If the pressure on a gas is constant and temperature is decreased, what will happen to its volume?

The volume of the gas increases as the temperature increases.

As temperature increases, the molecules of the gas have more kinetic energy. They strike the surface of the container with more force.

If the container can expand, then the volume increases until the pressure returns to its original value.

Charles' Law summarizes this observation mathematically.

http://socratic.org/questions/what-is-the-relationship-between-temperature-and-volume-in-charles-law

Hope this helps.