Drink - Victoria Moore dons a boffin's hat to find out why playing with hot water produces icy effects
If you need to make some ice cubes urgently, is it better to pour hot or cold water into the ice tray before slipping it into the freezer? The need to pose the question betrays the answer. When stiff gin and tonics are called for and there is no ice to hand, always freeze hot water. It's quicker.
This curious fact was discovered not by a desperate soak with good drinking manners, or an earnest and infinitely patient lab technician, but by a Tanzanian schoolboy called Erasto B Mpemba. Erasto experimented at home with identical containers, into which he poured equal quantities of water (although some people claim it was actually ice-cream mixture) at different temperatures. Every time he did it, he found that the water which had initially been hotter froze first.
This puzzled Erasto, whose grasp of the thermal laws of physics was good enough to tell him that this didn't make sense. The rate of the flow of heat between two bodies is proportional to the difference in temperature between them, so you would expect the hot water to begin losing heat more quickly.
But both physics and common sense dictate that, at some point, the hot water would reach the initial temperature of the cooler water, and that the time it would take to freeze from this moment would be the same as the total time taken by the cooler water. In other words, you'd expect the hot water always to be playing catch-up. So Erasto's physics teacher didn't believe him. Nor did his school friends. Nor, at first, did the British physicist Professor Osborne, who came to speak on physics at Erasto's school and to whom Erasto described his experiment, asking why this should be. Nor did I when I read about it in the physicist Peter Barham's recent book, The Science of Cooking (Springer, £19.95), but I've tried it at home and it really is true.
Ice is a very peculiar substance. Every schoolboy knows that the molecules in solids have less kinetic energy than those in liquids. That is what makes them wriggle about less, which is why the molecules settle more closely together so that, by rights, an ice cube should take up less space than the water it was frozen from. Virtually all substances have a lower volume in solid form than they do as liquids.
But when water freezes in pipes, it expands and bursts them. When ice-pops freeze, they are fatter than were the squishy plastic packets of coloured water. And ice cubes have a greater volume than the water from which they are frozen. (Once this is accepted, it is easy to deduce that ice must be less dense than water, explaining why ice cubes bob on the surface of a drink. It is almost as if a universal creator had bent the laws of physics in order to please us.)
In fact, water has many peculiar properties, among them a very high specific heat - the amount of energy required to raise the temperature of one gram of a substance through 1oC. But while physicists have figured out why water expands on freezing (it's to do with ice's crystalline structure, which is itself dictated by the hydrogen bonds between oxygen molecules), no one, as far as I can discover, has worked out why hot water freezes faster than cold.
The debate was aired about 30 years ago by Erasto himself, in the pages of the New Scientist, whose readers wrote in for months to offer possible explanations. Barham summarises them thus: "They include the idea that the evaporation of hot water leads to an additional cooling effect; that the concept that the convection currents caused by the high temperature difference of the hot water precipitates some small impurities that act as precipitating sites for ice crystals; and the idea that a hot container put in a freezer [on a layer of frost] can melt some ice on the surface it is in contact with, leading to a better thermal contact with its surroundings."
But there is no scientific consensus, and the problem remains one of the great unsolved mysteries of physics. Yet this is of no consequence to someone who only wants to make ice cubes for his drink. Just put the kettle on.
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