The three states of matter — AQA GCSE Combined Science
Test yourself on The three states of matter with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
The three states of matter explained
Matter exists as solid, liquid or gas depending on the energy of its particles and the strength of the forces between them.
Read the full explanation
In a solid, particles are closely packed in a regular arrangement and can only vibrate. In a liquid, particles are close but can move past one another. In a gas, particles are far apart and move rapidly in all directions. Melting is the change from solid to liquid and occurs at the melting point; freezing is the reverse change, liquid to solid, at the same temperature. Boiling is the change from liquid to gas and occurs at the boiling point; condensing is the reverse change, gas to liquid, at the same temperature. For example, ice melts at 0 °C and water boils at 100 °C at standard atmospheric pressure. These temperatures are fixed for a pure substance at a given pressure.
The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres. Particle theory can help to explain melting, boiling, freezing and condensing.
The simple particle model pictures a substance as tiny solid spheres. In a solid the spheres are close together in a regular arrangement and vibrate about fixed positions, so the shape and volume stay fixed. In a liquid they are still close but randomly arranged and can slide past one another, giving a fixed volume but the shape of the container. In a gas they are far apart, moving quickly and randomly, filling any container. Heating transfers energy to the particles, so they vibrate or move faster. At melting the regular solid arrangement breaks down to a liquid; at boiling particles escape throughout the liquid to form a gas; freezing and condensing are the reverse changes as energy is transferred away. For example, ice melts at 0 °C and water boils at 100 °C at normal atmospheric pressure.
The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance. The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles the higher the melting point and boiling point of the substance.
Changing state requires energy to overcome the forces between particles, not to break the particles themselves. Melting needs enough energy to weaken the regular solid arrangement so particles can slide past one another; boiling needs more energy to separate particles completely into a gas. The energy required therefore depends on how strong those forces are. The forces depend on the type of bonding and the structure: a giant ionic lattice such as sodium chloride has strong electrostatic forces between oppositely charged ions, giving a high melting point, while simple molecular substances such as iodine have weak forces between molecules, giving low melting and boiling points. In general, the stronger the forces between particles, the more energy is needed to change state, so the higher the melting point and boiling point.
(HT only) Limitations of the simple model above include that in the model there are no forces, that all particles are represented as spheres and that the spheres are solid.
The particle model pictures matter as tiny spheres arranged in solids, liquids and gases. It explains melting, boiling, diffusion and gas pressure, but it is a simplification. Higher tier students must state its limitations: the model includes no forces between particles, so it cannot explain why particles attract or repel or why energy is needed to melt or boil beyond breaking arrangements; every particle is drawn as a sphere, although real particles such as atoms, ions and molecules have different shapes; and the spheres are solid, so the model ignores internal structure and empty space inside atoms. For example, drawing water molecules as identical solid balls cannot show hydrogen bonding or the bent shape of H₂O.
predict the states of substances at different temperatures given appropriate data
Every substance has a melting point and a boiling point at a stated pressure. Given data, compare the temperature with these values. Below the melting point the substance is solid; at or above the melting point but below the boiling point it is liquid; at or above the boiling point it is gas. For example, if a metal melts at 660 °C and boils at 2470 °C, then at 25 °C it is solid, at 1000 °C it is liquid and at 3000 °C it is gas. At exactly the melting point, solid and liquid coexist; at exactly the boiling point, liquid and gas coexist. Pressure matters, so check whether the data apply to the pressure in the question.
explain the different temperatures at which changes of state occur in terms of energy transfers and types of bonding
Melting and boiling temperatures differ between substances because the energy needed to overcome the forces between particles differs. In a simple molecular substance, such as iodine, the molecules are held together by weak intermolecular forces, so only a small energy transfer is needed to separate them; melting and boiling points are low. In a giant ionic lattice, such as sodium chloride, strong electrostatic forces between oppositely charged ions must be overcome, so the melting point is high. In a giant covalent structure, such as diamond, many strong covalent bonds must be broken, giving a very high melting point. Metals have strong metallic bonding and generally melt at high temperatures. Energy transferred during heating increases the kinetic energy of particles until the forces can no longer hold them in place.
recognise that atoms themselves do not have the bulk properties of materials
Bulk properties are the large-scale behaviours we observe, such as melting point, boiling point, density, conductivity and solubility. They emerge from how enormous numbers of atoms or molecules are arranged and how they interact, not from a single atom. One isolated atom has no melting point, no boiling point and no fixed density because these ideas only make sense for a collection of particles. For example, a single copper atom cannot conduct electricity through a wire, yet a copper wire does because delocalised electrons move through the whole lattice. Similarly, one water molecule does not boil at 100 °C; boiling describes molecules escaping from a bulk liquid. When you explain a material's properties, refer to the particles, their arrangement, and the forces or bonds between them, rather than saying the atoms themselves are hard, shiny or dense.
