Free GCSE Physics lesson: Particles and Density
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Free Lessons -> GCSE / Key Stage 4 -> Physics -> Particles and Density

Lesson 5 · GCSE / Key Stage 4 · Physics

Particles, density and internal energy

Use the particle model to explain density, changes of state, internal energy and heating.

Qualification: GCSE Subject: Physics Particles

Particle model

Lesson overview

This lesson introduces the core physics idea, the useful equipment and the calculation or data skills used on this page.

FocusParticle model, density and internal energy
Time45-60 minutes
EquipmentCalculator, ruler and practical notes on measuring mass and volume.
Practical linkNo separate practical method focus
Maths tagsM1 substitution with units

What you will learn

  • Use density = mass divided by volume.
  • Explain solids, liquids and gases using particle arrangement and motion.
  • Distinguish temperature from internal energy.
  • Explain changes of state without saying particles disappear.

Material samples supplied on this page

Use the cube, liquid and heating examples below to practise density calculations and particle explanations.

Particle model and states of matter infographic

Infographic explaining the particle model and states of matter, including density, internal energy, heating, changes of state and gas pressure links.
Use this visual to connect particle arrangement with energy and density.Download visual

Clear explanation

Density tells you how much mass is packed into each unit volume. The same material has the same density even when the sample is larger or smaller.

The particle model explains states of matter. In solids, particles vibrate around fixed positions. In liquids, particles stay close but can move past each other. In gases, particles are far apart and move randomly.

Heating changes the internal energy of a substance. During a change of state, energy changes the arrangement of particles rather than raising the temperature.

Worked examples

Calculating density

A metal cube has mass 216 g and volume 27 cm3.

density = mass ÷ volume

density = 216 ÷ 27 = 8

Answer: The density is 8 g/cm3.

Quick checks

Choose an answer, then check your thinking.

1. A sample has mass 60 g and volume 20 cm3. What is its density?

2. During melting, why can temperature stay constant?

Practice questions

Question 1

A liquid has mass 160 g and volume 200 cm3. Calculate its density.

Reveal answer and marking guidance

Answer: 0.8 g/cm3.

Marking: Credit density = mass ÷ volume and 160 ÷ 200 = 0.8 g/cm3.

Question 2

A block has density 2.5 g/cm3 and volume 40 cm3. Calculate its mass.

Reveal answer and marking guidance

Answer: 100 g.

Marking: Credit mass = density x volume and 2.5 x 40 = 100 g.

Question 3

Explain why gases are easy to compress compared with liquids.

Reveal answer and marking guidance

Answer: Gas particles are much further apart, so there is space for them to be pushed closer together.

Marking: Credit particle spacing and compression; do not credit particles shrinking.

Question 4

What changes in the particles when a solid is heated but does not melt?

Reveal answer and marking guidance

Answer: The particles vibrate more and the internal energy increases.

Marking: Credit increased vibration and internal energy without a change of state.

Practice ladder

FluencyRecall the key definition, unit, equation or model before using the lesson questions.
ApplicationApply particle model, density and internal energy to an unfamiliar device, practical setup or data description.
Practical interpretationUse evidence, graph features, uncertainty, method quality or conclusion wording where the question asks you to evaluate.
Maths skillM1 substitution with units

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. For this lesson, marks come from using the correct physics model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as transfer, resultant force, acceleration, evidence and uncertainty.

Common mistakes

  • Multiplying mass by volume when asked for density.
  • Forgetting density units such as g/cm3 or kg/m3.
  • Saying particles expand when a substance expands.
  • Confusing temperature with total internal energy.

Extension challenge

Compare two objects with the same mass but different volumes. Explain which has greater density and why.

Reveal answer

Example answer: A strong extension response names the physics model, uses accurate units and explains why the evidence supports the conclusion.

Exam-board guidance

Short board notes only. Learn the core physics above first.

AQA GCSE Physics

AQA GCSE Physics: often rewards clear physics explanations, correct equations, units and practical evidence for particle model, density and internal energy.

OCR GCSE Physics

OCR GCSE Physics: often values precise definitions, clear working, graph interpretation and links between models and evidence.

Pearson Edexcel GCSE Physics

Pearson Edexcel GCSE Physics: often combines the concept with equation use, data handling and practical interpretation.

Eduqas GCSE Physics

Eduqas GCSE Physics: learn the core explanation and practise applying it to unfamiliar contexts, data and practical questions.

WJEC Wales

WJEC Wales: often expects accurate terms, units and evidence-based explanations using the shared physics idea.

CCEA GCSE Physics

CCEA GCSE Physics: connect the idea to your current unit and use the same practical method language your class uses.

Next lesson

Next, continue with Waves and Radiation.

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