Gases and the absolute scale of temperature — Describe

In terms of particles, what happens to the pressure of a fixed mass of gas (a) when its temperature is increased at constant volume, and (b) when its volume is decreased at constant temperature?

(a) At constant volume, increasing the temperature increases the average kinetic energy and speed of the particles, so they hit the container walls more often and with greater force, increasing the pressure. (b) At constant temperature, decreasing the volume reduces the space available, so particles (moving at the same average speed) collide with the walls more frequently, increasing the pressure.