Why can鈥檛 we cool water as quickly as we can heat it? (continued)
Guy Cox
Sydney, Australia
Looking at the wider world, absolute zero, the lowest possible temperature, is 0 kelvin (-273掳C), while we can create temperatures of millions of degrees. So, heating is more readily available than cooling. Liquid helium (at -269掳C) gives us the closest commercial approach to absolute zero, but it is a pretty expensive way to cool your cocktails!
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Humans have been able to heat water for millennia, but prior to the invention of refrigeration, cooling it was more difficult. In a suitable climate, we could attempt to store ice through the summer, which would give us 0掳C (32掳F) as long as it lasted. More generally, we used cellars and caves, which are typically at ground temperature; the annual average is around 10掳C (50掳F) in temperate climates. Another approach was evaporative cooling. Keeping water in a porous earthenware container allows some to evaporate, so cooling the remainder. Welcome in summer, but not substantial.
Alex McDowell
London, UK
We can cool water as quickly as we heat it: if, after heating the water we pour liquid nitrogen into it, or add ice cubes, it will cool very quickly.
When we heat water in a kettle, we put in heat (usually 2.2 kilowatts). When water is left to cool, heat leaks away to its surroundings; the higher its temperature is compared with its surroundings, the faster it cools. To boil a kettle, we must, at all times, put in heat faster than it escapes. If the element wasn鈥檛 powerful enough, the water鈥檚 temperature would rise until it reached equilibrium, whereby the heat from the element equalled the heat escaping and the temperature would rise no further.
If a cup of water at 100掳C (212掳F) is left standing in a room at 20掳C (68掳F), and we assume Isaac Newton鈥檚 law of cooling (that the rate of cooling is proportional to the excess temperature), initially the excess temperature is 80掳C (144掳F). However, it would be 120掳C (216掳F) if the water were put in a typical domestic freezer at a temperature of -20掳C (-4掳F) 鈥� i.e. the initial excess temperature would be 1.5 times higher and it would initially only cool at 1.5 times the rate it would if the ambient temperature was 20掳C. Hence, reducing ambient temperature has limited effect on the cooling rate.
We can speed up the cooling rate by placing the cup of water in a breeze. Air isn鈥檛 very dense and doesn鈥檛 carry heat away very well. Hence, we can cool the cup of water faster by standing it in a cold liquid.
It takes a long time to make ice in the freezer because of water鈥檚 latent heat of fusion. It will cool down to 0掳C fairly quickly, but latent heat is freed as ice forms, so it will stay at 0掳C, with ice and water together, until all the water has frozen.
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