Eli5 If putting something under pressure increases the temperature why is the bottom of the ocean so cold?
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It’s not just putting “something” under pressure; you may be over-generalizing an effect that happens to gases to things that are not gases.
When you compress a gas, its temperature increases, unless it can conduct away perfectly or unless the compression happens slowly enough where the imperfect conduction available to that gas is sufficient to remove the heat. If the compression of a volume of gas is done fast enough where there isn’t enough time for the heat to leave the gas, that is called *adiabatic compression*. That is the kind of compression that is used to ignite the fuel that is sprayed into diesel engines. You can see it demonstrated here, in a transparent fire-piston, where sudden rapid compression heats the air in the cylinder up to the temperature where it can ignite char cloth (between 660˚F-850˚F):
#Bushcraft Tools | [Transparent Fire Piston – Explosive Flash](https://youtu.be/-39wmSBO2FM?t=5)
As far as I understand, compressing liquids does not result in the same heating. Liquids are largely incompressible. This doesn’t mean they can’t be under high pressure, just that the pressure doesn’t result in much volume change at all. And without much volume change, there won’t be much compressive heating, because this kind of heating is contingent on both pressure and volume reduction, not merely pressure.
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