Density is mass per volume
Dense materials pack more mass into each unit of volume. Use mass and volume measurements with consistent units, and explain changes in density using particle spacing rather than saying particles themselves become heavier.
GCSE Physics resources
AQA GCSE Particle model of matter
Density and Particle Arrangement
Dense materials pack more mass into each unit of volume. Use mass and volume measurements with consistent units, and explain changes in density using particle spacing rather than saying particles themselves become heavier.
Start here
The revision snapshot
Dense materials pack more mass into each unit of volume. Use mass and volume measurements with consistent units, and explain changes in density using particle spacing rather than saying particles themselves become heavier.
By the end, you should be able to:
- Explain why density is mass per volume.
- Apply the particle model of matter model, evidence or method to an unfamiliar exam context.
- Recognise and correct this wrong turn: Mixing g/cm³ and kg/m³.
Before you revise
Diagnostic question
Choose an answer from memory. Your result tells you what to watch for in the guide.
Core revision guide
Learn the model, then use it
Density and Particle Arrangement questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.
Build the physics picture
Density compares how much mass is packed into a volume. Heating usually makes particles move further apart; the particles do not grow. For a fixed mass, a larger volume gives a lower density, which explains why warm fluids can rise.
The relationship for this guide
Density: ρ = m ÷ V (kg/m³, kg, m³). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.
Misconception clinic
Replace the tempting answer
Read each belief, say what is wrong with it, then compare your correction.
Tempting idea: Mixing g/cm³ and kg/m³.
Use instead: Dense materials pack more mass into each unit of volume. Use mass and volume measurements with consistent units, and explain changes in density using particle spacing rather than saying particles themselves become heavier.
Tempting idea: Using final water volume instead of the rise.
Use instead: Density compares how much mass is packed into a volume. Heating usually makes particles move further apart; the particles do not grow. For a fixed mass, a larger volume gives a lower density, which explains why warm fluids can rise.
Tempting idea: Saying particles expand.
Use instead: Choose a direct or displacement volume measurement, calculate ρ = m/V and compare values using the same units.
Retrieval practice
Quick checks
Worked examples
See the method being built
These examples expose the difference between a secure density is mass per volume method and the common error “Mixing g/cm³ and kg/m³.”.
Example 1 · Regular object
A block has mass 0.54 kg and dimensions 0.030 m × 0.020 m × 0.010 m. Calculate density.
- Find volume: 0.030 × 0.020 × 0.010 = 6.0 × 10⁻⁶ m³.
- Use ρ = m/V.
- Give kg/m³.
Show the answer
ρ = 0.54 ÷ 6.0 × 10⁻⁶ = 9.0 × 10⁴ kg/m³.
Example 2 · Displacement volume
Water rises from 40 cm³ to 65 cm³ when an irregular stone is submerged. Its mass is 75 g. Find density.
- Volume = 65 − 40 = 25 cm³.
- Use ρ = m/V.
- Keep g and cm³ together.
Show the answer
ρ = 75 ÷ 25 = 3.0 g/cm³.
Example 3 · Particle explanation
Explain why heating a liquid usually lowers its density.
- Keep mass fixed.
- State what heating changes.
- Link larger volume to density.
Show the answer
Particles move more and are on average farther apart, increasing volume. The same mass in a larger volume has lower density.
Exam precision
Exam technique: Density is mass per volume
Do this: Choose a direct or displacement volume measurement, calculate ρ = m/V and compare values using the same units.
Independent practice
Exam-style questions
Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.
1. State the SI unit of density.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: kg/m³.
2. A 200 g sample occupies 80 cm³. Calculate density in g/cm³.
[3 marks]Show marking guidance and model answer
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: ρ = 200 ÷ 80 = 2.5 g/cm³.
3. Describe how to find the volume of an irregular solid.
[3 marks]Show marking guidance and model answer
Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.
Model answer: Submerge it in water and use the increase in volume, ensuring no air bubbles are trapped.
4. Explain why particles do not themselves get larger when heated.
[3 marks]Show marking guidance and model answer
Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.
Model answer: Heating increases their motion and average separation; it does not create larger particles.
5. A mass is unchanged but volume doubles. State what happens to density.
[2 marks]Show marking guidance and model answer
Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.
Model answer: Density halves because ρ = m/V.
6. Give one way to reduce uncertainty in a displacement measurement.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: Read the bottom of the meniscus at eye level and use a measuring cylinder with finer divisions.
Questions pupils ask
Density and Particle Arrangement FAQs
What is the main idea in Density and Particle Arrangement?
Dense materials pack more mass into each unit of volume. Use mass and volume measurements with consistent units, and explain changes in density using particle spacing rather than saying particles themselves become heavier.
What mistake should I avoid in Density and Particle Arrangement?
Mixing g/cm³ and kg/m³. Density compares how much mass is packed into a volume. Heating usually makes particles move further apart; the particles do not grow. For a fixed mass, a larger volume gives a lower density, which explains why warm fluids can rise.
Useful next steps
Connect this guide to your next task
Density Required PracticalMeasure mass with a balance. For a regular object, calculate volume from dimensions; for an irregular object, use water displacement. Read scales at eye level and remove trapped air bubbles.Internal Energy and Heating CurvesInternal energy is the total kinetic and potential energy of the particles. Sloped sections of a heating curve show increasing average kinetic energy; flat sections show a change of state while separation changes.How to Answer GCSE Physics Calculation QuestionsA reliable calculation answer is a short chain of decisions. Make the physics visible so a marker can award method marks even if arithmetic slips.
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