Particle model of states of matter

The particle model represents matter as small particles. Solids, liquids and gases differ in how their particles are arranged, how far apart they are and how they move.

In the diagrams below, each dot represents a particle.

Solid

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The particles in a solid are closely packed in a regular arrangement. They vibrate about fixed positions rather than moving past one another.

Liquid

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The particles in a liquid are close together but arranged irregularly. They are not fixed in place and can move past one another.

Gas

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The particles in a gas are widely separated and arranged randomly. They move rapidly in all directions.

A static particle diagram shows arrangement and separation, but it cannot show movement on its own. Arrows can be added to indicate particle motion.

Temperature and particle motion

The particles in a substance are constantly moving. Their motion gives them kinetic energy, the energy of movement. Temperature is related to the particles’ average kinetic energy: a higher temperature means a greater average kinetic energy.

When a substance is heated and its temperature rises, its particles gain average kinetic energy and move more vigorously. When its temperature falls, their average kinetic energy decreases and their motion becomes less vigorous. The temperature describes the average; individual particles do not all have the same kinetic energy.

Absolute zero

Absolute zero is the lowest possible temperature, approximately $$-273,^\circ\mathrm{C}$$. At this temperature, particles have their least possible kinetic energy.

Gas pressure and particle collisions

Gas particles move randomly in all directions. When a particle collides with a surface, such as a container wall, its motion changes. The particle exerts a force on the wall during the collision.

A great many particles collide with the wall repeatedly. Their combined force, spread over the wall’s area, produces gas pressure. Pressure is the average force per unit area:

Here $$p$$ is pressure, $$F$$ is the average force on the surface and $$A$$ is the area of the surface. Pressure increases if collisions with the surface become more frequent or exert a greater force.

Gas pressure and temperature

For a fixed amount of gas in a container of constant volume, raising the temperature makes the particles move faster on average. They reach the walls more often, so collisions become more frequent. Each collision also produces a greater change in the particle’s motion and exerts a greater force on the wall. The pressure therefore increases.

Gas pressure and volume

For a fixed amount of gas at constant temperature, reducing the volume gives the particles less space to move in. They reach the walls more often, so wall collisions become more frequent. The increased collision frequency raises the pressure.

Brownian motion

A microscopic particle suspended in a gas or liquid moves in an irregular, unpredictable way. This movement is called Brownian motion.

The suspended particle is continually struck by the much smaller particles of the surrounding gas or liquid. These particles collide with it from different directions, but their impacts are uneven at any moment. As a result, the suspended particle changes speed and direction unpredictably.

Brownian motion provides evidence for the kinetic particle model. The visible movement of the suspended particle shows the effects of the continuous, random motion of the much smaller particles around it.

Particle forces and properties of states

The properties of a solid, liquid or gas depend on how far apart its particles are, how they move and how strongly they attract one another. The particles making up matter may be atoms, molecules or ions. Electrons are components of atoms and contribute to interactions between particles.

Solids

In a solid, attractive forces hold particles close together in fixed positions. The particles vibrate about these positions but do not move freely past one another. This is why a solid retains both its shape and volume. There is little space between its particles, so a solid is difficult to compress.

Liquids

In a liquid, attractive forces keep particles close together, but the particles can move past one another. A liquid therefore flows and takes the shape of its container. It retains its volume and is difficult to compress because its particles remain close together.

Gases

In a gas, particles are far apart and the attractive forces between them are weak. The particles move freely, so a gas flows and spreads to fill its container. The large spaces between its particles allow them to be pushed closer together, making a gas readily compressible.