GCSE Physics revision

Changes of state

Changes of state and the particle model

AQA 4.3.1.2

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Describe how, when substances change state (melt, freeze, boil, evaporate, condense or sublimate), mass is conserved.

MOST 🎯🎯

Apply the scientific explanation of changes of state to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving changes of state and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

The Particle Model: The Big Idea

Everything around us — solids, liquids, and gases — is made up of tiny particles that are always in motion.
The arrangement and movement of these particles determines which state of matter a substance is in.
Understanding the particle model is the key to explaining a huge range of everyday physical phenomena.

1. The Particle Model: The Big IdeaAQA

Notes

Everything around us — solids, liquids, and gases — is made up of tiny particles that are always in motion.
The arrangement and movement of these particles determines which state of matter a substance is in.
Understanding the particle model is the key to explaining a huge range of everyday physical phenomena.

Properties of Solids

In a solid, particles are arranged in a neat, regular pattern and are held tightly together.
Solid particles can only vibrate on the spot, which is why solids keep a fixed shape.
Because particles are so closely packed, solids are typically very dense.

Properties of Liquids

In a liquid, particles have more energy than in a solid and can break free from the rigid structure.
Liquid particles are still close together but can slide past one another, which is why liquids can flow.
Liquids do not have a fixed shape but do have a fixed volume.

Properties of Gases

In a gas, particles have the most energy and move rapidly in all directions, spread far apart.
Because gas particles are so spread out, gases will expand to fill whatever container they are placed in.
The large spaces between gas particles mean gases have a much lower density than solids or liquids.

Density and the Particle Model

Density is defined as the mass of a substance per unit volume — a measure of how much matter is packed into a given space.
The formula for density is ρ = m / V, where ρ is density, m is mass, and V is volume.
Solids are denser than gases because their particles are packed tightly together, fitting more mass into the same volume.
The unit of density is kg/m³ (or g/cm³ for smaller objects).

5. Density and the Particle ModelAQA

Notes

Density is defined as the mass of a substance per unit volume — a measure of how much matter is packed into a given space.
The formula for density is ρ = m / V, where ρ is density, m is mass, and V is volume.
Solids are denser than gases because their particles are packed tightly together, fitting more mass into the same volume.
The unit of density is kg/m³ (or g/cm³ for smaller objects).

Measuring Density in the Lab

To find the density of an irregularly shaped object, first measure its mass using a balance.
To measure the volume, use a displacement can filled with water to the spout, then lower the object in and collect the displaced water in a measuring cylinder.
Once mass and volume are known, calculate density using ρ = m / V.
This required practical is a common exam question, so knowing the method in detail is essential.

6. Measuring Density in the LabAQA

Notes

To find the density of an irregularly shaped object, first measure its mass using a balance.
To measure the volume, use a displacement can filled with water to the spout, then lower the object in and collect the displaced water in a measuring cylinder.
Once mass and volume are known, calculate density using ρ = m / V.
This required practical is a common exam question, so knowing the method in detail is essential.

Changes of State

When a solid is heated, its particles gain energy and vibrate more vigorously until they break free of their fixed positions, forming a liquid — this is melting.
Changes of state are physical changes, meaning they are reversible; for example, cooling a liquid removes energy from the particles, causing them to slow down and reform a solid.
The particles themselves do not change during a change of state — only their arrangement and energy level change.
Common changes of state include melting, freezing, boiling, condensing, and sublimation.

7. Changes of StateAQA

Notes

When a solid is heated, its particles gain energy and vibrate more vigorously until they break free of their fixed positions, forming a liquid — this is melting.
Changes of state are physical changes, meaning they are reversible; for example, cooling a liquid removes energy from the particles, causing them to slow down and reform a solid.
The particles themselves do not change during a change of state — only their arrangement and energy level change.
Common changes of state include melting, freezing, boiling, condensing, and sublimation.

Conservation of Mass

When a substance changes state, the total mass remains exactly the same — this is known as conservation of mass.
Mass is conserved because no particles are added or removed; only their arrangement and energy change.
For example, 100 g of ice will produce exactly 100 g of liquid water when melted.
This principle applies to all changes of state, whether melting, boiling, freezing, or condensing.

8. Conservation of MassAQA

Notes

When a substance changes state, the total mass remains exactly the same — this is known as conservation of mass.
Mass is conserved because no particles are added or removed; only their arrangement and energy change.
For example, 100 g of ice will produce exactly 100 g of liquid water when melted.
This principle applies to all changes of state, whether melting, boiling, freezing, or condensing.

Energy During Changes of State

When a substance is changing state (e.g., ice melting at 0°C), the temperature remains constant even though energy is still being supplied.
The energy being added during a change of state is used to break the bonds between particles, not to increase the speed of the particles.
This energy, which changes state without changing temperature, leads into the concept of specific latent heat.

9. Energy During Changes of StateAQA

Notes

When a substance is changing state (e.g., ice melting at 0°C), the temperature remains constant even though energy is still being supplied.
The energy being added during a change of state is used to break the bonds between particles, not to increase the speed of the particles.
This energy, which changes state without changing temperature, leads into the concept of specific latent heat.

Key Exam Takeaways

Know the properties of solids, liquids, and gases in terms of particle arrangement and movement.
Memorise the density formula ρ = m / V and be able to apply it to calculations.
Be able to explain why solids are denser than gases using the particle model.
Remember that changes of state are physical (reversible) changes.
Mass is always conserved during a change of state because the number of particles stays the same.

10. Key Exam TakeawaysAQA

Notes

Know the properties of solids, liquids, and gases in terms of particle arrangement and movement.
Memorise the density formula ρ = m / V and be able to apply it to calculations.
Be able to explain why solids are denser than gases using the particle model.
Remember that changes of state are physical (reversible) changes.
Mass is always conserved during a change of state because the number of particles stays the same.