States of Matter

This lesson covers the three states of matter — solids, liquids and gases — as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You need to understand the arrangement, movement and energy of particles in each state, and how this explains the properties we observe. Being able to describe and compare all three states using the particle model is a fundamental skill that underpins much of chemistry.

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The Three States of Matter

All substances can exist in one of three states of matter: solid, liquid or gas. The state a substance is in at a given temperature depends on the strength of the forces between its particles and the amount of energy those particles possess.

Solid

In a solid:

• Particles are held in fixed positions in a regular arrangement (a lattice).
• Particles are very close together — they are touching their neighbours.
• Particles vibrate about their fixed positions but do not move from place to place.
• The forces of attraction between particles are strong and hold them firmly in place.

Because of this arrangement, solids have:

• A fixed shape — they do not flow to fill a container.
• A fixed volume — they cannot be easily compressed.
• A high density compared to gases (particles are closely packed).

Liquid

In a liquid:

• Particles are close together but arranged randomly (no regular pattern).
• Particles can move past each other — they slide and flow.
• Particles move at moderate speeds and have more energy than in a solid.
• The forces of attraction between particles are weaker than in a solid but still significant.

Because of this arrangement, liquids have:

• No fixed shape — they take the shape of the container they are in.
• A fixed volume — they cannot easily be compressed.
• A density that is usually slightly less than the solid form (water is a notable exception).

Gas

In a gas:

• Particles are far apart and arranged randomly.
• Particles move quickly in all directions.
• Particles have much more energy than in a solid or liquid.
• The forces of attraction between particles are very weak (essentially negligible at normal pressures).

Because of this arrangement, gases have:

• No fixed shape — they spread out to fill any container.
• No fixed volume — they expand to fill the available space.
• Low density compared to solids and liquids.
• They are easily compressible because there is so much space between the particles.

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Particle Diagrams

You may be asked to draw or interpret particle diagrams. Here is a text description of what each looks like:

• Solid: Particles drawn as circles packed closely together in neat, regular rows (like stacked oranges). All particles are touching. Draw them in a grid-like pattern.
• Liquid: Particles drawn as circles still mostly touching but in an irregular, random arrangement. There are small gaps, and the arrangement is not orderly.
• Gas: Particles drawn as circles with large, irregular spaces between them. The particles are spread far apart and scattered randomly throughout the container.

Exam Tip: When drawing particle diagrams, make sure you show the correct spacing. For solids, particles must touch in a regular pattern. For liquids, particles still mostly touch but are disordered. For gases, particles must be far apart. A common mistake is to draw liquid and gas particles too close together or to draw solid particles in a random pattern.

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Comparing the Three States

The table below summarises the key differences between solids, liquids and gases:

Property	Solid	Liquid	Gas
Particle arrangement	Regular, closely packed	Random, closely packed	Random, far apart
Particle movement	Vibrate about fixed positions	Move past each other (flow)	Move quickly in all directions
Energy of particles	Lowest	Moderate	Highest
Forces between particles	Strong	Moderate	Very weak
Shape	Fixed	Takes shape of container	Fills container
Volume	Fixed	Fixed	Variable (fills container)
Compressibility	Cannot be compressed	Cannot easily be compressed	Easily compressed
Density	High	High (usually slightly less than solid)	Low

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Explaining Properties Using the Particle Model

You should be able to use the particle model to explain everyday observations:

Why are solids hard and rigid?

The particles are held in fixed positions by strong forces of attraction. They can only vibrate and cannot move past each other, so the solid holds its shape.

Why do liquids flow?

The forces between particles in a liquid are weaker than in a solid. Particles can slide past each other, allowing the liquid to take the shape of its container.

Why are gases compressible?

Gas particles are far apart with large spaces between them. When pressure is applied, the particles can be pushed closer together, reducing the volume.

Why do gases have low density?

The particles are spread far apart, so there is less mass per unit volume compared to solids and liquids.

Why do gases exert pressure on the walls of their container?

Gas particles are constantly moving in random directions at high speed. When they collide with the walls of their container, they exert a force on the walls. The combined effect of many billions of collisions per second produces gas pressure.

Exam Tip: When explaining a property of a state of matter, always refer to the particles. Do not say "the solid is hard because it is a solid." Instead, explain it in terms of particle arrangement, forces, and movement. For example: "The solid is hard because its particles are held in fixed positions by strong forces of attraction and can only vibrate."

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State Symbols in Chemical Equations

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

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

For example:

CaCO\u2083(s) + 2HCl(aq) \u2192 CaCl\u2082(aq) + H\u2082O(l) + CO\u2082(g)

Exam Tip: Always include state symbols when asked to write a balanced equation. You lose marks if they are missing when the question asks for them. Remember that (aq) means "dissolved in water" — it is not the same as (l).

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Temperature and Kinetic Energy

The temperature of a substance is related to the average kinetic energy of its particles:

• At higher temperatures, particles have more kinetic energy and move faster.
• At lower temperatures, particles have less kinetic energy and move more slowly.
• At absolute zero (−0 K or −273 \u00b0C), particles have the minimum possible energy and essentially stop moving.

This is why heating a solid eventually causes it to melt (particles gain enough energy to break free of their fixed positions) and then boil (particles gain enough energy to escape the liquid entirely).

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Summary

• All matter exists as a solid, liquid or gas.
• The state depends on the arrangement, movement and energy of the particles.
• Solids have a fixed shape and volume; particles vibrate in fixed positions.
• Liquids have a fixed volume but no fixed shape; particles can slide past each other.
• Gases have no fixed shape or volume; particles move freely and are far apart.
• Gases are easily compressed because of the large spaces between particles.
• State symbols (s), (l), (g) and (aq) are used in chemical equations.
• Temperature is a measure of the average kinetic energy of particles.

Exam Tip: A common 6-mark question asks you to compare the three states of matter using the particle model. Structure your answer by discussing arrangement, movement, energy and forces for each state. Use the comparison table as a framework and make sure you refer to particles throughout — not just to the substance itself.