States of Matter and Internal Energy

This lesson explores the three states of matter using the particle model, explains how the arrangement and energy of particles differ between states, and introduces the concept of internal energy. These ideas underpin the entire Particle Model section of the Edexcel GCSE Combined Science specification (1SC0).

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The Three States of Matter

All substances can exist as a solid, liquid or gas. The state depends on the temperature and pressure.

Property	Solid	Liquid	Gas
Particle arrangement	Regular, closely packed	Irregular, close together	Random, far apart
Particle motion	Vibrate about fixed positions	Move around each other	Move rapidly in all directions
Spacing	Very small	Small	Very large
Forces between particles	Strong	Moderate	Very weak (almost none)
Shape	Fixed	Takes shape of container	Fills the container
Volume	Fixed	Fixed	Variable (fills container)
Can be compressed?	No (virtually)	No (virtually)	Yes — easily

graph LR
    subgraph "Solid"
    S1["● ● ●"] --- S2["● ● ●"] --- S3["● ● ●"]
    end
    subgraph "Liquid"
    L1["●  ●"] --- L2[" ● ●"] --- L3["●  ●"]
    end
    subgraph "Gas"
    G1["●"] --- G2["     ●"] --- G3["  ●       ●"]
    end

Exam Tip: In the exam, draw the particles in a solid in neat rows, in a liquid slightly disordered and touching, and in a gas widely spaced and randomly arranged. Diagrams often earn marks.

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Changes of State

When a substance changes state, the arrangement, spacing and energy of the particles change, but the mass remains the same — no particles are created or destroyed.

Change of State	From → To	Energy
Melting	Solid → Liquid	Energy absorbed (endothermic)
Freezing	Liquid → Solid	Energy released (exothermic)
Boiling / Evaporation	Liquid → Gas	Energy absorbed (endothermic)
Condensation	Gas → Liquid	Energy released (exothermic)
Sublimation	Solid → Gas	Energy absorbed (endothermic)

graph LR
    Solid -->|"Melting"| Liquid
    Liquid -->|"Freezing"| Solid
    Liquid -->|"Boiling / Evaporation"| Gas
    Gas -->|"Condensation"| Liquid
    Solid -->|"Sublimation"| Gas

Key Point

Changes of state are physical changes — they are reversible and do not produce new substances. The mass is conserved.

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Internal Energy

The internal energy of a system is the total energy stored by the particles. It is the sum of:

1. Kinetic energy — energy due to the movement (vibration, rotation, translation) of the particles.
2. Potential energy — energy stored in the bonds / forces between particles.

$\text{Internal energy} = \text{Kinetic energy of particles} + \text{Potential energy of particles}$Internal energy=Kinetic energy of particles+Potential energy of particles

Heating a Substance

When you heat a substance and its temperature rises:

• The kinetic energy of the particles increases — they move or vibrate faster.
• The internal energy increases.

When you heat a substance at its melting point or boiling point and it is changing state:

• The temperature stays constant.
• The energy supplied goes into breaking or weakening the bonds between particles (increasing potential energy).
• The kinetic energy does not change during a change of state.

Exam Tip: A very common 4–6 mark question asks you to explain what happens to the energy and particles as a substance is heated from solid to gas. Always mention both kinetic and potential energy, and explain that during a change of state the temperature is constant because energy goes into breaking bonds, not increasing kinetic energy.

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Heating Curves

A heating curve is a graph of temperature against time (or energy supplied) for a substance being heated at a constant rate.

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

Section of the curve	What happens	Temperature
Sloped sections	Substance is heating up in one state — kinetic energy of particles increases	Rising
Flat sections (plateaus)	Substance is changing state — potential energy increases as bonds break	Constant

Reading a Heating Curve

• The first flat section corresponds to the melting point.
• The second flat section corresponds to the boiling point.
• The length of the flat section tells you how much energy is needed for the change of state.

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Evaporation vs Boiling

Both evaporation and boiling convert a liquid to a gas, but they are different processes.

Feature	Evaporation	Boiling
Temperature	Occurs at any temperature below the boiling point	Occurs only at the boiling point
Where it occurs	At the surface of the liquid only	Throughout the whole liquid
Bubbles?	No	Yes — bubbles of gas form inside the liquid
Rate	Gradual	Rapid
Energy source	Fastest particles at the surface escape	External heating supplies energy to all particles

Why Does Evaporation Cause Cooling?

During evaporation, the fastest (highest kinetic energy) particles escape from the surface. This lowers the average kinetic energy of the remaining particles, so the temperature of the liquid decreases.

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Factors Affecting the Rate of Evaporation

Factor	Effect
Higher temperature	More particles have enough energy to escape → faster evaporation
Larger surface area	More particles are at the surface → faster evaporation
Moving air (draught)	Carries away vapour, preventing particles returning → faster evaporation
Lower humidity	Less vapour in the air → faster evaporation

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Common Misconceptions