Why gases can be compressed

1. The molecules in a gas are very far apart, with mostly empty space between them.
2. Pushing on a gas forces its molecules closer together, so the same amount of gas fits into a smaller volume.
3. This decrease in volume is called compression.
4. The molecules themselves are not squashed, because only the spaces between them get smaller.
5. If the push on a gas is reduced, its molecules spread out into a larger volume, which is called expansion.
6. Liquids and solids are very hard to compress, because their particles are already touching and there is almost no space left to close up.
7. Gases can be compressed into a much smaller volume, which is how a diver's tank holds enough air for a long dive.

Pressure changes and volume

1. A gas changes its volume until the pressure it exerts balances the pressure acting on it from outside.
2. If the pressure outside a gas increases, the gas is compressed until its own pressure rises to match.
3. If the pressure outside a gas decreases, the gas expands until its own pressure falls to match.
4. Pushing the plunger of a sealed syringe increases the pressure on the trapped air, so the air is compressed into a smaller volume.
5. A sealed packet of crisps puffs up on an aeroplane, because the air pressure in the cabin is lower than on the ground and the air inside the packet expands.
6. A partly blown-up balloon placed in a sealed bell jar grows larger as a vacuum pump removes air from the jar, because the pressure around the balloon falls.
7. When air is let back into the jar, the pressure outside the balloon rises again and the balloon shrinks to its original size.

Force at right angles to a surface

1. Gas pressure pushes on every surface the gas touches, including the walls of its container and the face of a piston or plunger.
2. The force produced by pressure always acts at right angles to the surface, which is also called normal to the surface.
3. This happens because molecules strike the surface from every direction, so on average their sideways pushes along the surface cancel and only the push straight into the surface remains.
4. On the flat face of a plunger, the force from the trapped gas therefore acts straight along the barrel of the syringe.
5. On the curved inside of a balloon, the force acts outwards at right angles to the rubber at every point, which is why a balloon inflates into a rounded shape.
6. Most surfaces have gas pressing on both sides, so each side receives a force at right angles to it.
7. If the pressures on the two sides are equal, the two forces cancel and there is no net force on the surface.
8. If the pressures are different, there is a pressure difference across the surface, and the force from the higher-pressure side is larger.
9. This gives a net force at right angles to the surface, directed towards the side with the lower pressure.
10. A syringe plunger stays still while the pressure of the trapped air equals the pressure of the air outside, and it moves as soon as one of these pressures changes.