Liquid

Summary: The state of matter with a fixed volume but no fixed shape, in which closely packed particles in a random arrangement slide past one another, held by forces weaker than those in a solid. Tags: igcse chemistry Created: 2026-07-17

A liquid is the state of matter in which a substance has a fixed volume but no fixed shape, taking the shape of its container up to the level of its surface. According to the kinetic particle theory, the particles in a liquid are still close together and touching, but they are arranged randomly rather than in the regular pattern found in a solid. The particles have more energy than those in a solid, which allows them to move around and slide past one another; this movement is what enables liquids to flow and to be poured. The forces of attraction between the particles are weaker than in a solid but still strong enough to hold the particles close together, so a liquid keeps a constant volume and is only very slightly compressible. Because the particles remain closely packed, liquids have high densities, usually only slightly lower than the corresponding solid. When a liquid is heated to its boiling point, the particles gain enough energy to overcome the remaining forces of attraction completely and escape as a gas, while at the surface some higher-energy particles can escape at any temperature by evaporation.

Properties of Liquids

The characteristic properties of liquids place them between solids and gases: like solids, they have a fixed volume, but like gases, they can flow and change shape. Each property is explained by the arrangement, motion, and energy of the particles.

Particle arrangement and motion

The particles in a liquid are close together but arranged randomly, with no long-range order. Unlike the particles in a solid, they are not held in fixed positions: they move continuously, sliding past one another while remaining in contact with their neighbours. This combination of closeness and mobility explains why liquids flow, mix by diffusion (more slowly than gases), and adopt the shape of any container. As the temperature increases, the particles move faster and slightly further apart, so most liquids expand a little when heated.

Energy and forces between particles

The particles in a liquid have more kinetic energy than in the solid state of the same substance — enough to overcome the forces that would hold them in fixed positions, but not enough to escape from one another entirely. The forces of attraction between particles are therefore weaker than in a solid but much stronger than in a gas. These intermediate forces keep the particles together in a definite volume while still permitting movement. At the surface of a liquid, particles with above-average energy can break free of these forces and evaporate, which is why liquids gradually evaporate below their boiling point; at the boiling point, particles throughout the liquid gain enough energy to escape and bubbles of gas form within the bulk of the liquid.

Shape of Liquid

A liquid has no fixed shape — it takes the shape of its container below the level of its own surface. The particles in a liquid are closely packed but arranged randomly, and they can slide past one another freely. Because particles are not locked in fixed positions (unlike a solid), the liquid flows and adopts the shape of whatever holds it. However, a liquid does not expand to fill the entire container like a gas — it only fills up to its surface level, because the forces of attraction between particles are still strong enough to keep them together.

Exam point: “No fixed shape” means the liquid takes the shape of its container, but only up to its surface level.

Volume of Liquid

A liquid has a fixed volume — it does not change when poured from one container to another. The particles in a liquid remain close together and touching, held by forces of attraction. Although particles can slide past one another, they cannot move apart significantly. Thus the volume stays constant regardless of container shape. Liquids are only very slightly compressible because there is very little empty space between particles.

Exam point: “Fixed volume” means the liquid keeps the same volume no matter what container it is in.

Density of Liquid

Liquids have high density — typically only slightly lower than the density of the corresponding solid. The particles are still closely packed, so a large number of particles occupies a small volume. The slight decrease in density compared to solids occurs because the particles are arranged randomly rather than in a compact regular lattice, slightly increasing the average spacing. A few substances (e.g. water) are unusual in that the liquid is actually denser than the solid.

Exam point: Liquids have high density because particles are still closely packed and touching.

Sources

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

Common Misconceptions