Why does a gas fill all the space available to it?

Answer 1

Gases exert pressure against anything around them, because of something called Brownian motion: every particle is moving in a random direction with a speed based on temperature. These particles rebound millions of times a second off each otehr and off container walls, changing direction and speed as they do. But for something to change direction, a force must be applied. This force acts on everything, the gas and the container.

That "anything" includes other parts of the gas volume. Now, if they don't fill up all of the available space, there is zero pressure exerted on one side, while there is nonzero positive pressure exerted on all others. The net result is pressure towards the empty volume.

This pulls/pushes the gas into the empty volume, filling it up. When there is enough gas in that space to counteract the pressure form the rest of the gas - which will be when the entire volume has the same number of molecules per volume unit - the pressures in all directions are balanced, and the gas in general stops moving.

Answer 2

A gas's particles are spread far apart and bounce around freely in linear patterns (or, in other words, in a straight line). So they'll keep moving until they hit something, no matter how big or small that volume is.

This isn't true in liquids and solids because their particles are held closer together and, in solids, the particles don't even "bounce", but rather vibrate in place.

Answer 3

Gases fill entire space available to it because due to high kinetic energy(movement of particles) and negligible forces of attraction, the particles of a gas are moving with high speeds in all directions.

Answer 4

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Answer 5

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