Any Heat&Air People here? I have a question about Thermodynamics...

Mainly two factors involved,the barometric pressure which is based on the term,pressure at sea level.the other factor is temperature.
Its not a quick change of phase as some atoms have an already existing higher energy level,usually from heat.an atom at the bottom of a jar is colder than an an atom at the top of jar.

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I worked in HVAC for several years. Nearly all liquids have a vaporization point -- a temperature at which they go from liquid to gas. In HVAC, you're using refrigerants which are non-volatile chemicals that have relatively low vaporization points (near normal comfortable room temperature which is why it's used - the less heat you have to add/subtract, the more efficiebt the system).

When you vaporize a liquid into a gas, you're adding heat to it. When you're condensing it from gas to liquid, you're removing heat from it. Also, you can pressurize or depressurize a liquid with pumps & raise or lower that liquids boiling point/condensing point.

Circulate that refrigerant through condensing & evaporating coils & push air across the coil to add/remove heat (as in a heatpump). Simple as that.

The other application in HVAC is using water to mostly heat (but sometimes cool) with boilers & radiators. Water can absorb lots of heat before vaporizing (the opposite of refrigerants).