Solids, Liquids & Gases (AQA GCSE Physics)

The Particle Model

• All matter is made up of very small particles, or atoms

• The particle model is a model that describes the arrangement and movement of particles in a substance

• The particle model can be used to explain

  • The different states of matter e.g. solids, liquids and gases

  • Physical properties e.g. differences in density

Solids, Liquids & Gases

• Matter can exist in one of three different states: solid, liquid, or gas

Solids

• In a solid:

  • The particles are closely packed

  • The particles vibrate about fixed positions

• Solids have:

  • A definite shape (they are rigid)

  • A definite volume

Liquids

• In a liquid:

  • The particles are closely packed

  • The particles can flow over one another

• Liquids have:

  • No definite shape – they are able to flow and will take the shape of a container

  • A definite volume

Gases

• In a gas:

  • The particles are far apart

  • The particles move randomly

• Gases have:

  • No definite shape – they will take the shape of their container

  • No fixed volume – if placed in an evacuated container they will expand to fill the container

• Gases are highly compressible, this is because:

  • There are large gaps between the particles

  • It is easier to push the particles closer together than in solids or liquids

Diagram showing the three states of matter in terms of shape and volume

Solid, Liquid, Gas Summary Table

Differences in Density

Solids & Liquids

• In solids and liquids, the molecules are tightly packed together

  • The difference is, in a liquid, the molecules have enough energy to push past each other

• As a result of this, the density of solids and liquids are roughly the same

The molecules in solids and liquids are tightly packed, giving them a high density

Gases

• In a gas, the molecules are widely separated

  • As a result of this, gases have significantly lower densities than solids or liquids

• At room temperature, the distance between molecules in a gas is roughly ten times (in each direction) the distance between molecules in a solid or liquid

• As a result, the density of a gas is typically around one-thousandth (1/1000) of the density of a solid or liquid, for example:

  • The density of water is 1000 kg/m³

  • The density of air at sea level and room temperature is 1.3 kg/m³

The molecules in a gas are widely spaced, giving it a much lower density