Introduction

Look around you. Everything you can see, touch, or hold is made of matter. The air you breathe, the water you drink, the desk you sit at, and even your own body are all forms of matter. Although these substances appear very different, scientists understand them using a single idea called the particle model of matter.

The particle model explains that all matter is made of tiny particles that are constantly moving. The way these particles are arranged and how they move determines whether a substance is a solid, liquid, or gas. This simple model helps explain many everyday observations, from ice melting in a drink to the smell of perfume spreading across a room.

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What Is Matter?

Matter is anything that:

• has mass
• occupies space (has volume)

Almost everything around us is matter.

Examples include:

• air
• water
• rocks
• metals
• plants
• animals
• plastic

Things that are not matter include:

• light
• sound
• heat
• electricity

These are forms of energy, not matter.

Definition:
Matter is anything that has mass and occupies space.

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The Particle Model of Matter

Scientists explain the behaviour of matter using the particle model.

The particle model states that:

• all matter is made of tiny particles,
• particles are constantly moving,
• there are spaces between particles,
• particles attract one another,
• heating increases particle movement,
• cooling decreases particle movement.

Although we cannot usually see individual particles, their behaviour explains many properties of matter.

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Solids

In a solid, particles are packed very closely together in a regular arrangement.

Properties of Solids

• fixed shape
• fixed volume
• particles vibrate about fixed positions
• strong attractive forces between particles
• difficult to compress

Examples include:

• ice
• wood
• iron
• glass

Because the particles cannot move past one another, solids keep their shape.

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Liquids

In a liquid, particles remain close together but are free to move around each other.

Properties of Liquids

• fixed volume
• no fixed shape
• flow easily
• slightly compressible
• particles move continuously

Examples include:

• water
• milk
• oil
• mercury

Liquids take the shape of their container because their particles can slide past one another.

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Gases

In a gas, particles are far apart and move rapidly in all directions.

Properties of Gases

• no fixed shape
• no fixed volume
• highly compressible
• particles move freely
• fill any container

Examples include:

• oxygen
• nitrogen
• carbon dioxide
• helium

Because gas particles are widely spaced, gases can expand easily.

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Comparing the Three States

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Changes of State

Matter can change from one state to another when energy is added or removed.

During these changes:

• particles themselves do not change,
• only their energy, spacing, and movement change.

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Particle Behaviour During Changes of State

Melting

When a solid is heated:

• particles gain energy,
• vibrations become stronger,
• attractive forces weaken,
• particles begin moving past one another.

The solid becomes a liquid.

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Boiling

Further heating causes:

• particles to gain even more energy,
• particles to overcome attractive forces,
• particles to spread far apart.

The liquid becomes a gas.

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Freezing and Condensation

Cooling removes energy.

Particles:

• move more slowly,
• come closer together,
• become more strongly attracted.

Liquids freeze into solids, and gases condense into liquids.

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Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

This occurs because particles are constantly moving.

Examples include:

• perfume spreading through a room,
• food colouring spreading through water,
• oxygen moving into the bloodstream,
• carbon dioxide leaving the lungs.

Diffusion occurs fastest in gases because gas particles move most rapidly.

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Everyday Example of Diffusion

If someone sprays perfume at one side of a room, people across the room can smell it a short time later.

Why?

The perfume particles move randomly through the air until they become evenly spread throughout the room.

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Expansion

Heating causes particles to move faster.

Although the particles themselves do not become larger, the spaces between them increase, causing most substances to expand.

Examples include:

• railway tracks with expansion gaps,
• bridges with expansion joints,
• hot air balloons rising,
• metal lids loosening when warmed.

Cooling has the opposite effect, causing particles to move more slowly and come closer together.

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Worked Example 1

A metal rod becomes slightly longer after being heated.

Question

Does each metal particle become larger?

Solution

No.

The particles remain the same size.

Heating increases their kinetic energy, causing them to vibrate more and move slightly farther apart.

This increases the overall length of the rod.

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Worked Example 2

A student notices that food colouring spreads through a glass of still water without stirring.

Question

Use the particle model to explain this observation.

Solution

Water particles and food-colouring particles are constantly moving.

The food-colouring particles move randomly from areas of high concentration to areas of low concentration until they are evenly distributed throughout the water.

This process is called diffusion.

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Real-World Connection

The particle model helps scientists and engineers design many everyday technologies. Refrigerators and air conditioners rely on changes of state to transfer heat, while aerosol sprays work because gases expand to fill available space. Engineers include expansion joints in bridges and railway tracks to allow materials to expand safely on hot days. Understanding particle behaviour is also essential in chemistry laboratories, medicine, food preservation, and manufacturing.

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Did You Know?

Scientists estimate that a single breath of air contains trillions upon trillions of particles. Because gas particles move continuously, some of the air you breathe today may contain particles once breathed by dinosaurs, ancient Egyptians, or famous historical figures. Over millions of years, atmospheric gases become thoroughly mixed through diffusion and global air circulation!

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Key Terms

• Matter — anything that has mass and occupies space.
• Particle model — a model describing matter as tiny particles in constant motion.
• Solid — a state of matter with fixed shape and fixed volume.
• Liquid — a state of matter with fixed volume but no fixed shape.
• Gas — a state of matter with neither fixed shape nor fixed volume.
• Diffusion — the movement of particles from high concentration to low concentration.
• Expansion — an increase in volume caused by particles moving farther apart.
• Change of state — the process by which matter changes between solid, liquid, and gas.

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Key Takeaways

• Matter is anything that has mass and occupies space.
• The particle model explains that all matter is made of tiny particles that are constantly moving.
• Solids, liquids, and gases differ because of the arrangement and movement of their particles.
• During changes of state, particles gain or lose energy, changing their movement and spacing without changing their identity.
• Diffusion and thermal expansion can both be explained using the particle model.
• The particle model provides the foundation for understanding many topics in chemistry, physics, biology, and everyday life.