Question

Why is it necessary to cool the gas while applying pressure to liquefy

Answer 1

Some gases can be turned into a liquid just by compressing them. Propane is a good example. Other gases need to be cooled because it would take too much pressure to turn them into a liquid in a practical manner. Liquid nitrogen, liquid oxygen, liquid helium, liquid hydrogen are all examples of cryogenic liquids that are keep liquid by keeping them cold. Once they are turned into a liquid and transferred into an insulated container (called a dewar), they keep cold by using evaporative cooling. They boil off a small fraction of their mass every day to keep the rest of the liquid cold. Helium boils off the fastest, so you try not to let it sit around for too long before you use it otherwise, it may all be gone.

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