3. Gases Are Compressible:

The volume occupied by a gas is almost entirely empty space because the volume filled by the gas molecules is very small. Applying a small pressure will move gas molecules closer together, decreasing the volume.
	Steel cylinders containing gases under pessure are widely used in industry. When full, they can contain 100 times as many gas molecules as would be contained in a nonpressurized container.

The volume of gas in the syringe (below) is reduced when the plunger is pushed inward: