How does the particle model explain the three states of matter and changes of state?
The particle model of solids, liquids and gases, density, and changes of state as physical changes.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on the particle model, covering the arrangement of particles in solids, liquids and gases, density, and changes of state as physical changes.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 6 wants you to describe the particle model of solids, liquids and gases, explain density, and describe changes of state as physical changes.
The particle model
The hotter a substance, the more kinetic energy the particles have, so they move (or vibrate) faster.
Density and the states
Because particles are closest together in a solid and furthest apart in a gas, solids are usually the most dense and gases the least dense. Density is the mass per unit volume: , measured in kg/m cubed or g/cm cubed. The same mass of gas takes up far more volume than as a solid, so it has a much lower density.
Changes of state
A change of state is a physical change, not a chemical one, because:
- no new substance is made (ice, water and steam are all water),
- only the arrangement and movement of the particles change,
- the change is reversible by heating or cooling.
Naming the changes of state
It is worth knowing all the names and directions. Melting is solid to liquid, and freezing (solidifying) is the reverse, liquid to solid - both happen at the melting point. Boiling/evaporating is liquid to gas, and condensing is the reverse, gas to liquid - boiling happens at the boiling point. Heating gives the particles energy to spread out (solid to liquid to gas), while cooling removes energy so they come closer (gas to liquid to solid). Being able to name each change and say whether it needs heating or cooling is a common recall question.
Mass is conserved in a change of state
When a substance changes state, its mass stays the same, because the same particles are still present - they are just arranged differently. For example, if 10 g of ice melts, you get 10 g of water; if water boils away, the mass of steam equals the mass of water lost. This is because no particles are created or destroyed in a physical change. Recognising that mass is conserved during melting, boiling and freezing is a useful check and a common exam point, linking the particle model to the conservation of mass.
Try this
Q1. In which state are the particles furthest apart? [1 mark]
- Cue. A gas.
Q2. Name the change of state from a gas to a liquid. [1 mark]
- Cue. Condensing (condensation).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the arrangement and movement of the particles in a solid and in a gas.Show worked answer →
A Unit 6 describe question worth 4 marks. Reward: in a solid, the particles are close together in a regular arrangement and vibrate about fixed positions (2); in a gas, the particles are far apart, arranged randomly, and move quickly in all directions (2). Markers credit the spacing, arrangement and movement for each. A common error is to say solid particles do not move at all (they vibrate).
WJEC style3 marksExplain why a substance changing state (such as ice melting) is a physical change, not a chemical change.Show worked answer →
A Unit 6 explain question. Reward: when a substance changes state, only the arrangement and movement of the particles change, not the particles themselves (1); no new substance is made (the water is the same substance as ice and steam) (1); and the change is reversible by heating or cooling (1). Markers credit no new substance, the same particles, and reversibility. A common error is to call melting a chemical change.
Related dot points
- Specific heat capacity and the equation for energy, internal energy, and the energy needed for a change of state.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on heating, covering specific heat capacity and its equation, internal energy, and the energy needed during a change of state when the temperature stays constant.
- Gas pressure in terms of particle collisions, and how pressure changes with temperature and with volume.
A focused answer to the WJEC GCSE Science Double Award Unit 6 topic on gases, covering how gas pressure is caused by particle collisions, and how the pressure of a gas changes with temperature and with volume.
- Heat transfer by conduction, convection and radiation, and methods of reducing energy loss from a house such as insulation.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on reducing energy loss, covering heat transfer by conduction, convection and radiation, and the methods used to reduce energy loss from a house such as loft insulation and double glazing.
- Diffusion, osmosis and active transport as ways substances cross cell membranes, the factors affecting diffusion, the effect of osmosis on cells, and the osmosis required practical.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on movement across membranes, covering diffusion and the factors affecting it, osmosis and its effect on plant and animal cells, active transport and its energy demand, and the osmosis required practical.