(HT only) explain the limitations of the particle theory in relation to changes of state when particles are represented by solid inelastic spheres which have no forces between them.
The simple particle model represents particles as solid, inelastic spheres with no forces between them. While useful for visualising states of matter, Higher Tier students must explain its limitations during changes of state. Real particles are not solid spheres; they are atoms, ions, or molecules with internal structures that can deform. Furthermore, the model assumes no intermolecular forces. In reality, attractive and repulsive forces exist between particles. Because the model omits these forces, it cannot explain why different substances require different amounts of energy to melt or boil, nor can it explain varying melting and boiling points, such as why ionic compounds have higher melting points than simple molecules.
Your focus
- Name the three states of matter and describe the arrangement and motion of particles in each.
- Identify melting, freezing, boiling and condensing as changes of state and state the temperature at which each occurs.
- Explain why temperature remains constant during a change of state.
Show all 24 objectives
- Describe the arrangement and motion of particles in solids, liquids and gases using the simple particle model.
- Explain melting, boiling, freezing and condensing in terms of energy transfer and changes in particle arrangement and motion.
- Evaluate the simple particle model by identifying what it represents well and what it does not show.
- Explain why energy is needed to change state in terms of overcoming forces between particles.
- Relate the strength of forces between particles to the melting point and boiling point of a substance.
- Compare the melting and boiling points of substances with different bonding and structures using the particle model.
- State the three stated limitations of the simple particle model.
- Explain how each limitation reduces the model's explanatory power.
- Apply the limitations to a named substance or observed property.
- Locate the melting point and boiling point of a substance in given data.
- Compare a stated temperature with those fixed points to assign the correct state.
- Describe what is present at a temperature exactly equal to a melting point or boiling point.
- Describe how heating transfers energy to particles and increases their kinetic energy.
- Relate the strength of forces between particles to the temperature at which a substance changes state.
- Compare the melting or boiling points of substances with different types of bonding or structure.
- Identify that bulk properties such as melting point and density apply to a material made of many atoms.
- Describe how particle arrangement and forces between particles produce a named bulk property.
- Use a specific example to contrast the behaviour of a single atom with the behaviour of a bulk material.
- Explain that the simple particle model represents particles as solid, inelastic spheres and omits forces between them.
- Relate the model’s omitted forces and idealised particles to its limitations when explaining changes of state.
- Evaluate which features of changes of state the simple particle model can represent and which it cannot explain.
The three states of matter exam tips
Marking Points
- The three states of matter are solid, liquid and gas, distinguished by the arrangement, separation and motion of their particles.
- Melting is the change of state from solid to liquid, and freezing is the change from liquid to solid; both occur at the melting point of a pure substance.
- Boiling is the change of state from liquid to gas, and condensing is the change from gas to liquid; both occur at the boiling point of a pure substance.
- The temperature remains constant during a change of state because the energy supplied is used to overcome forces between particles rather than to increase kinetic energy.
- Impurities or changes in pressure can alter the melting and boiling points of a substance.
- States the arrangement and motion of particles in each of solid, liquid and gas, linking closeness of particles to the volume and shape observed.
- Uses the model to explain melting as the breakdown of the regular solid arrangement when particles gain enough energy to move past one another.
- Uses the model to explain boiling as particles throughout the liquid gaining enough energy to escape and form a gas.
- Explains freezing and condensing as the reverse changes in which particles lose energy and become more closely and regularly arranged.
- Recognises that the spheres are a simple representation and that the model does not show the true nature of the particles.
- States that energy is needed to overcome the forces between particles when a substance melts or boils, not to break the particles themselves.
- Links the strength of the forces between particles to the amount of energy required for a change of state.
- Explains that the nature of the particles and the forces between them depend on the type of bonding and the structure of the substance.
