The particulate nature of matter

As you know, there are three states of matter that you need to study during your IGCSE chemistry course:

  1. Solids
  2. Liquids
  3. Gases

Here is a detailed comparison between solids, liquids and gases.

  Solids Liquids Gasses
Description Fixed volume.

Fixed shape.

Fixed volume.

Takes the shape of its container.

Variable volume.

Takes the shape of its container.

Arrangement of particles Arranged in a regular pattern called a lattice. Random. Random.
Separation of particles (intermolecular gap) Close together.

Tightly packed.

A little close together

Slightly further apart than in solid phase.


Far apart.

Movement of particles Vibration about a fixed position. Slow movement in a random way from place to place, sliding past each other. Fast, random movement.
Attractive forces between particles Stronger than in the liquid phase. Slightly weaker than in the solid phase. No attractive forces between the particles.
Image <image for a solid here> <image for a liquid here>


<image for a gas here>


Changes of state

When the temperature of a solid increases, so does the energy of the particles. At the melting point, the particles have enough energy to move past each other and change positions. The liquid, unlike the solid, can flow and change its shape.

Similarly, when a liquid is heated, the energy of the particles increases. At the boiling point, the intermolecular forces can no longer hold the particles together, so the particles separate and become a vapour or gas. The particles can change position and move apart. Hence gases can flow and expand to fill any space.


  Change of state from
Melting A solid to a liquid
Boiling A liquid to a gas
Condensation A gas to a liquid
Freezing A liquid to a solid
Sublimation A solid to a gas
De-sublimation A gas to a solid



In both the liquid and gaseous states, the particles have random translational motion, which is movement from place to place. This results in the particles spreading out to occupy any available space. This is called as diffusion.

Study tip!

(Fusion is the fusing of two particles; diffusion is the spreading out of two particles)

Rates of diffusion

  • Diffusion does not occur in solids as the particles can only vibrate about a fixed position; they cannot spread apart or move across each other.
  • Diffusion in liquids is much slower than in gases because of the particles of a liquid move much more slowly than the particles of a gas.
  • The rate at which a gas diffuses depends on its relative molecular mass. Lighter molecules will move and diffuse faster than heavier molecules.

Try this:

  1. Which gas- carbon dioxide or nitrogen would diffuse faster? Give a reason to your answer. (3 marks)

(Answer: Nitrogen gas, as nitrogen Mr = 28 has lighter molecules than carbon dioxide, Mr = 44, which is heavier.)

  1. A balloon contains a mixture of methane and hydrogen. After several hours, the percentage of methane in the balloon has increased. Can you explain why? (2 marks)

(Answer: Hydrogen has a lower Mr than methane (which has a higher Mr) Hence hydrogen will diffuse out of the balloon at a higher diffusion rate than methane, which is heavier.)

Note: such type of questions are frequently asked in paper 3 and paper 1. Be careful!

PHET Simulation for The Particulate nature of matter!

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