States of Matter

This lesson covers the three states of matter — solid, liquid and gas — and the changes of state between them, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the particle model, explain the properties of each state, and describe what happens during state changes in terms of energy and particle arrangement.

---

The Three States of Matter

All substances can exist as solids, liquids or gases depending on the temperature and pressure. The particle model helps explain the properties and behaviour of each state.

Particle Arrangements

Property	Solid	Liquid	Gas
Arrangement	Regular pattern (lattice)	Irregular, close together	Random, far apart
Movement	Vibrate about fixed positions	Move around each other	Move rapidly in all directions
Spacing	Very close together	Close together (slightly more spaced than solid)	Very far apart
Energy	Lowest	Medium	Highest
Density	Highest	Medium to high	Very low

graph LR
    A["SOLID<br/>• Fixed shape<br/>• Fixed volume<br/>• Particles vibrate<br/>in fixed positions"] -->|"Melting<br/>(energy in)"| B["LIQUID<br/>• No fixed shape<br/>• Fixed volume<br/>• Particles move<br/>around each other"]
    B -->|"Boiling / Evaporation<br/>(energy in)"| C["GAS<br/>• No fixed shape<br/>• No fixed volume<br/>• Particles move<br/>rapidly, far apart"]
    C -->|"Condensing<br/>(energy out)"| B
    B -->|"Freezing<br/>(energy out)"| A

    style A fill:#2980b9,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e67e22,color:#fff

---

Properties of Each State

Solids

• Have a fixed shape and a fixed volume.
• Particles are held in a regular arrangement by strong forces.
• Particles vibrate about fixed positions — they do not move from place to place.
• Cannot be compressed because particles are already close together.

Liquids

• Have no fixed shape (take the shape of their container) but a fixed volume.
• Particles are close together but in an irregular arrangement.
• Particles can move around each other — this is why liquids flow.
• Cannot be easily compressed.

Gases

• Have no fixed shape and no fixed volume (they spread out to fill their container).
• Particles are far apart with lots of space between them.
• Particles move rapidly in random directions, colliding with each other and with the walls of their container.
• Can be compressed easily because there is lots of space between particles.

Exam Tip: When describing gases, always mention that particles are "far apart" and "move rapidly in random directions." Both points are needed for full marks.

---

Changes of State

A change of state is a physical change — no new substances are formed and the process is reversible.

The State Changes

Change of State	Direction	Energy
Melting	Solid → Liquid	Energy is taken in (endothermic)
Boiling	Liquid → Gas	Energy is taken in (endothermic)
Evaporation	Liquid → Gas (at surface, below boiling point)	Energy is taken in
Condensation	Gas → Liquid	Energy is given out (exothermic)
Freezing	Liquid → Solid	Energy is given out (exothermic)
Sublimation	Solid → Gas (directly)	Energy is taken in

What Happens to the Particles During State Changes?

• During melting, particles gain enough energy to overcome some of the forces holding them in fixed positions — they begin to move around each other.
• During boiling, particles gain enough energy to completely overcome the forces of attraction between them — they escape as a gas.
• During freezing, particles lose energy and slow down until the forces of attraction pull them into a regular arrangement.

Exam Tip: During a change of state, the temperature stays constant even though energy is being supplied. This is because the energy is being used to overcome the forces between particles, not to increase their kinetic energy.

---

Heating Curves

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

Key Features of a Heating Curve

Section	What Happens	Temperature
Solid being heated	Particles vibrate faster	Temperature rises
Melting	Forces between particles partly overcome	Temperature stays constant (flat section)
Liquid being heated	Particles move faster	Temperature rises
Boiling	Forces between particles fully overcome	Temperature stays constant (flat section)
Gas being heated	Particles move even faster	Temperature rises

The flat sections represent changes of state — the energy is used to break intermolecular forces rather than increase temperature.

---

State Symbols

In chemical equations, the state of each substance is shown using state symbols:

Symbol	State
(s)	Solid
(l)	Liquid
(g)	Gas
(aq)	Aqueous (dissolved in water)

Example: NaCl(s) → NaCl(aq) when salt dissolves in water.

Exam Tip: You may be asked to add state symbols to equations. Remember that (aq) means dissolved in water — it is NOT a gas. Students often confuse (aq) with (g).

---

Summary

• Matter exists in three states: solid, liquid and gas.
• The particle model explains the properties of each state in terms of the arrangement, movement and spacing of particles.
• Changes of state are physical changes that are reversible — melting, boiling, evaporation, condensation, freezing and sublimation.
• Energy is taken in during melting, boiling and evaporation; energy is given out during condensation and freezing.
• During a change of state, the temperature remains constant as energy overcomes forces between particles.
• State symbols (s), (l), (g) and (aq) are used in equations to show the physical state of each substance.

---

Worked Example — Interpreting a Heating Curve

Imagine heating a block of ice from −20 °C until it has fully boiled into steam.