How does the particle model explain solids, liquids, gases and changes of state?
The particle model of solids, liquids and gases, density, changes of state as physical changes, and internal energy.
A focused answer to WJEC GCSE Physics topic 1.8 on kinetic theory, covering the particle model of solids, liquids and gases, density and the density equation, changes of state as reversible physical changes, and internal energy.
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What this topic is asking
WJEC wants you to use the particle model to describe solids, liquids and gases, calculate density, and explain changes of state and internal energy. This is part of topic 1.8 Kinetic theory in Unit 1 of WJEC GCSE Physics (3420).
The three states of matter
The forces between particles are strongest in a solid and weakest in a gas, which is why solids hold their shape and gases spread out.
Density
Changes of state
Internal energy
Heating a substance transfers energy to its particles, increasing its internal energy, which is the total kinetic energy (from the motion of the particles) and potential energy (from the forces between them). Raising the temperature increases the kinetic part, making the particles move faster; changing the state changes the potential part, as the bonds between particles are broken or made.
The particle model is a simplified picture, but it explains a great deal. It explains why solids hold their shape (strong forces, fixed positions), why liquids flow and take the shape of a container (particles free to move past one another), and why gases spread out to fill any space (weak forces, fast random motion). It explains diffusion, the way a smell spreads across a room as gas particles mix with the air, and Brownian motion, the random jiggling of tiny smoke particles seen under a microscope as they are knocked about by fast-moving air particles. When the model is asked about in the exam, the marks come from describing the spacing, the arrangement and the movement of the particles, and then linking those three features to the property being explained.
Try this
Q1. Describe the arrangement of particles in a liquid. [2 marks]
- Cue. Close together but able to move past one another; no fixed shape, fixed volume.
Q2. A solid has mass and density . Find its volume. [2 marks]
- Cue. .
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 20193 marksA block of metal has a mass of and a volume of . Calculate its density.Show worked answer →
A topic 1.8 Calculate question on density. Use with mass and volume (1 mark for the equation). Substitute: (2 marks for the calculation and the unit). Markers reward the equation, the substitution and a sensible unit; this density suggests aluminium. A common error is to invert the equation or to mix grams with cubic metres.
WJEC 20224 marksUse the particle model to explain why a gas can be compressed easily but a solid cannot.Show worked answer →
A topic 1.8 Explain question. In a gas the particles are far apart with large spaces between them and move randomly at high speed (1 mark), so there is plenty of empty space for them to be pushed closer together, making the gas easy to compress (1 mark). In a solid the particles are packed closely together in a fixed, regular arrangement and only vibrate about fixed positions (1 mark), so there is almost no space to push them into, making a solid very hard to compress (1 mark). Markers reward the spacing in each state and how that spacing allows or prevents compression. A common error is to say solids have no particles moving at all.
Related dot points
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A focused answer to WJEC GCSE Physics topic 1.8 on kinetic theory, covering gas pressure as the result of particle collisions, the link between temperature and average particle speed, and how the pressure, volume and temperature of a fixed mass of gas are related.
- Specific heat capacity and its equation, specific latent heat of fusion and vaporisation, and heating and cooling curves.
A focused answer to WJEC GCSE Physics topic 1.8 on kinetic theory, covering specific heat capacity and its equation, specific latent heat of fusion and vaporisation, why temperature stays constant during a change of state, and heating curves.
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A focused answer to WJEC GCSE Physics topic 1.3 on making use of energy, covering conduction, convection and radiation, the factors affecting heat loss from a house, insulation methods, and how to compare them using cost-effectiveness and payback time.
- Energy stores and transfers, the conservation of energy, wasted energy, and the calculation of efficiency as the useful output over the total input.
A focused answer to WJEC GCSE Physics topic 1.3 on energy, covering energy stores and transfers, the conservation of energy, energy wasted as heat, and how to calculate efficiency as the useful energy or power output divided by the total input.
Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)