Explain in terms of the motion of the molecules of the gas why the volume of gas must increase if the pressure is to remain constant as the gas is heated.

- When gas is heated, the molecules gain kinetic energy. 

• This causes more collisions of greater force per unit time with the wall of the container. 
  
• If the pressure is constant, then fewer collisions per unit time are required. 
  
• Therefore a bigger volume of the container is required so that there are fewer collisions per unit time.