Question: How does a fridge keep food cold?

Ciarán O'Brien answered on 17 Nov 2014:

Fridges are awfully clever little devices. They take advantage of how a liquid’s boiling point is affected by pressure (this is part of a physical law called Boyle’s Law): Basically, the boiling point of a liquid is lower when the surrounding air pressure is lower. At the top of Mt. Everest, where the air is very thin, your kettle would boil at about 60 degrees instead of 100, absolutely ruining your tea.

A fridge is a little pump. Inside the pump, some liquid is pushed around in a little pipe under pressure. Then all of a sudden the liquid is pushed into a much bigger pipe where the pressure is much lower. The sudden decrease in pressure causes the liquid to boil and turn into a gas, and that sucks heat in from the surroundings to balance out the change. The fridge is designed so that most of this heat comes from the inside of the fridge. Once the gas has been pumped away from the fridge, it’s pushed into a tiny pipe again, which causes it to turn back into liquid. This releases the heat it had sucked up before, but it’s far away from the inside of the fridge now. If you ever get a look at the back of a running fridge you’ll find that it’s actually very warm. That’s because the fridge is just moving the heat from inside the fridge to the outside!