AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Density of materials: differences in density.
Density is the mass per unit volume of a material, calculated as density = mass ÷ volume, with units such as kg/m³ or g/cm³.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Density is the mass per unit volume of a material, calculated as density = mass ÷ volume, with units such as kg/m³ or g/cm³. Differences in density arise because the same volume of different materials contains different amounts of matter, and because the spacing and arrangement of particles changes between solids, liquids and gases. For example, 1 cm³ of iron has a mass of about 7.9 g, while 1 cm³ of water has a mass of about 1.0 g, so iron is denser. In general, solids are densest because particles are closely packed, liquids are slightly less dense, and gases are much less dense because particles are far apart. Density also changes with temperature for gases and with pressure.
What earns the marks
- Density is defined as mass per unit volume and is calculated using density = mass ÷ volume.
- The same volume of different materials can have different masses, which is what is meant by a difference in density.
- Particle spacing and arrangement explain why solids are usually densest, liquids slightly less dense, and gases much less dense.
- Typical values can be used to compare materials, for example iron at about 7.9 g/cm³ and water at about 1.0 g/cm³.
- Density can be measured by finding mass with a balance and volume by displacement or by measuring dimensions.
- Changes in temperature or pressure can change the density of a gas because the particles move further apart or closer together.
Common mistakes and how to put them right
Confusing density with mass.
Put it right: Compare mass per unit volume, not total mass.
Writing density as g/cm² or kg/m².
Put it right: Use cubic units.
Assuming all solids are denser than all liquids.
Put it right: Compare particle arrangement and actual values rather than state of matter alone.
Exam tips
- Always show the equation density = mass ÷ volume and substitute values with units before calculating.
- When comparing two materials, quote a typical density value for each and state which is denser and why in terms of particles.
- Check unit consistency: convert cm³ to m³ or g to kg before dividing if the question uses mixed units.
Try the lesson
The start of the Density of materials: differences in density. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement best uses the particle model to explain why a gas is generally much less dense than a solid?
Listen
Density is the mass per unit volume of a material, calculated as density = mass ÷ volume, with units such as kg/m³ or g/cm³. Differences in density arise because the same volume of different materials contains different amounts of matter, and because the spacing and arrangement of particles changes between solids, liquids and gases. For example, 1 cm³ of iron has a mass of about 7.9 g, while 1 cm³ of water has a mass of about 1.0 g, so iron is denser. In general, solids are densest because particles are closely packed, liquids are slightly less dense, and gases are much less dense because particles are far apart. Density also changes with temperature for gases and with pressure. A strong answer earns marks for these points. Density is defined as mass per unit volume and is calculated using density = mass ÷ volume. The same volume of different materials can have different masses, which is what is meant by a difference in density. Particle spacing and arrangement explain why solids are usually densest, liquids slightly less dense, and gases much less dense. Typical values can be used to compare materials, for example iron at about 7.9 g/cm³ and water at about 1.0 g/cm³. A common slip: Confusing density with mass: a large light object can have a lower density than a small heavy object. Instead, compare mass per unit volume, not total mass. Exam tip: Always show the equation density = mass ÷ volume and substitute values with units before calculating.
Key words
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Density
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Particle spacing and arrangement
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Typical values
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Practical measurement of density
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See every definition
- Density
- The mass per unit volume of a material, calculated as density = mass ÷ volume, with units such as kg/m³ or g/cm³.
- Particle spacing and arrangement
- How far apart particles are and how they are arranged, which explains why solids are usually densest, liquids slightly less dense, and gases much less dense.
- Typical values
- Values such as iron at about 7.9 g/cm³ and water at about 1.0 g/cm³ that can be used to compare materials.
- Practical measurement of density
- Finding mass with a balance and volume by displacement or by measuring dimensions.
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