Coolant, by definition, is something that cools down otherwise hot (or just plain non-cold) places or items. They can be gases (for example oxygen or freons) or liquids (liquid nitrogen is pretty cold stuff, right?)

A point of clarification. A coolant is used to lower the temperature of an object, fluid flow or area by simple heat exchange. Generally, the coolant will transfer the energy from a warm area to a cooler area. However, it is assumed that the coolant is used strictly to passively transfer this heat. If a substance is used inside an active cooling system (like, say, a refrigerator or air conditioner) and is used to turn motive power into a heat differential, it is called a refrigerant. Refrigerants are substances or compounds which, in the course of their heat exchange loop, move from gas to liquid state and back again (a phase change) due to energy from outside their cooling loop (such as that provided usually by a compressor). The latent heat of the liquid-to-gas phase change is absorbed from the device or area to be cooled - in essence, the 'power' applied by the compressor is used to pump heat.

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