In a closed system as a substance changes from a liquid to a gas what is happening?

Answer 1

The particles of the liquid are colliding with each other at a rate/force that allows them to separate considerably further than when they were a liquid, thus making them a gas.

The temperature at which this occurs can vary depending on the material in question, and the pressure of the container. The presence of substances that would "lower" the boiling temperature (such as boiling stones) do not actually lower the material's boiling temperature, but rather allow the temperature to spread more smoothly, and thus cause it to boil faster with the same heat input.

Even though the individual particles still "boil" at the same temperature as before, the more even distribution of heat causes them all to reach boiling point much faster than before.

Answer 2

As a substance changes from a liquid to a gas in a closed system, energy is being absorbed by the molecules, causing them to gain enough kinetic energy to overcome intermolecular forces and transition to the gaseous phase. This process is known as vaporization or evaporation. The temperature and pressure within the system remain constant during this phase change.