Changes of State and Internal Energy

This lesson covers the changes of state between solids, liquids and gases, and the concept of internal energy, as required by the AQA GCSE Combined Science Trilogy specification (8464, section 6.3.1). Understanding internal energy is key to explaining why temperature stays constant during a change of state.

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Changes of State

A change of state is a physical change — no new substances are formed, and the process is reversible. The mass of the substance is conserved (stays the same).

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

Change of State	Direction	Energy
Melting	Solid → Liquid	Energy absorbed
Freezing	Liquid → Solid	Energy released
Boiling / Evaporating	Liquid → Gas	Energy absorbed
Condensing	Gas → Liquid	Energy released
Sublimation	Solid → Gas (directly)	Energy absorbed

Exam Tip: Changes of state are physical changes, not chemical. No new substances are formed, and the change can be reversed. The total mass does not change because no particles are added or removed.

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What Is Internal Energy?

Internal energy is the total kinetic energy and potential energy of all the particles in a system.

$\text{Internal energy} = \text{total kinetic energy of particles} + \text{total potential energy of particles}$Internal energy=total kinetic energy of particles+total potential energy of particles

• Kinetic energy of particles depends on their speed — faster particles have more kinetic energy.
• Potential energy is associated with the forces (bonds) between particles — weakening or breaking bonds increases potential energy.

When a substance is heated:

• The internal energy of the system increases.
• Either the temperature increases (kinetic energy of particles increases)...
• ...or a change of state occurs (potential energy increases, bonds are broken or weakened).

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Why Temperature Stays Constant During a Change of State

During a change of state (e.g. melting or boiling), the temperature remains constant even though energy is being supplied. This is because:

• All the energy supplied goes into breaking or weakening the bonds between particles (increasing the potential energy stored between them).
• No energy goes into increasing the kinetic energy of the particles, so the temperature does not rise.
• Once the change of state is complete, the temperature begins to rise again.

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Heating Curves

A heating curve shows how the temperature of a substance changes as energy is supplied at a constant rate.

graph LR
    A["Solid heating<br/>(temp rises)"] --> B["Melting<br/>(temp constant)"]
    B --> C["Liquid heating<br/>(temp rises)"]
    C --> D["Boiling<br/>(temp constant)"]
    D --> E["Gas heating<br/>(temp rises)"]

Section	What happens	Temperature	Energy goes into
Solid heating	Particles vibrate faster	Rises	Increasing kinetic energy
Melting	Particles break free from fixed positions	Constant	Increasing potential energy (breaking bonds)
Liquid heating	Particles move faster	Rises	Increasing kinetic energy
Boiling	Particles escape from the liquid	Constant	Increasing potential energy (breaking bonds)
Gas heating	Particles move faster still	Rises	Increasing kinetic energy

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Evaporation vs Boiling

Feature	Evaporation	Boiling
Temperature	Occurs at any temperature below the boiling point	Occurs at the boiling point only
Where it happens	At the surface of the liquid only	Throughout the liquid (bubbles form)
Rate	Slower	Faster
Energy source	Particles at the surface with enough energy escape	Continuous energy supply heats the entire liquid

Why does evaporation cause cooling?

The fastest (most energetic) particles at the surface escape. This means the average kinetic energy of the remaining particles decreases, so the temperature of the liquid falls.

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Worked Example

Explain what happens to the internal energy of water as it is heated from 90 °C to 110 °C (going through boiling at 100 °C).

1. From 90 °C to 100 °C: the internal energy increases because the kinetic energy of the water particles increases (they move faster). The temperature rises.
2. At 100 °C (boiling): the internal energy continues to increase, but now the energy goes into breaking the bonds between liquid particles (increasing potential energy). The temperature stays constant at 100 °C.
3. From 100 °C to 110 °C (as steam): the internal energy increases again because the kinetic energy of the gas particles increases. The temperature rises.

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Common Misconceptions

Misconception	Correction
Temperature always rises when you heat a substance	During a change of state, temperature stays constant while energy is used to break bonds
Particles "get bigger" when heated	Particles do not change size — they move faster and the spacing between them increases
Mass is lost when water evaporates	Mass is conserved — the water particles become gas but are not destroyed
Boiling and evaporation are the same	Boiling occurs at a fixed temperature throughout the liquid; evaporation occurs at any temperature at the surface

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Summary

• Changes of state are physical, reversible changes where mass is conserved.
• Internal energy = total kinetic energy + total potential energy of all particles.
• Heating a substance increases its internal energy.
• During a change of state, temperature stays constant because energy goes into breaking bonds (increasing potential energy), not increasing kinetic energy.
• Evaporation occurs at any temperature at the surface; boiling occurs throughout the liquid at its boiling point.
• Evaporation causes cooling because the fastest particles escape, lowering the average kinetic energy.

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Specific Heat Capacity — A Quick Reminder

When a substance is heated without changing state, the energy supplied raises its temperature according to:

$E = m \times c \times \Delta\theta$E=m×c×Δθ

Symbol	Quantity	Unit
$E$E	Energy transferred	J
$m$m	Mass	kg
$c$c	Specific heat capacity	J/kg °C
$\Delta\theta$Δθ	Temperature change	°C