The Particle Model and States of Matter

Everything around you — the chair you sit on, the water you drink, the air you breathe — is made of tiny particles far too small to see. The particle model pictures all matter as built from these particles and uses their arrangement, spacing, motion and energy to explain the everyday behaviour of solids, liquids and gases. With this one model we can explain why a solid keeps its shape, why a liquid flows and takes the shape of its container, why a gas fills any space it is put in, and why gases can be squashed but solids and liquids cannot. This lesson, part of Topic P1 (Matter) of OCR Gateway Science A, sets out the model, uses it to explain the properties of each state, and is honest about the points where this simple model breaks down.

By the end of this lesson you should be able to describe the arrangement, spacing, motion and energy of particles in solids, liquids and gases, explain the properties of each state using the particle model, and state the limitations of the simple particle model.

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The Three States of Matter

Matter is usually found in one of three states: solid, liquid or gas. The particle model explains the differences between them entirely in terms of how the particles are arranged and how they move. The diagram below shows the three arrangements side by side.

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Particles in a Solid

In a solid, the particles are:

• Arrangement: packed closely together in a regular, fixed pattern (a lattice), all touching their neighbours.
• Spacing: very small — the particles are as close as they can be.
• Motion: the particles cannot move from place to place; they only vibrate about fixed positions.
• Energy: they have the least energy of the three states.

Because the particles are locked in position by strong forces of attraction, a solid has a fixed shape and a fixed volume, and it cannot be compressed (squashed) because there is almost no space between the particles. The vibrations get bigger as the solid is heated, but the particles stay in their lattice positions until the solid melts.

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Particles in a Liquid

In a liquid, the particles are:

• Arrangement: still close together and touching, but in a random, irregular arrangement with no fixed pattern.
• Spacing: small, similar to a solid (a liquid is only slightly less dense than its solid).
• Motion: the particles can move around and slide past one another, though they stay in contact.
• Energy: they have more energy than in a solid but less than in a gas.

Because the particles can move past one another, a liquid can flow and takes the shape of its container, yet because they are still close together it keeps a fixed volume and is almost incompressible. This ability to flow is why liquids (and gases) are together called fluids.

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Particles in a Gas

In a gas, the particles are:

• Arrangement: far apart with large empty spaces between them; completely random.
• Spacing: very large — typically the particles are about ten times further apart than in a liquid.
• Motion: the particles move quickly in all directions (constant random motion), colliding with each other and the walls of the container.
• Energy: they have the most energy of the three states.

Because the particles are far apart and free to move anywhere, a gas has no fixed shape and no fixed volume — it spreads out to fill any container completely. And because of the large gaps between particles, a gas can be compressed (squashed) easily by pushing the particles closer together.

Property	Solid	Liquid	Gas
Arrangement	Regular, fixed lattice	Random, close	Random, far apart
Spacing	Very close	Close	Very far apart
Motion	Vibrate in place	Slide past each other	Move fast in all directions
Energy	Least	Medium	Most
Shape	Fixed	Takes container's shape	Fills container
Volume	Fixed	Fixed	Not fixed
Compressible?	No	Almost no	Yes
Density	High	Slightly lower	Very low

Exam Tip: Learn the three columns as a block — arrangement, spacing, motion, energy — for each state. Almost every "explain the property" question is answered by quoting the right particle behaviour, e.g. "a gas can be compressed because there are large spaces between the particles".

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Explaining Properties with the Particle Model

The power of the particle model is that each everyday property follows directly from how the particles behave. It is worth practising the because statements:

• A solid keeps its shape because its particles are held in fixed positions by strong forces and can only vibrate.
• A liquid flows and takes the shape of its container because its particles can move and slide past one another.
• A gas fills its container because its particles move freely in all directions with large spaces between them.
• Gases are easily compressed but solids and liquids are not because gases have large gaps between particles to squash, while in solids and liquids the particles are already touching.
• Solids and liquids are much denser than gases because their particles are packed closely, whereas a gas has its particles spread thinly through the space.

Exam Tip: When you "explain" a property, name the particle feature and the consequence: "the particles are far apart (feature), so the gas can be compressed (consequence)". A bare statement of the feature without the consequence often loses the mark.

Worked Example — Explaining why a gas can be compressed but a liquid cannot

Use the particle model to explain why a gas can be squashed into a smaller volume but a liquid cannot.

Step 1 — describe the gas particles: in a gas the particles are far apart, with large empty spaces between them.