How does the particle model explain the states of matter, and how do we separate mixtures?
The particle model and the three states of matter, changes of state, the difference between a pure substance and a mixture, and the techniques used to separate mixtures.
A focused answer to Edexcel GCSE Combined Science Topic 2 (CC2), covering the particle model and the three states of matter, changes of state, the difference between pure substances and mixtures, and the main techniques for separating mixtures.
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What this dot point is asking
Edexcel wants you to use the particle model to describe the three states of matter and changes of state, distinguish a pure substance from a mixture, and describe the techniques used to separate mixtures.
The particle model and states of matter
Solids have a fixed shape and volume; liquids have a fixed volume but take the shape of their container; gases fill their container and can be compressed.
Changes of state
Heating gives particles energy:
- Melting (solid to liquid) and boiling (liquid to gas) happen when particles gain enough energy to overcome the forces holding them.
- Freezing (liquid to solid) and condensing (gas to liquid) happen on cooling, as particles lose energy.
Pure substances and mixtures
A pure substance contains only one type of element or compound and melts and boils at a fixed temperature. A mixture contains two or more substances that are not chemically joined, so they keep their own properties and can be separated by physical means. A mixture melts and boils over a range of temperatures.
Separating mixtures
The technique used depends on the properties of the parts:
- Filtration: separates an insoluble solid from a liquid (for example sand from water).
- Crystallisation / evaporation: obtains a soluble solid from its solution (for example salt from salt water).
- Simple distillation: separates a liquid (solvent) from a dissolved solid by evaporating then condensing it (for example pure water from salt water).
- Fractional distillation: separates two or more liquids with different boiling points (for example ethanol from water, or the fractions of crude oil).
- Chromatography: separates substances in a mixture (for example dyes in an ink) based on how strongly they are attracted to the paper and how soluble they are in the solvent. A substance that is more soluble and less attracted to the paper travels further up.
Try this
Q1. Describe the arrangement of particles in a gas. [1 mark]
- Cue. Far apart and moving quickly in all directions.
Q2. Name the technique used to separate an insoluble solid from a liquid. [1 mark]
- Cue. Filtration.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksUse the particle model to explain what happens to the particles of a solid as it is heated until it melts and then boils.Show worked answer →
A 4-mark explain question using the particle model.
In a solid the particles are held in fixed positions in a regular arrangement and vibrate (1 mark). As the solid is heated, the particles gain energy and vibrate more, until at the melting point they have enough energy to break free from their fixed positions and the solid melts into a liquid, where particles are close together but can move past each other (1 mark). On further heating, the particles gain more energy and move faster (1 mark), until at the boiling point they have enough energy to overcome the forces between them and escape to form a gas, where particles are far apart and move quickly (1 mark).
Markers reward describing the arrangement and movement at each state and the energy change at melting and boiling.
Edexcel 20214 marksA mixture contains sand and salt. Describe how you could obtain pure, dry samples of both the sand and the salt from this mixture.Show worked answer →
A 4-mark practical question on separating mixtures.
Add water and stir to dissolve the salt; the sand does not dissolve (1 mark). Filter the mixture: the sand stays on the filter paper as the residue, and the salt solution passes through as the filtrate (1 mark). Rinse and dry the sand to obtain a pure, dry sample (1 mark). Evaporate the water from the salt solution (heating to crystallisation) to obtain dry salt crystals (1 mark).
Markers reward dissolving, filtering to separate the insoluble sand, and evaporating or crystallising to recover the salt.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)