1.3 Density
Summary: Figure 1 shows a set of weights, which athletes lift to get stronger. The weights are made from iron. Why not make weights from another metal, like aluminium? Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- define mass, volume, and density
- explain why the same substance has different densities in each of its three states
- explain why different substances in the solid state have different densities.
Think back
- Give the state of matter of ice.
- In which two states of matter do particles touch their neighbours?
- Write four factors from the particle model that give a substance its properties.
Key idea Density is the mass of a substance in a certain volume.
Key words mass, volume, density
Figure 1 shows a set of weights, which athletes lift to get stronger. The weights are made from iron. Why not make weights from another metal, like aluminium?
Figure 1Weights made from iron.
What is density?
The iron weights are heavy. They have a mass of 20 kg. Aluminium weights of the same size are less heavy. They have a mass of only 7 kg. Mass is the amount of matter (stuff) in an object. It is measured in grams (g) or kilograms (kg).
The volume of an object is the amount of space it takes up. In Figure 2, the block of iron on the right has the greater volume. Volume is measured in cm3.
Figure 2The block of iron on the right has the greater volume.
The density of a substance is its mass in a certain volume. It tells you how heavy something is for its size. Iron has a greater density than aluminium, so iron weights are heavier than aluminium weights of the same size.
A Write definitions for mass, volume, and density.
Maths skills You can calculate density using mass and volume. density = mass ÷ volume A block of silver has a mass of 100g. Its volume is 9.5 cm3. Calculate the density of silver.
How does the particle model explain density?
The particle model explains why different substances have different densities. Table 1 shows the densities of aluminium and iron. Table 1 The densities of aluminium and iron. Substance
Density in the solid state in g/cm3
Relative mass of one particle of the substance
aluminium
3
27
iron
8
56
Iron has a greater density than aluminium. This is partly because iron particles have a greater mass than aluminium particles. The density of a substance depends on the mass of its particles.
B Explain why aluminium has a lower density than iron.
How does state affect density?
The same substance has different densities in its three states, even though the particles are identical. Table 2 shows the density of gold in two states. Table 2 The densities of solid and liquid gold. Substance
Density in the solid state in g/cm3
Density in the liquid state in g/cm3
gold
19
17
Solid gold has a greater density than liquid gold. This is because the particles are much closer together in solid gold. In the gas state, the particles are far apart. Therefore, gold gas has a lower density than liquid gold.
Most substances are like gold. Their density is greater in the solid state than in the liquid state. Their density in the gas state is lowest of all.
C Which factor explains why solid gold has a greater density than liquid gold: what the particles are like, or how the particles are arranged?
How is water different?
Water is unusual. Below 4°C, its density is greater in the liquid state than in the solid state. This is because the particles are closer together in the liquid state. This density difference means that ice floats on liquid water below 4°C (see Figure 3).
Figure 3Solid water (ice) is less dense than liquid water. This is why ice floats.
Stretch zone
- Copy and complete the sentences. The amount of matter in an object is its blank. The amount of space an object takes up is its blank. The mass of a substance in a certain volume is its blank.
- Use the particle model to explain why: the density of liquid gold is less than the density of solid goldthe density of liquid water is greater than the density of steamthe density of solid gold is greater than the density of solid aluminium.
Stretch zone
- Compare the reasons for: different substances in the same state having different densitiesthe same substance in different states having different densities. Use the particle model in your answers.