AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Density of materials: The density of a material is defined by the equation:
Density describes how much mass is packed into each unit of volume for a material.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Density describes how much mass is packed into each unit of volume for a material. It is a property of the material itself, so a solid block and a powder of the same pure substance have the same density if measured without trapped air. You calculate density by dividing mass by volume, but you must first measure mass with a balance and volume by geometry or displacement. For example, a 54 g cube of side 3 cm has volume 27 cm³, so density is 54 g ÷ 27 cm³ = 2 g/cm³. In SI units, 2 g/cm³ equals 2000 kg/m³. Density explains floating and sinking: less dense objects float on denser fluids. When a material changes state, its mass stays constant but volume changes, so density changes.
What earns the marks
- State that density is the mass per unit volume of a material.
- Use the equation ρ = m / V and substitute measured mass and volume correctly.
- Measure mass in grams or kilograms using a balance and volume in cm³ or m³ using a ruler or displacement can.
- Convert between g/cm³ and kg/m³ by multiplying or dividing by 1000.
- Explain that density is a property of the material and does not depend on the size of the sample.
- Use density values to predict whether an object will float or sink in a given fluid.
Common mistakes and how to put them right
Confusing mass with density.
Put it right by dividing mass by volume.
Forgetting to convert units before calculating.
Put it right by converting all measurements to consistent units first.
Assuming a larger piece of the same material has greater density.
Put it right by comparing density values rather than mass or volume alone.
Exam tips
- Write the equation, substitute values with units, then give the answer with the correct unit, such as g/cm³ or kg/m³.
- Show unit conversions clearly, for example 1 g/cm³ = 1000 kg/m³, to avoid losing accuracy marks.
- When explaining floating, compare the density of the object with the density of the fluid, not just the mass of the object.
Try the lesson
The start of the Density of materials: The density of a material is defined by the equation: lesson, made from checked teaching notes for this specification point.
Have a go first
A block of material has a mass of 300 g and a volume of 100 cm³. Using the density equation, what is the density of the material?
Listen
Density describes how much mass is packed into each unit of volume for a material. It is a property of the material itself, so a solid block and a powder of the same pure substance have the same density if measured without trapped air. You calculate density by dividing mass by volume, but you must first measure mass with a balance and volume by geometry or displacement. For example, a 54 g cube of side 3 cm has volume 27 cm³, so density is 54 g ÷ 27 cm³ = 2 g/cm³. In SI units, 2 g/cm³ equals 2000 kg/m³. Density explains floating and sinking: less dense objects float on denser fluids. When a material changes state, its mass stays constant but volume changes, so density changes. A strong answer earns marks for these points. State that density is the mass per unit volume of a material. Use the equation ρ = m / V and substitute measured mass and volume correctly. Measure mass in grams or kilograms using a balance and volume in cm³ or m³ using a ruler or displacement can. Convert between g/cm³ and kg/m³ by multiplying or dividing by 1000. A common slip: Confusing mass with density: mass is the amount of matter in an object, while density is mass per unit volume. Instead, dividing mass by volume. Exam tip: Write the equation, substitute values with units, then give the answer with the correct unit, such as g/cm³ or kg/m³.
Key words
Say what each word means before you turn the card.
Density
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Mass
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Floating and sinking
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See every definition
- Density
- The mass per unit volume of a material, calculated by dividing mass by volume.
- Mass
- The amount of matter in an object, measured with a balance in grams or kilograms.
- Floating and sinking
- Less dense objects float on denser fluids, so comparing the density of an object with the density of the fluid predicts whether it floats or sinks.
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