The relationship between the pressure and the volume of a gas is described in a gas law known as Boyle's Law, which states that for a fixed mass of gas held at a constant temperature, the product of the pressure and the volume is a constant, as demonstrated by the equation PV = C. So, when the volume goes down, pressure goes up, and when volume goes up, pressure goes down.
The atoms, molecules or ions that make up solids, liquids and gases are moving even when it is not apparent to the naked eye. In solids, particles vibrate but do not flow easily and are not easily compressed. In liquids, the particles are still close together, but move more freely and allow the liquid to take the shape of a container. In gases, however, there is a lot of space between particles, and the gas particles vibrate and move around freely at high speeds. As the gas molecules bounce around freely in a container, they frequently collide with each other and the walls of the container. The force of these collisions exerts a force on the interior surface of the container and is called the gas pressure. As the container gets larger, or the volume increases, there are fewer collisions between gas molecules and the container's interior, making the gas pressure lower