Gas

Summary: The state of matter with no fixed shape or volume, in which widely spaced particles move rapidly and randomly in straight lines with negligible forces between them. Tags: igcse chemistry Created: 2026-07-17

A gas is the state of matter in which a substance has neither a fixed shape nor a fixed volume, expanding to fill the entire container in which it is placed. According to the kinetic particle theory, the particles in a gas are very far apart compared with their own size and are arranged completely randomly, with the space between particles being mostly empty. Gas particles possess the highest energy of the three states of matter and move rapidly and randomly in straight lines, changing direction only when they collide with other particles or with the walls of the container. The forces of attraction between gas particles are negligible, which is why a gas does not hold together in a fixed volume and why its particles spread out to fill all available space. The large empty spaces between particles mean that gases have very low densities and are highly compressible, since the particles can easily be pushed closer together. The constant collisions of gas particles with the walls of their container produce gas pressure, and heating a gas makes its particles move faster, increasing the pressure in a fixed volume or expanding the gas if it is free to do so.

Properties of Gases

Gases behave very differently from solids and liquids because their particles are widely separated and essentially independent of one another. Each macroscopic property follows directly from the particle model.

Particle arrangement and motion

The particles in a gas are far apart, in a completely random arrangement, and in constant rapid motion. Each particle travels in a straight line until it collides with another particle or with the wall of the container, after which it moves off in a new direction. This ceaseless, random, straight-line motion explains why gases diffuse quickly and why they fill any container completely and evenly. Increasing the temperature increases the average speed and kinetic energy of the particles, so the particles collide with the walls more often and with greater force; this raises the pressure of a gas in a sealed container and causes an unconfined gas to expand.

Forces between particles

The forces of attraction between gas particles are negligible — so weak that, for most purposes at IGCSE level, they can be ignored. Because the particles are not attracted to one another, they move completely independently, spreading apart until they are evenly distributed throughout the available space. This absence of significant attractive forces is the fundamental reason why gases have no fixed shape or volume. When a gas is cooled, its particles slow down until, at the condensation point, the weak attractive forces become significant enough to draw the particles close together, and the gas condenses into a liquid.

Pressure and collisions

Gas pressure is caused by the collisions of particles with the walls of the container. Each collision exerts a tiny force on the wall, and the enormous number of collisions per second produces a steady overall pressure. Compressing a gas into a smaller volume increases the frequency of these collisions, which increases the pressure; heating a gas at constant volume makes the particles strike the walls faster and more often, which also increases the pressure.

Shape of Gas

A gas has no fixed shape — it takes the entire shape of its container. Gas particles are far apart and move rapidly and randomly in all directions. With negligible forces of attraction between them, nothing holds the particles together, so they spread out until they uniformly fill whatever container they are in. Unlike liquids, gases do not stop at a surface level — they expand to occupy all available space.

Exam point: “No fixed shape” means the gas spreads to fill the whole container, not just the bottom.

Volume of Gas

A gas has no fixed volume — it expands to fill the entire volume of its container. The particles are widely separated with large amounts of empty space between them. Because the forces of attraction between gas particles are negligible, nothing prevents them from spreading apart. Changing the container size or shape changes the gas volume accordingly. Compressing a gas reduces the empty space and decreases its volume; heating a gas increases particle speed and causes expansion.

Exam point: “No fixed volume” means the gas volume is the same as the container volume — the gas does not have its own volume.

Density of Gas

Gases have low density — much lower than solids and liquids of the same substance. The particles are very far apart, with most of the space between them being empty. Only a comparatively small number of particles occupies a given volume. For example, the density of water vapour is about 1/2000 the density of liquid water at the same mass.

Exam point: Gases have low density because particles are far apart with large amounts of empty space between them.

Compressibility of Gas

Gases are highly compressible — they can be squashed into a much smaller volume with relatively little pressure. The particles in a gas are widely separated, so there is an enormous amount of empty space that can be reduced. When a gas is compressed, the particles are simply forced closer together; the particles themselves do not change size. This is fundamentally different from solids and liquids, where particles are already touching and cannot be pushed significantly closer.

Exam point: “Highly compressible” means gas volume can be greatly reduced because of the large empty spaces between particles.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education

Common Misconceptions