Edexcel GCSE Combined Science CC2 States of matter and mixtures: a complete overview of the particle model, changes of state and separation techniques

What CC2 actually demands

States of matter and mixtures covers the particle model and the practical chemistry of separating substances. The examiners reward clear particle-model explanations of changes of state and confident choice of the right separation technique for a given mixture, both of which appear on Chemistry Paper 1.

This guide walks through the topic and ties together the matching dot-point page, which has its own practice questions.

The particle model and changes of state

The particle model describes solids (particles fixed in a regular pattern, vibrating), liquids (close together, free to move) and gases (far apart, fast-moving). Heating gives particles energy so a solid melts and a liquid boils; cooling causes freezing and condensing. These are physical changes that make no new substance and can be reversed.

Pure substances and mixtures

A pure substance melts and boils at a fixed temperature; a mixture contains substances that are not chemically joined and melts over a range. The parts of a mixture keep their own properties and can be separated physically.

Separation techniques

The method depends on the properties of the parts:

• Filtration - insoluble solid from a liquid.
• Crystallisation / evaporation - soluble solid from its solution.
• Simple distillation - a liquid from a dissolved solid.
• Fractional distillation - liquids with different boiling points.
• Chromatography - substances in a mixture, by solubility and attraction to the paper.

How CC2 is examined

• Particle model. Explaining the arrangement, movement and energy of particles in each state and during changes of state.
• Pure versus mixture. Using melting and boiling behaviour to tell them apart.
• Choosing techniques. Selecting and justifying the right separation method.
• Practical detail. Describing how to recover both parts of a mixture, such as sand and salt.

Check your knowledge

A mix of recall and application questions covering CC2. Attempt them under timed conditions, then check against the solutions.

1. Describe the arrangement of particles in a solid. (1 mark)
2. Name the change of state from gas to liquid. (1 mark)
3. State whether changes of state are physical or chemical. (1 mark)
4. How does a pure substance behave when it melts? (1 mark)
5. Name the technique to separate sand from water. (1 mark)
6. Name the technique to obtain salt from salt water. (1 mark)
7. Which technique separates liquids with different boiling points? (1 mark)
8. What does chromatography separate a mixture by? (1 mark)