Gases can be compressed

1. The particles of a gas are widely separated, so most of the space a gas occupies is empty.
2. That empty space can be taken away, which is why a gas can be squeezed into a much smaller volume.
3. Pushing a piston inwards increases the pressure applied to the trapped gas, so the gas is compressed into a smaller volume.
4. The compression continues until the pressure of the gas has risen enough to balance the pressure being applied to it.
5. The number of particles does not change while a gas is compressed, so the same particles are simply packed closer together.
6. Solids and liquids can barely be compressed at all, because their particles are already in contact and there is almost no space to remove.
7. This difference is put to work in machinery, because compressed air can be stored in a cylinder and released to drive a tool, while hydraulic systems rely on a liquid that will not compress.
8. A diver's cylinder is another example, holding a large mass of air in a small volume because the air inside has been compressed to a high pressure.

Gases can be expanded

1. Reducing the pressure acting on a gas allows it to expand into a larger volume.
2. Pulling the plunger of a sealed syringe outwards lowers the pressure inside, and the trapped gas expands to fill the extra space.
3. The expansion continues until the pressure of the gas has fallen enough to balance the reduced pressure pushing in on it.
4. A weather balloon swells as it climbs, because atmospheric pressure falls with height and so the gas inside meets less opposition.
5. A gas has no volume of its own, so its volume is set by the pressure acting on it and by the container holding it.
6. The number of particles is again unchanged, so an expanded gas is the same gas with its particles spread further apart.
7. Compression and expansion are opposite processes, and a gas can be taken back and forth between them by raising and lowering the pressure applied to it.

Pressure acts at right angles

1. Gas particles reach a surface from every direction, because their motion inside the container is random.
2. For every particle striking the surface at an angle from one side, another arrives at a matching angle from the other side.
3. The sideways effects of all those collisions therefore cancel out on average, leaving no overall push along the surface.
4. What remains is a net force at right angles to the surface, in other words along the normal to it.
5. This holds for every surface the gas touches, whatever the angle at which that surface is set.
6. On a curved surface the direction of the net force changes from point to point, staying perpendicular to the surface at each point.
7. An inflated balloon shows this clearly, since the gas pushes outwards over the whole of the inner surface and the balloon settles into a rounded shape.
8. The size of that outward force grows with the area of the surface, so a large flat panel in a pressurised container carries a very large total force.