What causes gases to expand and fill available space?

Gases expand and fill available space due to the constant, random motion of their particles.

In more detail, gases are composed of a large number of tiny particles, which are in constant, random motion. This is a fundamental principle of the kinetic theory of gases. These particles are moving in all directions and at different speeds. They collide with each other and with the walls of their container, if there is one.

When a gas is confined to a container, its particles are constantly hitting the walls of the container. If the container is opened or if the gas is not confined, the particles will continue their motion in a straight line until they collide with something else. Therefore, if there is more space available, the gas particles will move to fill it. This is why gases expand to fill the volume of whatever container they are in, or if not confined, they will spread out indefinitely.

The speed and direction of the gas particles are influenced by temperature. Higher temperatures increase the speed of the particles, which means they will hit the walls of the container with more force and more often. This is why heating a gas will cause it to expand, if it is able to.

In addition, gas particles are far apart from each other and there are no significant forces of attraction between them. This means that there is a lot of empty space for the particles to move into. This lack of attraction between particles, combined with their constant, random motion, is what causes gases to expand and fill any available space.