Which crystal absorbs more heat?

How do hand warmers work?

Even gloves are often not enough to keep your hands comfortably warm in the cold winter months. So-called latent heat storage can provide a short-term remedy in such moments - by using a phenomenon of thermodynamics.

Hand warmers, which work on the principle of so-called latent heat storage, usually consist of a plastic bag containing a special liquid and a small metal plate. If you press this metal plate and thereby deform it, it jumps back to its original shape shortly afterwards - accompanied by a faint cracking sound. Suddenly the liquid crystallizes starting from the platelet and gives off heat in the process.

This trick works because the fluid is in a special thermodynamic state. Because actually the phase change - from liquid to solid - should have taken place a long time ago. Just as water freezes to ice at temperatures below zero degrees Celsius, the liquid in the hand warmer should also solidify as soon as the temperature drops below the melting temperature of around 58 degrees Celsius. “The phase transition can be delayed,” says Dieter Herlach from the German Aerospace Center in Cologne. “This is exactly the case with the liquid in the hand warmer. One speaks of a supercooled liquid. "

This phenomenon occurs when so-called crystal nuclei are missing. “That could be another crystal, for example, or the liquid container, which itself has a crystalline structure,” explains Herlach. The first crystals can form on such structures and thus trigger a phase change: The crystal growth continues - until the liquid has turned into a solid.

In a hand warmer, however, there are no crystallization nuclei on the plastic cover, which does not have a crystalline structure, or in the solution of water and water-containing salts. The liquid therefore does not solidify even at temperatures far below the melting point. "If you bend the metal plate, however, a sound wave propagates in the liquid, which leads to densities at the atomic level," reports Herlach. "Such a compression can set the crystallization in motion, with which the supercooled liquid changes from its metastable state to the state of a solid."

Why the desired heat is released during this phase change also explains why this type of hand warmer is called latent heat storage. "Depending on their condition, substances can store different amounts of energy," says Herlach. If this specific energy for a substance is higher in the liquid state than in the solid state, the excess energy is released - this is also the case in the case of the supercooled melt. "The release of this latent heat during crystallization leads to the warming of the hand warmer."

As soon as the hand warmer has completely solidified, it cools down. By re-melting in hot water, the latent heat is supplied again, which is then stored in the heat cushion. After a few minutes, the latent heat storage is ready for use again - and can provide cozy warmth again in cold temperatures.