In a fixed volume when temperature increases, pressure does what?

also

in a fixed volume when temperature decreases, pressure does what?

When the volume is constant the pressure increases with temperature.

Explanation:

Gay Lussac's law precisely explains the relation between pressure and temperature of a system which has constant volume. In a system which has constant volume an increase in temperature indicates increase in temperature as well.

The reason behind the observation of this trend is the change in randomness of particles of the system.

With increase in temperature the particle movement becomes random and fast. The particles hit the container walls at an increased rate and the pressure of the system increase.