When the system's number of molecules and pressure are held constant, then the volume is directly proportional to any change in temperature. This can clearly be seen on the graph as the temperature is changed. As the temperature increases the volume of the gas increases. We can understand this best by noticing the particles move faster when the temperature is increased. The faster the particles move the greater the force of the collisions with the walls of the container and the greater number of collisions with the walls of the container. This increase in the force of the collision and with the number of collisions means the internal pressure increases. Since the external pressure is constant the volume of the container must increase until the internal pressure equals the external pressure.

To further explore the relationship between pressure, volume, moles of gas and temperature look at the Gas Law MoLE. Be sure to print out a copy of the Gas Law Activity, or look at it in your Chemistry Activity Manual.