 So what is pressure? Particularly gas pressure. You may be familiar with measuring pressure when pumping up your bicycle tires or a basketball or maybe even with weather systems as low and high pressure systems move across Australia. So gas pressure is the measurement of the force of the gas particles colliding with the walls of the container. So here we have a container and some gas particles. So these are moving around freely and as they hit the side of the container that's what we're measuring. So pressure equals the force over the surface area. There's a few things that pressure depends on. So let's have a look at them now. So here we have a container with no particles at zero atmospheres. If we are to increase the amount of particles in the pressure will of course increase. As there's more gas particles there are more collisions and therefore more pressure. Another factor that pressure depends on is temperature. So if we heat up these molecules of course they'll move faster and because they're moving faster they're colliding more and because of that pressure also increases. Same thing happens but in reverse when you cool it down. Here we're dropping the temperature well below room temperature and we can see that these molecules have slowed down. The third one is the volume of the container. So the smaller the surface area is the more collisions will happen as there's less free space for the particles to move around in. Because of this there are more collisions and therefore more pressure.