Particulate Nature of Matter

Chemistry • Secondary School

1. Introduction

Everything around us is made of matter. The desk you are sitting at, the water you drink, the air you breathe and the materials used to make buildings are all forms of matter.

The particulate nature of matter explains that matter is not continuous. Instead, it is made up of extremely small particles. These particles are constantly moving and interact with one another.

Definition: The particulate nature of matter is the idea that all matter is made up of tiny particles that are in continuous motion.

Learning objectives

By the end of this topic, you should be able to:

Explain the particle theory of matter.
Describe the arrangement of particles in solids, liquids and gases.
Compare the movement of particles in different states.
Explain changes of state using the particle model.
Explain diffusion and Brownian motion.
Interpret simple particle diagrams.

2. Particle Theory of Matter

The particle theory can be summarised by the following ideas:

Matter is made up of very small particles.
There are spaces between the particles.
The particles are constantly moving.
Particles attract one another.
Heating a substance increases the kinetic energy of its particles.

Key idea: The differences between solids, liquids and gases are mainly due to the arrangement, separation and movement of their particles.

3. States of Matter

3.1 Solids

In a solid, particles are packed closely together and held by strong forces of attraction. They cannot move freely from one position to another but vibrate about fixed positions.

Fixed shape.
Fixed volume.
Particles are closely packed.
Strong attractive forces.
Particles vibrate about fixed positions.
Very difficult to compress.

3.2 Liquids

In a liquid, particles are still relatively close together, but the attractive forces allow particles to move past one another.

No fixed shape.
Fixed volume.
Particles are close together.
Particles can move around each other.
Can flow.
Only slightly compressible.

3.3 Gases

Gas particles are widely separated and move rapidly in random directions.

No fixed shape.
No fixed volume.
Particles are far apart.
Weak attractive forces compared with solids and liquids.
Particles move rapidly and randomly.
Highly compressible.

Property	Solid	Liquid	Gas
Shape	Fixed	Not fixed	Not fixed
Volume	Fixed	Fixed	Not fixed
Particle spacing	Very small	Small	Large
Particle movement	Vibration	Movement past one another	Rapid random movement
Compressibility	Very low	Very low	High

4. Changes of State

When matter is heated or cooled, its particles gain or lose kinetic energy. This can cause a change of state.

Change	From	To
Melting	Solid	Liquid
Freezing	Liquid	Solid
Boiling	Liquid	Gas
Evaporation	Liquid	Gas
Condensation	Gas	Liquid
Sublimation	Solid	Gas
Deposition	Gas	Solid

Important: During a change of state, the substance itself does not become a new chemical substance. The physical arrangement and energy of its particles change.

5. Diffusion

Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration until the particles become more evenly distributed.

Diffusion occurs because particles are constantly moving randomly.

Example

If a bottle containing perfume is opened in one corner of a room, the smell eventually reaches other parts of the room. Perfume particles move through the air by random motion.

Examination focus: Gas particles generally diffuse faster than liquid particles because gas particles have more space between them and can move more freely.

6. Factors Affecting Diffusion

Temperature

Increasing temperature gives particles more kinetic energy. They move faster, so diffusion becomes faster.

Particle mass

At the same temperature, lighter gas particles generally move faster than heavier gas particles.

Concentration difference

A larger concentration difference generally produces a faster initial rate of diffusion.

7. Brownian Motion

Brownian motion: The continuous random movement of small particles suspended in a fluid caused by collisions with the rapidly moving particles of the fluid.

Brownian motion provides evidence that particles in fluids are constantly moving.

Example: Tiny smoke particles observed under a microscope may appear to move in an irregular, zigzag pattern because they are continuously hit by moving air molecules.

8. Evidence for the Particulate Nature of Matter

Important evidence includes:

Diffusion.
Brownian motion.
Compression of gases.
Expansion of substances when heated.
Changes of state.
Mixing of substances without mechanical stirring.

MANEB-style point: When asked for evidence that matter is particulate, do not simply write “matter is made of particles.” Give an observable phenomenon, such as diffusion or Brownian motion, and explain how it supports the particle model.

9. Worked Examples

Example 1: Explain why a gas can be compressed much more easily than a liquid.

Solution:

Gas particles are separated by relatively large spaces. When pressure is applied, the particles can be forced closer together. Liquid particles are already close together, so there is much less empty space available for compression.

Example 2: Explain why diffusion becomes faster when temperature increases.

Solution:

Heating increases the kinetic energy of particles. The particles move faster and therefore spread through the medium more quickly.

10. MANEB Examination Focus

Be prepared to:

Compare solids, liquids and gases.
Draw and label particle diagrams.
Explain diffusion using particle movement.
Explain Brownian motion.
Explain changes of state.
Explain the effect of heating on particles.
Explain why gases are compressible.
Distinguish evaporation from boiling.

Common mistake: Do not say that particles “expand” when a substance is heated. Usually the particles themselves do not become larger; their average separation and movement change.

11. Practice Exercises

1. Define matter.

2. State four ideas of the particle theory.

3. Give three differences between solids and gases.

4. Explain why liquids can flow.

5. Define diffusion.

6. Explain why diffusion is faster at higher temperatures.

7. What is Brownian motion?

8. Explain why gases are highly compressible.

9. Distinguish evaporation from boiling.

10. Explain what happens to particles when a solid is heated until it melts.

Answers

Matter is anything that has mass and occupies space.
Examples: matter consists of particles; particles are in constant motion; there are spaces between particles; particles attract one another.
Solids have fixed shape while gases do not; solids have fixed volume while gases do not; solid particles are closely packed while gas particles are widely separated.
The particles in a liquid can move past one another.
The net movement of particles from higher concentration to lower concentration.
Particles gain kinetic energy and move faster.
Random movement of small suspended particles caused by collisions with moving fluid particles.
There are large spaces between gas particles that can be reduced when pressure is applied.
Evaporation occurs at the surface and can occur below the boiling point; boiling occurs throughout the liquid at its boiling point.
The particles gain kinetic energy and eventually overcome enough attractive forces to move past one another as a liquid.

12. Examination Tips

Use particle language when explaining physical properties.
Always distinguish particle movement, spacing and attraction.
For “explain” questions, give a cause and its effect.
Use labelled diagrams where appropriate.
Do not confuse evaporation with boiling.
When comparing states, mention more than one property.

13. Topic Summary

Matter consists of tiny particles that are continuously moving. Particles in solids are closely packed and mainly vibrate. Particles in liquids are close together but can move past one another. Gas particles are widely separated and move rapidly and randomly.

Changes in temperature alter particle kinetic energy and can cause changes of state. Diffusion and Brownian motion provide important evidence for the particulate nature of matter.