When a liquid changes into a gas, it's called vaporization. This is a physical change, because the chemical composition of the substance remains the same.

Here's why it's a physical change:

* No new substances are formed: The molecules of the liquid simply spread out and move faster when they become a gas. They are still the same molecules.

* The change is reversible: You can condense the gas back into a liquid by cooling it down.

There are two main types of vaporization:

* Evaporation: This happens slowly at the surface of a liquid at any temperature.

* Boiling: This happens throughout the liquid at a specific temperature called the boiling point.

Let me know if you'd like to know more about the processes of evaporation or boiling!