AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Density of materials: 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. In a solid, particles are closely packed in a regular arrangement, so a given volume contains many particles and density is high.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Density is mass per unit volume, ρ = m ÷ V. In a solid, particles are closely packed in a regular arrangement, so a given volume contains many particles and density is high. In a liquid, particles remain close but are arranged randomly and can slide past one another, so density is slightly lower than the solid but still much greater than a gas. In a gas, particles are far apart with large empty spaces, so the same volume contains far fewer particles and density is low. For example, 1 cm³ of water has a mass of about 1 g, while 1 cm³ of steam at atmospheric pressure has a mass of roughly 0.0006 g. Heating usually decreases density because the particles move further apart while the mass stays the same.
What earns the marks
- States that density is mass per unit volume and is measured in kg/m³ or g/cm³.
- Describes the arrangement of particles in a solid as closely packed and regular, giving a high density.
- Describes the arrangement in a liquid as close but random, allowing particles to slide, giving a density slightly less than the solid.
- Describes the arrangement in a gas as widely spaced with large gaps, giving a much lower density.
- Links the number of particles in a given volume to the density, keeping the mass of each particle constant.
- Explains that heating a substance usually decreases its density because the particles move further apart while the mass stays the same.
Common mistakes and how to put them right
Saying that gas particles are lighter than solid particles.
Put it right by explaining that the particles are the same, but they are further apart in a gas.
Confusing density with mass.
Put it right by stating that density compares mass with volume, so a large object can have a low density.
Claiming that particles in a liquid are far apart.
Put it right by stating that liquid particles are close together but arranged randomly and can move past each other.
Exam tips
- Always quote the equation ρ = m ÷ V and give the units you are using.
- Use the words arrangement and spacing when comparing the three states, because these are the ideas being tested.
- If a calculation is required, convert cm³ to m³ by multiplying by 1 × 10⁻⁶ before substituting.
Try the lesson
The start of the Density of materials: 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. lesson, made from checked teaching notes for this specification point.
Have a go first
How does the arrangement of particles explain why a substance is typically much denser in its solid state than in its gaseous state?
Listen
Density is mass per unit volume, ρ = m ÷ V. In a solid, particles are closely packed in a regular arrangement, so a given volume contains many particles and density is high. In a liquid, particles remain close but are arranged randomly and can slide past one another, so density is slightly lower than the solid but still much greater than a gas. In a gas, particles are far apart with large empty spaces, so the same volume contains far fewer particles and density is low. For example, 1 cm³ of water has a mass of about 1 g, while 1 cm³ of steam at atmospheric pressure has a mass of roughly 0.0006 g. Heating usually decreases density because the particles move further apart while the mass stays the same. A strong answer earns marks for these points. States that density is mass per unit volume and is measured in kg/m³ or g/cm³. Describes the arrangement of particles in a solid as closely packed and regular, giving a high density. Describes the arrangement in a liquid as close but random, allowing particles to slide, giving a density slightly less than the solid. Describes the arrangement in a gas as widely spaced with large gaps, giving a much lower density. A common slip: Saying that gas particles are lighter than solid particles. Instead, explaining that the particles are the same, but they are further apart in a gas. Exam tip: Always quote the equation ρ = m ÷ V and give the units you are using.
Key words
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Density
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Particle model of matter
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Effect of temperature on density
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See every definition
- Density
- Mass per unit volume, written as ρ = m ÷ V and measured in kg/m³ or g/cm³.
- Particle model of matter
- Used to explain the differences in density between the different states of matter in terms of the arrangement of atoms or molecules.
- Effect of temperature on density
- Heating usually decreases density because the particles move further apart while the mass stays the same.
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