Gases are made up of molecules that are separated by empty space, making them much less dense than solids. Pressure in gases is caused by molecular collisions with the container walls. Boyle's law states that for a fixed amount of gas, pressure and volume have an inverse relationship. Specifically, as the volume of a container decreases, the pressure increases because the molecules have less space and collide with the walls more frequently. Experiments can show how the microscopic behavior of gas molecules explains their macroscopic properties like compressibility and pressure.

1. Matter Very Simple Up, Up and Away

5. I can hardly stand up!

10. Collisions cause pressure on container walls But what happens if the volume of the container is changed???

11. Boyle’s Law – Pressure is proportional to ‘crowding’ of molecules pV = constant x Number of molecules