Why can’t we cool water as quickly as we can heat it? (continued) Looking at the wider world, absolute zero, the lowest possible temperature, is 0 kelvin (-273°C), while we can create temperatures of millions of degrees. So, heating is more readily available than cooling. Liquid helium (at -269°C) gives us the closest commercial approach to absolute zero, but it is a pretty expensive way to cool your cocktails!
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°C (32°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°C (50°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.
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’t powerful enough, the water’s 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°C (212°F) is left standing in a room at 20°C (68°F), and we assume Isaac Newton’s law of cooling (that the rate of cooling is proportional to the excess temperature), initially the excess temperature is 80°C (144°F).
However, it would be 120°C (216°F) if the water were put in a typical domestic freezer at a temperature of -20°C (-4°F) – 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°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.
Extract — continue reading at the source.
