Changes of State

This lesson covers changes of state as required by the AQA GCSE Physics specification (4.3.2). When a substance is heated or cooled, it can change from one state of matter to another. Understanding the names and nature of these changes, as well as how they relate to the particle model, is essential knowledge for your exam.

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The Three States of Matter

All matter exists in one of three states: solid, liquid, or gas. The state of a substance depends on the temperature and the arrangement and energy of its particles.

Property	Solid	Liquid	Gas
Particle arrangement	Regular, closely packed	Irregular, close together	Random, widely spaced
Particle movement	Vibrate around fixed positions	Move around each other	Move rapidly in all directions
Particle spacing	Very close	Close	Very far apart
Shape	Fixed	Takes the shape of the container	Fills the container
Volume	Fixed	Fixed	Expands to fill the container
Density	High	Medium	Low
Compressible?	No	Almost no	Yes

graph LR
    subgraph Solid
        direction TB
        S1["O O O O O"]
        S2["O O O O O"]
        S3["O O O O O"]
        S4["O O O O O"]
    end
    subgraph Liquid
        direction TB
        L1["O  O  O  O"]
        L2[" O  O  O "]
        L3["O  O  O  O"]
        L4[" O  O  O "]
    end
    subgraph Gas
        direction TB
        G1["O       O"]
        G2["    O       O"]
        G3["O         O"]
        G4["     O      "]
    end

    Solid -->|"Melting"| Liquid
    Liquid -->|"Evaporation /<br/>Boiling"| Gas

    style Solid fill:#3498db,color:#fff
    style Liquid fill:#2ecc71,color:#fff
    style Gas fill:#e74c3c,color:#fff

Exam Tip: When describing the three states of matter in an exam, always refer to the arrangement, movement, and spacing of particles. These are the three key features the examiners are looking for. Never say particles "expand" or "shrink" — the particles themselves do not change size.

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Names of the Changes of State

There are six changes of state, and you must know the names of all of them:

Change of State	From	To	Energy Change
Melting	Solid	Liquid	Energy absorbed (heating)
Freezing	Liquid	Solid	Energy released (cooling)
Boiling / Evaporation	Liquid	Gas	Energy absorbed (heating)
Condensation	Gas	Liquid	Energy released (cooling)
Sublimation	Solid	Gas	Energy absorbed (heating)
Desublimation	Gas	Solid	Energy released (cooling)

graph TD
    A["SOLID"] -->|"Melting"| B["LIQUID"]
    B -->|"Freezing"| A
    B -->|"Boiling / Evaporation"| C["GAS"]
    C -->|"Condensation"| B
    A -->|"Sublimation"| C
    C -->|"Desublimation"| A

    style A fill:#3498db,color:#fff
    style B fill:#2ecc71,color:#fff
    style C fill:#e74c3c,color:#fff

Key Definitions

• Melting point: The temperature at which a solid turns into a liquid. For pure water, this is 0 degrees C.
• Boiling point: The temperature at which a liquid turns into a gas throughout the liquid (not just at the surface). For pure water, this is 100 degrees C.
• Evaporation: The process where particles escape from the surface of a liquid and become a gas. This can happen at any temperature below the boiling point.

Exam Tip: Do not confuse boiling and evaporation. Boiling occurs at a specific temperature (the boiling point), happens throughout the liquid, and requires a heat source. Evaporation occurs at any temperature, only happens at the surface, and is caused by the fastest-moving particles escaping. This distinction is a very common exam question.

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Changes of State and the Particle Model

During a change of state:

• The energy of the particles changes — they gain or lose energy.
• The arrangement and spacing of the particles changes.
• The particles themselves do not change — they are the same particles, just arranged differently.
• The mass of the substance does not change — no particles are added or removed.

What Happens During Melting

When a solid is heated to its melting point:

1. The particles gain kinetic energy and vibrate more vigorously.
2. At the melting point, the particles have enough energy to overcome some of the forces of attraction holding them in their fixed positions.
3. The regular structure breaks down, and particles can move past each other — the substance becomes a liquid.

What Happens During Boiling

When a liquid is heated to its boiling point:

1. The particles gain more kinetic energy and move faster.
2. At the boiling point, particles throughout the liquid have enough energy to overcome all the forces of attraction between them.
3. The particles escape from the liquid and move freely as a gas.

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Physical Changes vs Chemical Changes

Changes of state are physical changes, not chemical changes. This is a crucial distinction:

Physical Change (Change of State)	Chemical Change
Reversible — can be reversed by heating or cooling	Often irreversible
No new substance is formed	A new substance is formed
The particles are the same — only their arrangement changes	The particles are rearranged into different substances
Mass is conserved	Mass is conserved
No change in the chemical properties of the substance	Chemical properties change

Conservation of Mass

When a substance changes state, the number of particles stays the same. Therefore, the mass stays the same. For example:

• If you melt 100 g of ice, you get 100 g of water.
• If you boil 50 g of water, you get 50 g of steam (water vapour).

Exam Tip: If a question asks "What happens to the mass when ice melts?", the answer is that the mass stays the same. Changes of state are physical changes — no particles are lost or gained. The mass is conserved. This is true for ALL changes of state.

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Heating and Cooling Curves

A heating curve shows how the temperature of a substance changes as it is heated over time. A cooling curve shows the reverse process.

graph LR
    A["Time -->"] --- B["Temperature"]
    style A fill:#fff,stroke:#000
    style B fill:#fff,stroke:#000

Features of a Heating Curve

Section of Curve	What Is Happening	Temperature Change
Rising section 1	Solid is being heated — particles vibrate faster	Temperature increases
Flat section 1 (melting)	Solid is melting — energy is being used to break bonds between particles	Temperature stays constant
Rising section 2	Liquid is being heated — particles move faster	Temperature increases
Flat section 2 (boiling)	Liquid is boiling — energy is being used to separate particles completely	Temperature stays constant
Rising section 3	Gas is being heated — particles move even faster	Temperature increases

Why Temperature Stays Constant During a Change of State