States of Matter

This lesson covers the three states of matter — solid, liquid, and gas — as required by the AQA GCSE Chemistry specification (4.2.4). You need to understand the particle model, state changes, and how to interpret heating and cooling curves. This topic links directly to bonding and structure, as the type of bonding determines the temperatures at which state changes occur.

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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 at a given temperature depends on the strength of the forces between its particles.

Property	Solid	Liquid	Gas
Arrangement of particles	Regular, closely packed	Irregular, close together but able to move	Random, widely spaced
Movement of particles	Vibrate in fixed positions	Move around each other (flow)	Move quickly in all directions
Energy of particles	Low	Medium	High
Distance between particles	Very close	Close (slightly further than solid)	Far apart
Shape	Fixed shape	Takes shape of container	Fills the container
Volume	Fixed volume	Fixed volume	No fixed volume (fills container)
Compressibility	Cannot be compressed	Cannot be compressed significantly	Easily compressed
Density	High	High (slightly less than solid, usually)	Low

Exam Tip: You must be able to draw particle diagrams for each state. Solids: regular rows of touching circles. Liquids: irregular but touching circles. Gases: widely spaced circles with arrows showing random movement. In the exam, always draw at least 8-10 particles for each state.

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State Symbols

In chemical equations, state symbols are used to show the physical state of each substance:

Symbol	State	Example
(s)	Solid	NaCl(s)
(l)	Liquid	H2O(l)
(g)	Gas	CO2(g)
(aq)	Aqueous (dissolved in water)	NaCl(aq)

You should always include state symbols in balanced equations when asked to do so.

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Changes of State

When a substance changes state, the particles gain or lose energy (usually in the form of heat). The changes of state are:

graph LR
    A["SOLID"] -->|"Melting<br/>(energy absorbed)"| B["LIQUID"]
    B -->|"Evaporation /<br/>Boiling<br/>(energy absorbed)"| C["GAS"]
    C -->|"Condensation<br/>(energy released)"| B
    B -->|"Freezing<br/>(energy released)"| A
    A -->|"Sublimation<br/>(energy absorbed)"| C
    C -->|"Deposition<br/>(energy released)"| A

    style A fill:#3498db,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff

Change of State	Process	Energy Change
Melting	Solid to liquid	Energy absorbed (endothermic)
Boiling / Evaporation	Liquid to gas	Energy absorbed (endothermic)
Condensation	Gas to liquid	Energy released (exothermic)
Freezing	Liquid to solid	Energy released (exothermic)
Sublimation	Solid directly to gas	Energy absorbed (endothermic)
Deposition	Gas directly to solid	Energy released (exothermic)

Key Point About Changes of State

During a change of state:

• The temperature remains constant while the state change is occurring.
• The energy being supplied is used to overcome the forces between particles (intermolecular forces or ionic/metallic bonds), NOT to increase the kinetic energy of the particles.
• The mass does not change — no atoms are created or destroyed.
• Changes of state are physical changes, not chemical changes. The substance can be changed back by reversing the conditions.

Exam Tip: A change of state is a physical change, NOT a chemical change. No new substances are formed — the particles remain the same; only their arrangement and energy change. If asked "Is melting a chemical or physical change?" always answer physical. No bonds between atoms are broken during changes of state for molecular substances — only the intermolecular forces are overcome.

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Heating Curves

A heating curve shows how the temperature of a substance changes as it is heated over time. It has characteristic flat sections (plateaus) where state changes occur.

Features of a Heating Curve

1. Rising sections: The temperature increases as energy is supplied. The kinetic energy of the particles increases.
2. Flat sections (plateaus): The temperature remains constant during a change of state. Energy is being used to overcome the forces between particles, not to raise the temperature.

Interpreting a Heating Curve

Section	What Is Happening	Particle Behaviour
First rising section	Solid is being heated	Particles vibrate faster
First plateau	Melting (solid to liquid)	Forces between particles being overcome; particles start to move freely
Second rising section	Liquid is being heated	Particles move faster
Second plateau	Boiling (liquid to gas)	Forces between particles being fully overcome; particles break free
Third rising section	Gas is being heated	Particles move even faster

Cooling Curves

A cooling curve is the reverse of a heating curve. The plateaus occur at the same temperatures but the state changes are reversed (gas to liquid = condensation, liquid to solid = freezing).

Exam Tip: When interpreting heating curves, remember that flat sections = state changes. If asked what is happening at a plateau, say: "The substance is changing state. Energy is being used to overcome the forces between particles rather than increasing the temperature." Do NOT say "the temperature is not changing" and leave it at that — you need to explain WHY it stays constant.

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Predicting States from Melting and Boiling Points

You can predict the state of a substance at a given temperature by comparing it to the melting point and boiling point:

• Below the melting point: solid
• Between the melting point and boiling point: liquid
• Above the boiling point: gas

Worked Example

Ethanol has a melting point of -114 degrees C and a boiling point of 78 degrees C.