Why can’t we cool water as quickly as we can heat it?

Why can’t we cool water as quickly as we can heat it? Hillary Shaw Newport, Shropshire, UK The reason is because our preferred temperature range is much nearer the freezing point of water than its boiling point. You can only raise or lower the temperature of something if you have something else that is hotter or cooler than the temperature you want to get to (second law of thermodynamics). Advertisement Room temperature is 15-25°C (59-77°F), our fridges are 0-5°C (32-41°F) and domestic freezers can be as low as -20°C (-4°F). A stove ring or kettle element can reach over 200°C (about 400°F), so we have no trouble getting water up to 100°C (212°F) given there is a temperature gradient of well over 100°C. But to cool water without freezing it, we only have the fridge, a temperature gradient of about 20°C. Also, an electric stove ring uses 1-2 kilowatts, whereas a fridge uses just 5 per cent of that, so heat removal via a fridge is slower than heat addition via a stove ring. You could put your glass of water in a large freezer, but the temperature gradient is still just 40°C, much less than that provided by a stove. You also risk freezing the water, possibly cracking the container. Much of the impression that cooling is slower is because we heat water actively, but often let it cool passively If terrestrial temperatures were 80°C (176°F), you could cool water (in a high-power fridge) quicker than you could heat it (before it boiled away). Mike Follows Sutton Coldfield, West Midlands, UK It has certainly felt that way during the recurring heatwaves many people in the northern hemisphere have experienced this summer. When we heat things, we usually do so actively. A kettle can dump a couple of kilowatts of power straight into a litre of water. Cooling is often more passive: put a warm object somewhere colder and thermal energy leaks into the surroundings and, as the temperature difference diminishes, that heat flow slows. A fridge speeds up cooling by using energy to pump heat from the cold interior into the warmer room. However, there is no fundamental rule that says water must cool more slowly than it heats. So, during our everyday uses of water, much of the impression that cooling is slower is simply because we heat it actively and quickly, but often leave it to cool passively and slowly. The more interesting asymmetry appears when we try to cool something below ambient temperature. Then we encounter a less familiar problem: entropy. Entropy is often described rather vaguely as “disorder”. Perhaps my fellow physicists will forgive me a little simplification here: for our purposes, think of entropy as baggage associated with transferring heat. If we want to cool something, that baggage has to go somewhere. This is where the numbers become rather unforgiving. For a given amount of heat transferred, the associated entropy change is greater at lower temperatures. Transferring 1 joule of heat reversibly at 30 kelvin (-243°C) involves 10 times the entropy change of transferring 1 joule at 300 K (27°C); at 3 K (-270°C), it is 100 times as much entropy change. At very low temperatures, even removing a tiny amount of energy therefore means finding a home for a relatively large amount of entropy. Entropy, unfortunately, can’t simply be thrown away. Once we want to cool something below the temperature of its surroundings, we have to move that entropy from somewhere cold to somewhere warmer, and that takes work. The colder things get, the harder that task becomes. My old professor, the late George Pickett, advised thinking about temperature in orders of magnitude. Room temperature is about 300 K, while the centre of the sun is around 15 million K. On a logarithmic temperature scale, a factor of 10 is always the same interval on a graph: from 15 million K to 1.5 million K, or from 3 K to 0.3 K. But the physics of achieving those drops is anything but the same. As we approach absolute zero, removing thermal energy and disposing of its entropy becomes progressively harder, demanding increasingly ingenious cooling techniques. I experienced that difficulty first-hand as part of Pickett’s ultra-low temperature physics group at Lancaster University, UK, when we achieved the lowest temperature ever recorded in the known universe – an achievement rather undersold, I always thought, by its billing as a Guinness World Record. To answer this question – or ask a new one – email [email protected].