What is the point at witch gas changes into a liquid?

The same as the point at which liquid changes to a vapor; i.e., the boiling point. It is sometimes called the condensation point, also. Temperature, wise, however, it is the same T.

To determine the point at which a gas changes into a liquid, you need to know the specific substance you are referring to. Each substance has its own specific boiling point or condensation point at which it transitions between the gas and liquid state.

One way to find the boiling point of a substance is to consult a reference book or an online database that provides physical properties of various substances. These sources will list the boiling point of each substance under standard conditions (usually at atmospheric pressure).

Alternatively, you can conduct an experiment to determine the boiling point. To do this, you would need a sample of the substance and a laboratory setup such as a heating apparatus and a thermometer. By gradually heating the substance and monitoring its temperature using the thermometer, you can observe the exact point at which it changes from a gas to a liquid.

Keep in mind that the boiling point of a substance can vary with changes in pressure. At higher elevations, where atmospheric pressure is lower, the boiling point of a substance will be lower as well. Conversely, at higher pressures, the boiling point will be higher.