Recently, for no real reason, I decided to buy myself some whisky stones. These are little cubes of rock which you put in the freezer before adding them to a drink (usually alcoholic) which you want to be chilled. The advantage over ice is that the rocks do not melt and dilute your expensive whisky (not that I drink that). I have used them a few times and was quite disappointed to discover they don’t cool a drink very well at all, at least compared to ice. The best they’ll do is stop it getting any warmer, but they are no substitute for ice cubes. Now I’ve thought about it, I should have figured out why: a drink is cooled mainly by the process of ice cubes melting rather than their being cold, and I learned all about this in physics class at school.
There is a physics term called Specific Latent Heat which is defined as the amount of heat energy required to change the phase of a substance, i.e. turn it from a solid to a liquid, or a liquid to a gas. This amount of heat is usually a hell of a lot more than that required simply to increase the substance’s temperature, and I remember my physics teacher telling us why you feel so cold when you step out of a shower: the heat taken from your body to evapourate the water is 2,265kJ per kg (which is, of course, why we have evolved to sweat to cool us down). He also gave us an example of the effect in reverse: burns from steam are so much worse than those from boiling water because that 2,265kJ per kg is being transferred from the water onto the skin.
I remembered this reverse effect when I was in Sakhalin, and I noticed that the air temperature dropped right down when the snow on the ground was starting to melt. Of course it would, because to change snow into water takes 334kJ/kg heat energy out of the air. But somehow I’d forgotten this when I ordered the whisky stones and expected them to work like ice cubes. My old physics teacher, wherever he may be now, would have given me a right bollocking for this.