Revision notes for AQA GCSE Chemistry The three states of matter. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
The three states of matter
What you'll learn
- How to describe solids, liquids and gases using the particle model.
- What melting, freezing, boiling and condensing mean.
- How to predict the state of a substance from its melting point, boiling point and temperature.
- Why stronger forces between particles lead to higher melting and boiling points.
Starting point: substances are made of particles
In GCSE Chemistry, a substance is a material with a particular chemical identity, such as water, sodium chloride or oxygen.
A particle is a tiny unit of matter. Depending on the substance, the particles might be atoms, molecules or ions. You will meet these in more detail across bonding and structure, but for this topic the key idea is simple: we explain the behaviour of substances by thinking about how their particles are arranged and how they move.
Definition
State of matter
A state of matter is the physical form a substance is in: solid, liquid or gas.
A substance can change state when energy is transferred to it or from it. For example, water can be solid ice, liquid water or gaseous steam.
The simple particle model
Chemists often use a model: a simplified picture or explanation that helps us understand something complicated. In the simple particle model, particles are shown as small solid spheres.
This diagram shows the GCSE particle model for the three states and the main changes of state.
Solids
In a solid:
- particles are very close together
- particles are arranged in a regular pattern
- particles vibrate about fixed positions
- the solid keeps its shape and has a fixed volume
The particles are not still — they are vibrating all the time.
Liquids
In a liquid:
- particles are close together
- particles are arranged randomly
- particles can move past each other
- the liquid has a fixed volume but takes the shape of its container
This is why a liquid can flow.
Gases
In a gas:
- particles are far apart
- particles are arranged randomly
- particles move quickly in all directions
- the gas fills the whole container
Gas particles have much more space between them than particles in solids or liquids.
Key Idea
Particle model summary
As you go from solid → liquid → gas, particles generally become less regularly arranged, further apart, and more free to move.
Common Mistake
Particles in solids are not stationary
Do not write that particles in a solid “do not move”. They vibrate about fixed positions.
Changes of state
A change of state happens when a substance changes between solid, liquid and gas. It is a physical change, not a chemical reaction.
That means the particles themselves do not change into different particles. For example, when ice melts, water molecules are still water molecules.
Definition
Melting point and boiling point
The melting point is the temperature at which a solid changes to a liquid. The boiling point is the temperature at which a liquid changes to a gas.
The key changes are:
- melting: solid → liquid
- freezing: liquid → solid
- boiling: liquid → gas
- condensing: gas → liquid
Melting and freezing happen at the melting point. Boiling and condensing happen at the boiling point.
Energy and movement during changes of state
When you heat a substance, energy is transferred to its particles. This usually makes them move more, but during a change of state the energy is used to overcome some of the forces holding particles together.
When you cool a substance, energy is transferred away from its particles. The particles move less and can become held more closely together.
Example
Explaining melting using particle theory
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Energy is transferred to the solid, so its particles vibrate more strongly about their fixed positions.
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At the melting point, the particles have enough energy to overcome some of the forces holding them in the regular solid arrangement.
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The particles are still close together, but they can now move past each other, so the substance has become a liquid.
Common Mistake
Particles do not expand
When a substance is heated, do not say “the particles expand”. The particles stay the same size. Usually, the spaces between particles increase and the particles move more.
Predicting the state from temperature data
You may be given a melting point, a boiling point and a temperature, then asked to predict the state of the substance.
Use this rule:
- below the melting point: solid
- between the melting point and boiling point: liquid
- above the boiling point: gas
This number line is a useful way to visualise the decision.
Common Mistake
At the exact melting or boiling point
At the exact melting point, solid and liquid can both be present while melting or freezing happens. At the exact boiling point, liquid and gas can both be present while boiling or condensing happens.
Example
Predicting states from melting and boiling points
A substance has a melting point of -114 °C and a boiling point of 78 °C. Predict its state at -120 °C, 25 °C and 100 °C.
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Compare -120 °C with the melting point. Since -120 °C is below -114 °C, the substance is a solid.
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Compare 25 °C with both points. Since 25 °C is above the melting point but below the boiling point, the substance is a liquid.
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Compare 100 °C with the boiling point. Since 100 °C is above 78 °C, the substance is a gas.
Tip
Quick method
Draw a small temperature line: melting point first, boiling point second. Then place the given temperature on the line and read off solid, liquid or gas.
Why different substances change state at different temperatures
Different substances have different melting points and boiling points because their particles are held together by different strengths of forces.
A force between particles is an attraction that holds particles near each other. The stronger these forces are, the more energy is needed to separate the particles enough for a change of state.
Key Idea
Forces and energy
The stronger the forces between particles, the higher the melting point and boiling point of the substance.
This links directly to bonding and structure:
- In some substances, the particles are molecules with relatively weak forces between molecules.
- In ionic substances, there are many oppositely charged ions held in a giant lattice by strong electrostatic attractions.
- In giant covalent substances, many atoms are joined by strong covalent bonds throughout a huge structure.
The “particles” in the simple model can therefore represent different things depending on the substance: atoms, molecules or ions.
Example
Explaining different melting points
Sodium chloride has a much higher melting point than chlorine.
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Sodium chloride has a giant ionic structure, so there are strong attractions between oppositely charged ions throughout the lattice.
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Chlorine is made of small molecules, so when chlorine melts, only the weak forces between molecules need to be overcome.
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Because sodium chloride has much stronger forces between its particles, much more energy is needed to melt it, so it has a much higher melting point.
Common Mistake
Breaking the wrong forces
For simple molecular substances, melting and boiling overcome forces between molecules. They do not usually break the covalent bonds inside the molecules.
Bulk properties: one particle is not a material
A bulk property is a property of a large sample of a substance, such as melting point, boiling point, hardness, density or electrical conductivity.
A single atom does not have the bulk properties of the material. For example, one copper atom is not “shiny” or “malleable” in the everyday sense. Those properties come from many copper atoms arranged and bonded together.
Key Idea
Particles vs materials
Atoms, molecules and ions do not individually have all the properties of the substance. Many particles together, with a particular structure and bonding, produce the bulk properties.
Limitations of the simple particle model
The simple particle model is very useful, but it is not perfect. If you are sitting Higher Tier, you should be able to explain its limitations.
In the model, particles are shown as solid spheres. This is a simplification. Real particles are not tiny hard balls: atoms have structure, molecules can have different shapes, and ions have charge.
The model also does not show the forces between particles. That is a big limitation because melting points and boiling points depend on the strength of these forces.
Another limitation is that the diagrams are flat, but real substances are three-dimensional. A GCSE particle diagram is a simplified two-dimensional representation of a 3D arrangement.
Tip
Higher Tier wording
A strong Higher Tier answer might say: “The simple particle model is limited because it represents particles as solid spheres with no forces between them, so it cannot fully explain why different substances have different melting and boiling points.”
Exam technique
In the exam
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For particle model questions, describe arrangement, spacing and movement of particles.
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For state prediction questions, compare the temperature with the melting point and boiling point, and remember the exact-point edge case.
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For melting and boiling point explanations, link the answer to energy needed to overcome forces between particles — stronger forces mean higher melting and boiling points.
Self review
Check yourself
- How are the particles arranged and moving in a liquid compared with a gas?
- A substance melts at 10 °C and boils at 90 °C. What state is it at 25 °C?
- Why does a substance with stronger forces between particles usually have a higher boiling point?
How bonding and structure are related to the properties of substances
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