JAMB - Chemistry (2007 - No. 29)

When a gas is compressed to half its volume, the average intermolecular distance decreases because the same number of gas molecules are now confined to a smaller space, forcing them closer together, resulting in a smaller average distance between each molecule. 

Conversely, when the volume of a gas is reduced, the frequency of collisions between molecules increases; this is because the molecules have less space to move around, leading to more frequent interactions with each other and the container walls.