize each other forming sodium acetate, waterfall And carbon dioxide — that foam we see during the reaction. Once the release of bubbles has finished, simply heat the solution to evaporate the water and obtain a concentrate very similar to that contained in commercial hand warmers.
Supersaturated solutions: water that dissolves “more than possible”
The real magic, however, happens when you study the solubility of sodium acetate. TO room temperature (approximately 20 °C), only 15 g of acetate dissolves in 30 g of water. But if yes warms up the solution up to 100°Cthe solubility increases enormously and up to 160 g can be dissolved.
Now, if this hot solution is left cool without disturbing her, he remains liquid even at room temperature: the water “forgets” that it cannot dissolve all that salt. It’s one supersaturated solutionan unstable condition in which molecules are ready to crystallize at any moment, but are just waiting for a “spark”.
That spark can be a small acetate crystalor even just an imperfection on the skin: just one contact and the entire solution solidifies instantly, releasing heat.
Why the gel hand warmer heats up and how to return it to the liquid state
During the crystallizationsodium acetate molecules go from a free, moving (liquid) state to a orderly and fixed (solid). By losing kinetic energy, that same energy is released in the form of heat: this is why the process is exothermic.
It is also reversible: once solidified, the hand warmer can be “regenerated” by simply immersing it in boiling water to dissolve the crystals again. Once cooled, it is ready for a new click and a new heat release.
