The behavior of gases and liquids under external force demonstrates a fundamental difference in their physical structure. A gas is a state of matter without a fixed shape or volume, and its ability to be compressed—reducing its volume by applying external pressure—is a defining characteristic. This high compressibility stems directly from the microscopic arrangement of its constituent particles, contrasting sharply with the molecular architecture of liquids.
The Defining Feature: Vast Intermolecular Space
The unique compressibility of gases is explained by the Kinetic Molecular Theory, which describes gases as collections of particles in constant, rapid, and random motion. A gas sample is predominantly empty space; the volume occupied by the actual gas particles is negligible compared to the total volume of the container. At standard temperature and pressure, the average distance between gas molecules can be up to ten times the diameter of the molecule itself.
This immense separation means gas molecules exert only extremely weak attractive forces on one another. When an external force is applied, the particles are easily pushed closer together because there is significant empty space to fill. This structural feature allows a gas to be squeezed into a much smaller volume, making high compressibility possible by reducing the vast void between the particles.
The Result of Compression: Increased Pressure and Density
Applying force to a gas decreases its volume and immediately changes its internal characteristics, specifically increasing both its pressure and its density. Pressure in a gas results from the constant collisions of gas particles with the container walls. As the volume shrinks, the same number of particles are confined to a smaller area.
This reduction in available space forces the particles to cover shorter distances between collisions, significantly increasing the frequency of impacts. This greater rate of collision translates directly into a higher measurable pressure, a relationship described by Boyle’s Law. Furthermore, since density is mass per unit volume, and the mass remains constant while the volume decreases, compression necessarily causes an increase in the gas’s density.
Contrast with Incompressible States of Matter
Liquids, along with solids, are referred to as condensed states of matter because their particles are already packed very closely together. In a liquid, the molecules are in continuous contact, allowing them to slide past one another, which is why a liquid can flow. The intermolecular forces are substantially stronger than those in gases, holding the molecules in a tight arrangement.
Because there is very little empty space between the molecules, applying external pressure cannot significantly reduce the overall volume. The molecules are essentially touching, and further compression would require overcoming powerful repulsive forces. While liquids are not perfectly incompressible, the volume reduction is so minimal that the change is considered negligible for most practical applications.