The Particle Model — Solids, Liquids and Gases

This lesson covers the particle model of matter, a fundamental part of the AQA GCSE Combined Science Trilogy specification (8464, section 6.3). You will learn how the arrangement, movement and energy of particles differ in solids, liquids and gases, and how the particle model explains the properties of each state.

The Three States of Matter

All matter exists in one of three states: solid, liquid or gas. The particle model is used to explain the properties of each state.

Property	Solid	Liquid	Gas
Arrangement	Regular, closely packed pattern	Close together but irregular (no fixed pattern)	Far apart, randomly arranged
Movement	Vibrate about fixed positions	Move around each other (slide/flow)	Move randomly at high speeds in all directions
Forces between particles	Strong	Moderate	Very weak (almost none)
Shape	Fixed	Takes the shape of the container	Fills the container
Volume	Fixed	Fixed	Expands to fill the container
Can be compressed?	No	Almost not	Yes, easily

Particle Diagrams

graph LR
    subgraph Solid["Solid"]
        S1["● ● ● ●"]
        S2["● ● ● ●"]
        S3["● ● ● ●"]
    end
    subgraph Liquid["Liquid"]
        L1["●  ●  ●"]
        L2[" ●  ●  ●"]
        L3["●  ●  ●"]
    end
    subgraph Gas["Gas"]
        G1["●      ●"]
        G2["   ●      "]
        G3["●      ●"]
    end

    style Solid fill:#3498db,color:#fff
    style Liquid fill:#2ecc71,color:#fff
    style Gas fill:#e74c3c,color:#fff

In a solid, particles are packed closely in a regular arrangement and vibrate about fixed positions. The strong forces of attraction hold them in place.
In a liquid, particles are close together but arranged irregularly. They can move past one another, which is why liquids flow and take the shape of their container.
In a gas, particles are far apart with almost no forces between them. They move at high speeds in all directions, which is why gases spread out to fill any container and can be compressed.

Energy and the Particle Model

The particles in all substances have kinetic energy (energy of movement). The amount of kinetic energy depends on the temperature:

Higher temperature → particles have more kinetic energy → they move faster (or vibrate more vigorously).
Lower temperature → particles have less kinetic energy → they move more slowly.

Temperature is related to the average kinetic energy of the particles. If two objects are at the same temperature, their particles have the same average kinetic energy.

Exam Tip: A very common mistake is to say that particles in a gas are "bigger" or "heavier" than those in a solid. The particles are the same — what changes is their spacing, arrangement and energy. AQA examiners specifically penalise answers that confuse spacing with particle size.

Why Solids Have a Fixed Shape

In a solid, the particles are held in place by strong forces of attraction. The particles can only vibrate about their fixed positions — they do not have enough energy to break free. This is why solids keep their shape and cannot be compressed.

Why Liquids Flow

In a liquid, the particles have enough energy to overcome some of the forces of attraction. They can move past each other while remaining close together. This explains why liquids:

Flow and take the shape of their container
Have a fixed volume (particles are still close together)
Cannot be easily compressed

Why Gases Fill Their Container

In a gas, the particles have enough energy to completely overcome the forces of attraction. They move randomly at high speeds in all directions. This explains why gases:

Spread out to fill any container
Can be compressed (large spaces between particles)
Exert pressure on the walls of their container (particles collide with the walls)

The Particle Model and Physical Properties

Physical property	Explanation using the particle model
Solids are hard to compress	Particles are already close together with no space to move closer
Liquids flow	Particles can slide past each other
Gases are easy to compress	Large gaps between particles allow them to be pushed closer together
Solids have the highest density	Particles are packed most closely together — most mass per unit volume
Gases have the lowest density	Particles are far apart — least mass per unit volume
Heating a gas increases its pressure	Particles gain kinetic energy, move faster, collide with walls more frequently and with greater force

Limitations of the Particle Model

The simple particle model is useful but has limitations:

It assumes all particles are identical spheres — in reality, atoms and molecules have different shapes and sizes.
It does not explain the exact nature of the forces between particles (intermolecular forces).
It does not account for the size of particles relative to the spacing between them accurately.
It oversimplifies the behaviour of real substances, especially near changes of state.

Exam Tip: AQA may ask you to evaluate the particle model. Acknowledge that it is a simplified model that helps explain observations, but has limitations — this shows higher-level thinking.

Common Misconceptions

Misconception	Correction
Gas particles are bigger than solid particles	Particles are the same size — they are just further apart in a gas
Particles expand when heated	Particles do not expand — they move faster and the spacing between them increases
There is "air" or "nothing" between gas particles	Between gas particles there is empty space (a vacuum at the molecular level)
Heating always increases temperature	Not during a change of state — energy goes into breaking bonds, not raising temperature

Summary

Matter exists in three states: solid, liquid and gas.
The particle model describes the arrangement, movement and energy of particles in each state.
In solids, particles are in a regular arrangement and vibrate about fixed positions.
In liquids, particles are close but irregular and can slide past each other.
In gases, particles are far apart and move randomly at high speed.
Temperature is related to the average kinetic energy of particles.
The particle model explains many physical properties but is a simplified representation of reality.