Compressibility is a measure of the relative volume change of a material under applied pressure. It is defined as the fractional change in volume per unit pressure.
- Gases are highly compressible because their molecules are far apart and have a lot of space to move closer together when pressure is applied. When pressure is applied to a gas, the gas molecules move closer together and the volume of the gas decreases. For example, if you push down on the plunger of a syringe filled with air, the volume of the air inside the syringe will decrease.
- Liquids are much less compressible than gases because their molecules are already packed closely together and there is not much space for them to move closer together when pressure is applied. When pressure is applied to a liquid, the liquid molecules can only move slightly closer together and the volume of the liquid decreases only slightly. For example, if you push down on the plunger of a syringe filled with water, the volume of the water inside the syringe will decrease only a small amount.
The compressibility of a substance is determined by its molecular structure and intermolecular forces. In gases, the intermolecular forces are weak and the molecules are far apart, so gases are highly compressible. In liquids, the intermolecular forces are stronger and the molecules are packed more closely together, so liquids are much less compressible.
