Increasing the temperature of the gas increases the average
speed at which molecules move, thereby also increasing
the pressure of the gas. There are two reasons for this effect.
First, if the molecules in the box are moving faster, they will
hit the walls more frequently (Figure 9.3c). More molecules
hitting the walls of the box each second means that the pressure
is higher. Second, faster-moving molecules hit the wall
harder, exerting more force. Together, these two effects mean
that doubling the temperature of a gas doubles the pressure
of the gas.