At a constant temperature, what will happen to the pressure inside a container of gas if the volume of the container is reduced?
The pressure will increase.
Study Boyle's Law. Pressure is inversely related to Volume. So if The volume is increased the pressure will decrease, and if the volume is decreased the pressure will increase.
Pressure is caused by the collisions between molecules. If the volume is decreased the molecules are pushed closer together. As the molecules are pushed closer together the molecule collide more frequently increasing the pressure. Study kinetic theory of molecular motion.