States of Matter and Changes of State: KS3 Science

Matter exists in three main states — solid, liquid, and gas — which differ in how particles are arranged and how much they move. Changes of state occur when energy is added or removed, and the process can be explained using the particle model. This is a core topic in KS3 chemistry, typically taught in Year 7.

What are the three states of matter?

Solid

In a solid, particles are packed closely together in a regular, ordered arrangement. They vibrate in fixed positions but cannot move past one another. This gives solids a fixed shape and a fixed volume. Solids are not easily compressed.

Liquid

In a liquid, particles are close together but arranged randomly. They can move past one another, which is why liquids can flow and take the shape of their container while keeping a fixed volume. Liquids are also not easily compressed.

Gas

In a gas, particles are spread far apart and move rapidly and randomly in all directions. Gases have no fixed shape and no fixed volume — they fill any container they are placed in. Gases are easily compressed because of the large spaces between particles.

Property | Solid | Liquid | Gas
Particle arrangement | Regular, close | Random, close | Random, far apart
Particle movement | Vibrate in place | Move past each other | Move rapidly, freely
Shape | Fixed | Takes shape of container | Fills container
Volume | Fixed | Fixed | Not fixed
Compressibility | Very low | Very low | High

What is the particle model of matter?

The particle model (also called the kinetic theory of matter) explains the properties of solids, liquids, and gases using the behaviour of particles:

All matter is made of tiny particles (atoms or molecules).

Particles are always in motion.

The temperature of a substance is related to the average kinetic energy (speed) of its particles.

There are attractive forces between particles, strongest in solids and weakest in gases.

The particle model is a simplification — it treats particles as small, hard spheres — but it is powerful enough to explain changes of state and many physical properties at KS3 level.

What are changes of state?

A change of state happens when a substance changes from one state to another due to a change in temperature. Changes of state are physical changes, not chemical changes — the substance itself does not change, only the arrangement of its particles.

The six changes of state

Change | Direction | Name
Solid → Liquid | Adding energy | Melting
Liquid → Solid | Removing energy | Freezing / solidification
Liquid → Gas | Adding energy | Evaporation or boiling
Gas → Liquid | Removing energy | Condensation
Solid → Gas | Adding energy | Sublimation
Gas → Solid | Removing energy | Deposition

Sublimation is worth knowing: some substances turn directly from solid to gas without passing through the liquid state. Dry ice (solid carbon dioxide) sublimates at −78.5°C at atmospheric pressure, producing a fog of condensed water vapour often seen at events.

A worked example: heating water from ice to steam

Consider 100 g of ice starting at −10°C being heated steadily. This is a classic KS3 heating curve experiment.

Stage 1: Ice warms (−10°C to 0°C) The temperature rises as particles in the ice vibrate faster — their kinetic energy increases.

Stage 2: Melting (at 0°C) The temperature stays constant at 0°C even though heat is still being added. The energy goes into breaking some of the attractive forces between particles, not into increasing their speed. This is the melting point of water. At 0°C, solid ice and liquid water can coexist.

Stage 3: Liquid water warms (0°C to 100°C) The temperature rises again as particles in the liquid gain kinetic energy and move faster.

Stage 4: Boiling (at 100°C) The temperature stays constant at 100°C. Energy goes into fully breaking the attractive forces between particles so they can escape as gas. This is the boiling point of water. At 100°C, liquid water and steam (water vapour) coexist.

Stage 5: Steam warms (above 100°C) The temperature of the steam rises as gas particles move faster still.

The flat sections of the heating curve at 0°C and 100°C are a key feature to recognise and explain in KS3 exams.

What is the difference between evaporation and boiling?

Both involve liquid turning to gas, but they differ:

Evaporation happens at the surface of a liquid, at temperatures below the boiling point. Faster-moving particles at the surface escape into the air. This is why puddles dry up on a warm day without reaching 100°C.

Boiling happens throughout the liquid, at a specific boiling point (100°C for water at standard pressure). Bubbles of gas form within the liquid itself.

Evaporation causes cooling: the faster particles escape, leaving behind the slower (cooler) ones. This is why sweating cools the body — evaporation of water from the skin removes heat.

Melting and boiling points as evidence for the particle model

The melting point and boiling point of a pure substance are fixed and characteristic — they can be used to identify it. For example, the melting point of pure ice is exactly 0°C and the boiling point of pure water is exactly 100°C (at sea level). Impurities raise the boiling point and lower the melting point — this is why salt is spread on roads in winter (it lowers the freezing point of water, preventing ice).

According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should understand the particle model and use it to explain changes of state and the properties of the three states. BBC Bitesize KS3 Chemistry notes that changes of state are physical changes and that the temperature stays constant during a change of state because energy is used to break particle bonds rather than increase temperature.

Frequently asked questions

What are the three states of matter?

The three states of matter are solid, liquid, and gas. In a solid, particles are in a fixed, ordered arrangement and vibrate in place. In a liquid, particles are close together but move past each other. In a gas, particles are spread far apart and move rapidly in all directions.

What happens to particles when a solid melts?

When a solid melts, energy is transferred to the particles, increasing their vibration until the attractive forces holding them in fixed positions are overcome. Particles then move past one another — the solid becomes a liquid. The temperature stays constant at the melting point until all the solid has melted.

Why does temperature stay constant during a change of state?

During a change of state, energy input goes into breaking or forming the attractive forces between particles rather than increasing their kinetic energy. Since temperature measures average kinetic energy, it stays flat until the change of state is complete.

What is sublimation?

Sublimation is when a solid changes directly into a gas without passing through the liquid state. Dry ice (solid carbon dioxide) sublimates at −78.5°C. Iodine also sublimates when heated gently. The reverse process — gas turning directly to solid — is called deposition.

How is a change of state different from a chemical change?

A change of state is a physical change: the substance stays the same, only the arrangement of its particles changes. For example, liquid water and ice are both H₂O. A chemical change produces a new substance with different properties — for example, burning magnesium in oxygen produces magnesium oxide, which is a different substance.

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