How does the particle model explain the three states of matter and the changes between them?
States of matter: the particle model of solids, liquids and gases, the changes of state and their names, state symbols, and the limitations of the simple particle model.
A focused answer to Edexcel GCSE Chemistry topic 2, covering the particle model of solids, liquids and gases, the arrangement, movement and energy of particles, the names of the changes of state, how to predict the state at a given temperature, and the limitations of the simple particle model.
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What this dot point is asking
Edexcel wants you to use the particle model to describe the arrangement, movement and energy of particles in solids, liquids and gases, name and explain the changes of state, predict the state of a substance at a given temperature from its melting and boiling points, use state symbols, and state the limitations of the simple particle model. This underpins the whole of Paper 1.
The particle model of the three states
The kinetic particle model describes matter as tiny particles in constant motion. The state depends on how much energy the particles have and how strongly they are held together.
This explains the everyday properties: solids have a fixed shape and volume and cannot be compressed; liquids take the shape of their container and have a fixed volume; gases fill their container and can be compressed because of the space between particles.
Changes of state
Changes of state are physical changes: no new substance forms and the change can be reversed. Heating adds energy, which overcomes the forces between particles; cooling removes energy.
- Melting: solid to liquid (at the melting point).
- Freezing: liquid to solid (at the freezing point, the same temperature as melting).
- Boiling and evaporating: liquid to gas (boiling at the boiling point throughout the liquid; evaporation at the surface below the boiling point).
- Condensing: gas to liquid.
- Sublimation: solid straight to gas (and the reverse).
Predicting the state at a given temperature
Compare the temperature with the melting and boiling points:
- Below the melting point, the substance is a solid.
- Between the melting and boiling points, it is a liquid.
- Above the boiling point, it is a gas.
State symbols
State symbols are written after a formula in an equation: (s) solid, (l) liquid, (g) gas, and (aq) for a substance dissolved in water (aqueous). For example, is liquid water and is salt solution.
Limitations of the simple particle model
The simple model treats particles as small, solid, inelastic spheres. Its limitations are that it:
- ignores the forces between particles,
- assumes the particles themselves have no volume (no real size),
- ignores that particles are not all solid spheres but can be atoms, molecules or ions of different shapes.
Try this
Q1. Name the change of state from gas to liquid. [1 mark]
- Cue. Condensing.
Q2. Describe the arrangement and movement of particles in a liquid. [2 marks]
- Cue. Close together but randomly arranged; able to move past one another.
Q3. A substance melts at and boils at . State its state at and explain. [2 marks]
- Cue. Liquid, because is above the melting point but below the boiling point.
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 marksDescribe the arrangement and movement of the particles in a solid and in a gas, and use this to explain why a gas can be compressed but a solid cannot.Show worked answer →
A 4-mark particle-model description.
In a solid the particles are packed closely together in a regular arrangement and vibrate about fixed positions (1 mark). In a gas the particles are far apart with no regular arrangement and move quickly in all directions (1 mark). A gas can be compressed because there is a lot of empty space between the particles, which can be pushed closer together (1 mark). A solid cannot be compressed because its particles are already touching with almost no space between them (1 mark).
Markers reward contrasting the spacing (close versus far apart) and using it to explain compressibility.
Edexcel 20213 marksA substance has a melting point of and a boiling point of . State its state at and explain your answer.Show worked answer →
A 3-mark state-prediction question.
At the substance is a liquid (1 mark). This is because is above the melting point of , so it has melted (1 mark), but below the boiling point of , so it has not yet boiled to a gas (1 mark).
Markers want the temperature compared to both the melting and boiling points, not just one.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)