AQA GCSE Physics

Revision Notes

3.1.2 Solids, Liquids & Gases

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

Shape and volume, IGCSE & GCSE Physics revision notes

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

solid-liquid-density, IGCSE & GCSE Physics revision notes

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

Gas Density, downloadable IGCSE & GCSE Physics revision notes

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

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