Density of materials — AQA GCSE Physics
Test yourself on Density of materials with AQA GCSE practice questions.
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Density of materials explained
The particle model is drawn as circles, and the marks come from arrangement and spacing rather than artistry.
Read the full explanation
In a solid, draw the particles touching, in a regular repeating pattern, filling the box; they vibrate about fixed positions. In a liquid, draw them still touching or nearly touching but irregularly arranged, with a few gaps, so they can slide past one another. In a gas, draw far fewer particles, well separated and randomly placed, moving quickly in all directions. Keep every circle the same size across all three diagrams, because the particles themselves do not shrink or swell when a substance changes state; only their spacing and arrangement change. Use the same number of particles in the solid and liquid boxes so the comparison is fair.
Students should be able to explain the differences in density between the different states of matter in terms of the arrangement of atoms or molecules.
Density is mass per unit volume, ρ = m ÷ V, so it depends on how closely the particles are packed rather than on how heavy each particle is. In a solid the particles touch and are regularly arranged, so a large number fits into a small volume and the density is high. A liquid of the same substance is usually only slightly less dense, because its particles still touch but are disordered with a few extra gaps. A gas is far less dense, often about a thousand times less, because the same number of particles is spread through a much larger volume with large empty spaces between them. Helium of mass 0.00254 kg filling 0.0141 m³ has a density of only 0.18 kg/m³, compared with about 1000 kg/m³ for water.
Required practical activity 5: use appropriate apparatus to make and record the measurements needed to determine the densities of regular and irregular solid objects and liquids. Volume shoul be determined from the dimensions of regularly shaped objects, and by a displacement technique for irregularly shaped objects. Dimensions to be measured using appropriate apparatus such as a ruler, micrometer or Vernier callipers.
Both methods start the same way: measure the mass on a balance and calculate density from ρ = m ÷ V. For a regular solid, measure the sides with a ruler, or with a micrometer or Vernier callipers for small dimensions, and calculate the volume from those dimensions, so for a cube V = side³. For an irregular solid such as a small statue you cannot calculate the volume, so use displacement: fill a eureka can to the spout, lower the object in until it is fully submerged, and collect the displaced water in a measuring cylinder, because the volume of water displaced equals the volume of the object. For a liquid, find the mass of an empty measuring cylinder, add the liquid, reweigh, and subtract.
Your focus
- Draw labelled particle diagrams for a solid, a liquid and a gas with the correct spacing and arrangement in each.
- Identify the state of matter shown by an unlabelled particle diagram and justify the choice.
- Explain why the particles are drawn the same size in all three diagrams.
Show all 9 objectives
- Explain why a gas has a much lower density than the same substance as a solid, using particle spacing.
- Calculate a density from a mass in kilograms and a volume in cubic metres and give the correct unit.
- Compare the densities of the solid and liquid forms of one substance and account for the small difference.
- Describe, in a logical sequence, a method for finding the density of a regularly shaped solid.
- Describe how to find the volume of an irregular object by displacement and say why the two volumes are equal.
- Calculate a density from measured mass and volume data and give the answer in a correct unit.
Density of materials exam tips
Marking Points
- one mark for solid particles drawn touching in a regular arrangement
- one mark for liquid particles drawn touching or close but irregularly arranged
- one mark for gas particles drawn far apart and randomly arranged
- one mark for particles drawn the same size in all three states
- one mark for density stated as mass per unit volume, ρ = m ÷ V
- one mark for solid particles being close together in a regular arrangement, giving a high density
- one mark for gas particles being far apart, so the same mass occupies a much larger volume
- one mark for a liquid being close in density to the solid because its particles still touch
- marked by levels out of 6, not by separate points: Level 1 = 1-2 marks, Level 2 = 3-4 marks, Level 3 = 5-6 marks for a method that would give a valid outcome with all key steps identified and logically sequenced
- credited within the level: measure the mass of the object on a balance
- credited within the level: find the volume of a regular object from its measured dimensions, using a ruler, a micrometer or Vernier callipers as the size requires
- credited within the level: lower an irregular object until it is fully submerged in a displacement can and collect the water that overflows in a measuring cylinder
- credited within the level: the volume of water displaced equals the volume of the object
- credited within the level: calculate the density from ρ = m ÷ V
Examiner Tips
- 💡Draw particles as identical circles and keep the same circle size in all three boxes.
- 💡Add a short label under each diagram naming the state, so no mark hangs on the drawing alone.
- 💡Use the same number of particles in the solid and liquid boxes to show that only the spacing changed.
- 💡Always answer in terms of the number of particles in a given volume, not the size of the particles.
- 💡Choose the unit carefully in multiple-choice unit questions, since density is mass divided by volume, so kg/m³.
- 💡Quote a real comparison, such as air against water, to show the scale of the difference.
- 💡Six-mark method answers are marked by levels, so put the steps in a logical order another person could follow.
- 💡Name the instrument for every measurement: balance, ruler or callipers, measuring cylinder.
- 💡Finish the method with the equation you would use, because the calculation step is part of the top level description.
Common Mistakes
- drawing gas particles larger than solid particles to show that they have expanded
- drawing a solid with gaps between its particles
- drawing a liquid identical to a gas, with widely separated particles
- describing the arrangement in words while leaving the drawing itself ambiguous
- saying a gas is less dense because its particles are lighter or smaller
- claiming a gas has less mass than the solid it came from, when mass is conserved on a change of state
- giving the unit as m³/kg instead of kg/m³
- stating that a liquid is always exactly halfway in density between the solid and the gas
- weighing the liquid in its container and forgetting to subtract the mass of the empty container
- using a ruler on an irregular object and calling the result its volume
- not submerging the object completely, so the volume displaced is too small
- mixing grams with cubic metres and reporting a density that is wrong by a factor of a thousand