Further physics - Magical heat-pack
Wong Wing-hung (Translation by Yip Ying-kin)

The magical heat-pack
Fig. 1   This kind of heat-pack requires neither electricity nor hot water. What is the magic behind it?
It is now the time that cold wind sweeps along the streets in Hong Kong. The bitterly cold gale remind us about the heat-pack sold in many department stores, which is filled with a colored liquid and can warm up our bodies in the long and cold winter (Fig. 1). The heat-pack can be bought from big department stores in Hong Kong. The heat-pack is very special because users do not need to add hot water or put batteries in it. All you have to do is to gently bend the metallic plate in the heat-pack and let it go. Then the liquid will begin to solidify around the metal plate and heat up automatically. What is the magic behind this amazing heat-pack?

The working principle of this magical heat-pack is derived from a phenomenon called undercooling. Let us firstly consider liquid water which we are most familiar with. Most people believe that water will freeze and form solid ice when its temperature drops below . Actually, this is not correct. In reality, pure water droplets suspended in air may not freeze. It may be still remain in liquid state even at a temperature as low as . We call liquid water with a temperature below as undercooled water. Undercooling happens not only in water but also in some semiconducting liquids. This phenomenon is one of the research topics in the Department of Physics, The Chinese University of Hong Kong.

In some colder regions like the Northern China, temperature may fall well below , and undercooled water droplets can be seen suspending in the air everywhere. When undercooled water droplets come into contact with solid surfaces or particles that have a crystal structure similar to that of ice (e.g. silver iodide), they will condense and give out latent heat. Scientists discovered this phenomenon long time ago and have put it into practical use. One example is the production of artificial rain. There are many tiny water droplets or ice inside the clouds. From a plane, scientists spread silver iodide powder over the clouds; the powder will then catalyze the condensation of water droplets and lead to rain.

Let us return to our discussion about the magical heat-pack. Actually the undercooled liquid inside the heat-pack is made of a substance which is a solid in normal conditions. Bending the small metallic plate in the heat-pack will easily solidify the liquid and give out latent heat. That is why the heat-pack warms up.