Using particle theory

Scientists use the idea that all matter is made of particles to explain the properties of solids, liquids and gases. In Stage 7 you learnt about the arrangement of particles in these three states of matter.

The particles in a gas do not touch each other

The particles in liquids are arranged with the particles touching each other but not in a pattern of rows

The particles in solids are arranged in regular rows with the particles touching each other

The particles in solids are held firmly in place by forces of attraction. The particles can only vibrate - they cannot move or change places.
The particles in liquids are held together by weaker forces of attraction. The particles can move past each other.
The particles in a gas are not attracted to each other and they can move freely.
In order for particles to move or vibrate, they need energy. The more energy they have, the more they can move or vibrate.

$A+I$: 1) Explain why a solid expands when it is heated.

$A+I$: 2) Explain how the liquid in a thermometer changes so that it can be used to measure a temperature.

$A+I$: 3) Use particle theory to explain why solids and liquids cannot be compressed (squashed into a smaller volume).

$A+I$: 4) Use particle theory to explain why liquids and gases can flow.

Changing state

Matter exists as solid, liquid or gas. Particle theory can explain changes of state from solid to liquid, liquid to gas, liquid to solid or gas to liquid.
For example, when a solid is heated, the particles vibrate more, because some of the energy is transferred to the particles. The particles may have enough energy to escape the strong forces holding them together in their places. The particles can now move past each other. The solid has melted to form a liquid.

melting freezing boiling condensing — The diagrams show how the arrangement of the particles changes when there is a change of state

$A+I$: 5) Explain, using particle theory, how a liquid changes to a gas.

$A+I$: 6) Explain, tising particle theory, how a liquid changes to become a solid.

$A+I$: 7) Explain, using particle theory, what happens when steam in the bathroom hits a cold surface, such as a mirror.

8) Copy the flow chart. The arrows represent the processes involved when matter changes state. Add the name for each process to your flow chart.

solid liquid gas solid liquid gas

Summary

• All matter is made up of particles.
• Particle theory can be used to explain the properties of the three states of matter.
• Particle theory can be used to explain the changes between the states of matter.