The box contains an ideal gas, drawn as nitrogen particles with no volume of their own and no forces between them. Temperature is in kelvin. Volume is the width of the container on a relative scale, not a litre measurement. Pressure is calculated from the ideal gas law, PV = nRT, and the particle colours show speed: slower particles are cooler colours, faster particles are warmer colours.
At constant temperature, pressure is inversely proportional to volume: P ∝ 1/V, or P₁V₁ = P₂V₂. Squashing the gas gives the particles less distance between collisions with the walls, so the collisions are more frequent and the pressure is higher.
At constant pressure, volume is proportional to absolute temperature: V₁/T₁ = V₂/T₂. Faster particles hit the piston harder. The piston moves out until the collisions are spread over a larger area and the pressure is back to its original value.
At constant volume, pressure is proportional to absolute temperature: P₁/T₁ = P₂/T₂. The container cannot expand, so faster, more frequent collisions raise the pressure. A temperature in Celsius does not give a proportional graph through the origin; kelvin does.
Pressure comes from particles colliding with the walls. Temperature is a measure of their average kinetic energy. In an ideal gas that energy is proportional to the kelvin temperature, which is why the RMS speed rises with the square root of T.
GCSE and IGCSE use Boyle’s law, and often the qualitative effect of temperature on pressure and volume. A level and IB combine them as the ideal gas equation and use the kinetic model to explain the three proportionalities. The mode tabs are the three constant-quantity experiments.
Particles are points, and forces between them are ignored, so this is an ideal gas rather than a real gas near liquefaction. The volume slider is a relative container scale. It is for comparing P, V and T, not for reading a laboratory burette.