To cause some gas to condense to a liquid, what must be done?

To cause some gas to condense to a liquid, removing heat is a fundamental requirement. When a gas is cooled, the kinetic energy of its molecules decreases, allowing intermolecular forces to take effect, leading to the formation of a liquid phase. As the temperature drops, the gas molecules come closer together and eventually condense into a liquid.

While increasing pressure and decreasing temperature can also contribute to gas condensation—since both can bring molecules closer together, promoting a phase change—removing heat is a direct cause of reducing energy in the system, which directly leads to condensation. It's essential to emphasize that simply cooling the gas through heat removal is a straightforward method for achieving condensation.

Adding heat, on the other hand, would increase the energy of the gas molecules, causing them to remain in the gaseous state or even transition into vapor, while increasing pressure alone isn't sufficient without also lowering temperature or removing heat to reach the dew point necessary for phase change.