- Uses the relationship that stronger forces between particles give higher melting and boiling points, and applies it to a named example.
- Distinguishes between energy needed to overcome forces between particles and energy needed to break chemical bonds within particles.
- States that the simple model has no forces between particles, so it cannot explain attractions, repulsions or the energy changes on melting and boiling.
- Explains that representing all particles as spheres ignores the different shapes of atoms, ions and molecules, such as the bent H₂O molecule.
- Explains that treating spheres as solid ignores internal structure and the empty space within atoms.
- Links each limitation to a property the model cannot explain, for example diffusion rates, gas pressure or changes of state.
- Uses the model's success in explaining the three states alongside its limitations, showing awareness that models are provisional.
- Applies the limitations to a named substance rather than describing them only in the abstract.
- Identify the melting point and boiling point of the substance from the data provided.
- Compare the given temperature with the melting point to decide whether the substance is solid or has melted.
- Compare the given temperature with the boiling point to decide whether the substance is liquid or has boiled.
- State the predicted state clearly, using the correct term solid, liquid or gas.
- Recognise that at the melting point solid and liquid are both present, and at the boiling point liquid and gas are both present.
- Check that the pressure stated in the data matches the pressure in the question before predicting.
- Link the temperature of a change of state to the energy transfer needed to overcome the forces between particles.
- Identify the type of bonding or structure present, for example simple molecular, giant ionic, giant covalent or metallic.
- Explain that weak intermolecular forces in simple molecular substances require less energy to overcome, giving lower melting and boiling points.
- Explain that strong electrostatic forces in giant ionic lattices or strong covalent bonds in giant covalent structures require more energy, giving higher melting and boiling points.
- Use the idea that heating transfers energy to particles, increasing their kinetic energy until the forces holding them in the solid or liquid arrangement are overcome.
- Compare two named substances and relate the difference in their melting or boiling points to the difference in bonding and forces.
- State that bulk properties belong to a material made of many atoms, not to one atom on its own.
- Give a named bulk property such as melting point, boiling point, density, conductivity or solubility.
- Explain that the property depends on the arrangement of particles and the forces or bonds between them.
- Use a specific example, such as copper conducting because of delocalised electrons throughout the lattice, or water boiling because many molecules escape together.
- Avoid describing an individual atom as having a property such as shiny, hard, dense or a fixed melting point.
- State that the simple model incorrectly assumes particles are solid, inelastic spheres.
- Identify that real particles (atoms, molecules, ions) have internal structures and are not perfectly spherical or inelastic.
- State that the model incorrectly assumes there are no forces between particles.
- Explain that because real particles have intermolecular forces, the model cannot account for the specific energy required to overcome these forces during melting or boiling.
- Explain that the absence of forces in the model means it cannot explain why different substances have different melting and boiling points.
Examiner Tips
- 💡When describing changes of state, name both the starting and finishing states, for example 'solid to liquid' for melting.
- 💡Use the correct terms 'melting point' and 'boiling point' rather than vague phrases like 'melting temperature'.
- 💡If a question asks about the effect of impurities, state that impurities lower the melting point and raise the boiling point.
- 💡When asked to explain a change of state, name the state change, describe the change in particle arrangement and motion, then link this to the energy transferred.
- 💡Use the words arrangement, energy and forces when describing particle behaviour, as these are the ideas the model is used to explain.
- 💡If a question asks about the limitations of the model, mention that the spheres do not show the forces between particles or the true scale of the particles.
- 💡When comparing melting points, name the substance, state the type of bonding and structure, then link the strength of the forces to the energy needed and the melting point.
- 💡Use comparative language such as stronger forces, more energy and higher melting point to make the relationship explicit.
- 💡If a question gives data, quote the values and use them to support the comparison rather than describing the trend in general terms only.
- 💡Read the command word: 'describe' needs the limitations stated, while 'explain' needs each limitation linked to an observation it cannot account for.
- 💡Use the phrase 'the model assumes' to signal that you are discussing a simplification rather than a fact.
- 💡Give one concrete substance, such as water or carbon dioxide, to make each limitation specific and creditworthy.
- 💡Remember that evaluating the limitations of the simple particle model is assessed at Higher Tier only.
- 💡Underline the melting point and boiling point in the data before you start.
