Final answer:
When a gas is heated, the speed of its molecules increases and they hit the walls with more force. The pressure can remain constant if the volume of the container increases.
Step-by-step explanation:
When a gas is heated, the speed of its molecules increases and thus they hit the walls with more force. The only way that pressure can remain constant under these conditions is if the volume of the container increases. According to Charles's Law, if the temperature of a gas is increased, a constant pressure may be maintained only if the volume occupied by the gas increases.
This will result in greater average distances traveled by the molecules to reach the container walls, as well as increased wall surface area. These conditions will decrease both the frequency of molecule-wall collisions and the number of collisions per unit area, balancing the effect of increased collision forces due to the greater kinetic energy at the higher temperature.