Definition: Liquid vs Gasses
In a liquid, molecules can move relative to each other, but the volume remains relatively constant because of the strong cohesive forces between the molecules.
As a result, a liquid takes the shape of the container it is in, and it forms a free surface in a larger container in a gravitational field.
A gas, on the other hand, expands until it encounters the walls of the container and fills the entire available space.
This is because the gas molecules are widely spaced, and the cohesive forces between them are very small.
Unlike liquids, gases cannot form a free surface.
