Changes of State

This lesson covers the changes of state between solids, liquids and gases, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You need to understand each type of change of state, how energy is involved, and how to interpret heating and cooling curves. A key point is that changes of state are physical changes — they are reversible, and no new substances are formed.

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Types of Change of State

There are six changes of state you need to know:

Change of State	From	To	Energy Change
Melting	Solid	Liquid	Energy absorbed (endothermic)
Freezing	Liquid	Solid	Energy released (exothermic)
Boiling / Evaporating	Liquid	Gas	Energy absorbed (endothermic)
Condensing	Gas	Liquid	Energy released (exothermic)
Sublimation	Solid	Gas (directly)	Energy absorbed (endothermic)
Deposition	Gas	Solid (directly)	Energy released (exothermic)

Melting

When a solid is heated, its particles gain kinetic energy and vibrate more and more. At the melting point, the particles have enough energy to overcome the forces holding them in their fixed positions. They break free and can move past each other — the solid becomes a liquid.

Freezing

When a liquid is cooled, its particles lose kinetic energy and slow down. At the freezing point (which is the same temperature as the melting point), the particles no longer have enough energy to move past each other. They become locked into fixed positions — the liquid becomes a solid.

Boiling and Evaporating

When a liquid is heated to its boiling point, particles throughout the liquid gain enough energy to overcome all the forces of attraction between them and escape as a gas. Bubbles of gas form within the liquid.

Evaporation is different from boiling:

• Evaporation happens at any temperature at the surface of a liquid.
• Boiling happens at a specific temperature (the boiling point) throughout the liquid.
• Evaporation is a slower process; boiling is rapid.

Condensing

When a gas is cooled, its particles lose kinetic energy. When they slow down enough, the forces of attraction between particles pull them together to form a liquid.

Sublimation

Some substances change directly from a solid to a gas without passing through the liquid state. This is called sublimation. Examples include:

• Solid carbon dioxide (dry ice) — sublimes at −78.5 \u00b0C at atmospheric pressure.
• Iodine crystals — when gently heated, they produce a purple vapour directly.

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State Change Diagram

The diagram below shows all the transitions between the three states:

graph LR
    S["Solid"] -->|"Melting<br/>(energy absorbed)"| L["Liquid"]
    L -->|"Freezing<br/>(energy released)"| S
    L -->|"Boiling / Evaporating<br/>(energy absorbed)"| G["Gas"]
    G -->|"Condensing<br/>(energy released)"| L
    S -->|"Sublimation<br/>(energy absorbed)"| G
    G -->|"Deposition<br/>(energy released)"| S

    style S fill:#3498db,color:#fff
    style L fill:#2ecc71,color:#fff
    style G fill:#e74c3c,color:#fff

Exam Tip: You do not need to know the term "deposition" for the Edexcel exam, but you do need to know sublimation. Make sure you can identify every change of state and whether energy is absorbed or released.

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Energy Changes During Changes of State

What Happens to Energy?

During a change of state, energy is used to break or form the bonds (forces) between particles. Crucially, this energy does not change the temperature of the substance during the change of state itself.

• Melting and boiling (endothermic): Energy is absorbed from the surroundings. This energy is used to overcome the forces of attraction between particles, not to increase the temperature.
• Freezing and condensing (exothermic): Energy is released to the surroundings. The forces of attraction between particles are re-established, and the energy associated with breaking them is given back.

Important Distinction

During a change of state:

• The temperature stays constant even though energy is being supplied or removed.
• The energy goes into breaking intermolecular forces (when heating) or is released when intermolecular forces form (when cooling).
• The kinetic energy of the particles does not change during the change of state.

Exam Tip: A very common exam mistake is to say that "the temperature increases during melting." It does not. During a change of state, the temperature remains constant. The energy being supplied is used to overcome the forces between particles, not to raise the temperature. This is a crucial distinction that examiners look for.

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Heating Curves

A heating curve is a graph of temperature against time as a substance is heated at a constant rate. It typically shows five sections:

Section 1: Solid being heated

• Temperature increases.
• Particles gain kinetic energy and vibrate more.

Section 2: Melting (solid to liquid)

• Temperature stays constant at the melting point.
• Energy is used to overcome forces between particles.
• The substance is a mixture of solid and liquid.

Section 3: Liquid being heated

• Temperature increases again.
• Particles gain kinetic energy and move faster.

Section 4: Boiling (liquid to gas)

• Temperature stays constant at the boiling point.
• Energy is used to overcome remaining forces between particles.
• The substance is a mixture of liquid and gas.

Section 5: Gas being heated

• Temperature increases again.
• Gas particles gain more kinetic energy and move even faster.

Interpreting the Flat Sections

The flat (horizontal) sections on a heating curve represent changes of state. During these sections:

• Energy is being supplied, but the temperature does not change.
• All the energy is used to break the forces between particles.
• The flat section at the lower temperature is the melting point.
• The flat section at the higher temperature is the boiling point.

Exam Tip: When asked to interpret a heating curve, always explain what is happening to the particles and the energy in each section. For the flat sections, state that "energy is being used to overcome the forces of attraction between particles, so the temperature remains constant even though heating continues."

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Cooling Curves

A cooling curve is the reverse. As a gas is cooled:

1. Temperature decreases (gas cooling).
2. Temperature stays constant at the boiling/condensation point (condensing).
3. Temperature decreases (liquid cooling).
4. Temperature stays constant at the melting/freezing point (freezing).
5. Temperature decreases (solid cooling).

The flat sections on a cooling curve indicate that energy is being released as forces between particles are re-formed.

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Changes of State Are Physical Changes

This is a critical point: