Question

What is the process by which a gas changes its state to become a liquid?

Answer 1

`"Condensation"` describes the phase transition: `"gas "rarr" liquid"`

Explanation:

And thus `"steam"`, `"water vapour"`, condenses to liquid water:

`H_2O(g) rarr H_2O(l) + Delta`

The process as written is exothermic, and of course the condensation reaction can be used to drive a steam engine.