A gas will always do what to fill its container?

A gas will always expand to fill its container due to the nature of gas particles and the forces acting among them. Gas particles are in constant, random motion and are widely spaced compared to atoms in liquids and solids. This characteristic allows gas to fill any volume available to it.

When a gas is in a container, the particles move in all directions and will spread out until they evenly occupy the entire space of that container. This behavior can be attributed to the kinetic molecular theory, which states that gas particles are in ceaseless motion and collide with each other as well as the walls of their container. These constant collisions exert pressure on the walls, effectively causing the gas to fill the entire volume.

In contrast, the other options refer to behaviors that are not characteristic of gases under normal conditions. For instance, condensation involves a gas transitioning to a liquid state under certain conditions, neither contracting nor remaining constant accurately describes the behavior of gases as they move to occupy the entire available space.