- 💡Write the comparison as an inequality, for example 25 °C < 660 °C, so your reasoning is visible.
- 💡If the question gives several temperatures, answer each one separately and name the state for each.
- 💡Name the structure and bonding type first, then link it to the energy needed to overcome the forces.
- 💡Use comparative language such as weaker or stronger forces to make the explanation explicit.
- 💡If asked to compare, give a sentence for each substance and a sentence linking the difference to bonding.
- 💡Read the question carefully: if it asks about a material or a sample, answer in terms of many particles, not one atom.
- 💡When asked to explain a property, name the property, then link it to particle arrangement and the forces or bonds present.
- 💡Use a quick contrast in your answer, such as 'one copper atom cannot conduct, but a copper wire can because delocalised electrons move through the lattice'.
- 💡As this is a Higher Tier only concept, expect questions to ask you to evaluate the model or explain why it fails to predict specific melting points.
- 💡Always link the limitation directly to a physical property; for example, link 'no forces between particles' to the inability to explain latent heat or varying boiling points.
Common Mistakes
- Confusing boiling with evaporation. Correction: boiling occurs throughout the liquid at a specific boiling point, while evaporation occurs at the surface at temperatures below the boiling point.
- Thinking that temperature increases during melting or boiling. Correction: the temperature stays constant during a change of state for a pure substance.
- Mixing up melting point and boiling point. Correction: melting point is the temperature at which a solid changes to a liquid; boiling point is the temperature at which a liquid changes to a gas.
- Saying particles in a solid do not move at all; correct this by stating that they vibrate about fixed positions.
- Saying particles expand or get bigger on heating; correct this by stating that the particles themselves do not change size, they move further apart or faster.
- Confusing boiling with evaporation; correct this by stating that boiling happens throughout the liquid at a specific temperature, whereas evaporation happens at the surface over a range of temperatures.
- Saying that melting breaks the particles apart; correct this by stating that the particles stay intact and the forces between them are overcome.
- Saying that all substances with strong bonding have high melting points without considering structure; correct this by linking the forces to the arrangement, for example simple molecular substances have weak forces between molecules.
- Confusing the energy needed to melt with the energy needed to boil; correct this by stating that boiling requires more energy because particles must be completely separated.
- Saying the model has no particles: the error is confusing 'no forces' with 'no particles'; the correction is that particles are present but forces between them are omitted.
- Claiming the spheres are solid because particles are hard: the error is treating a modelling choice as a fact; the correction is that real particles have internal structure and empty space.
- Stating that the model is wrong: the error is rejecting a useful model; the correction is that it is limited, not useless, and still explains many observations.
- Using the wrong inequality: a temperature below the melting point is solid, not liquid. Correct by writing the order solid < melting point ≤ liquid < boiling point ≤ gas.
- Ignoring the pressure at which the melting and boiling points were measured. Correct by checking the data label and only predicting for that pressure.
- Treating a temperature exactly equal to the melting point as only liquid. Correct by stating that solid and liquid coexist at that temperature.
- Saying that covalent bonds break when a simple molecular substance melts. Correct by stating that intermolecular forces between molecules are overcome, not the covalent bonds within molecules.
- Confusing the strength of bonding with the amount of bonding. Correct by referring to the strength of the forces or bonds that must be overcome, not simply how many there are.
- Describing ionic bonding as sharing electrons. Correct by stating that ionic bonding involves electrostatic attraction between oppositely charged ions such as Na⁺ and Cl⁻.
- Saying a single atom is hard or shiny: correct this by explaining that hardness and shininess are bulk properties arising from many atoms and their bonding.
- Claiming one atom has a melting point: correct this by stating that melting point describes a bulk sample changing state, not an isolated particle.
- Confusing the properties of an element with the properties of one atom: correct this by linking the material's behaviour to the particle arrangement and intermolecular or metallic bonding.
- Stating the model is completely useless: correct this by acknowledging it is a useful simplification for basic states of matter, but specify its limitations regarding forces and particle structure.
- Confusing intermolecular forces with the bonds inside molecules: correct this by specifying that the model fails to show the forces between particles (intermolecular forces), not the covalent bonds within them.
- Forgetting to mention the 'inelastic spheres' limitation: correct this by explicitly stating that real particles can vibrate, deform, and have complex shapes, rather than being rigid, solid spheres.