GCSE · Chemistry · AQA · Spec 8462

Three states of matter and particle theory

The ice in your drink and the steam from a kettle are made of exactly the same particles. So what's actually different?

Turn up the heat — then turn it back down

Temperature: -30 °C. State: Solid. static

Temperature-30 °C

Solid. The particles sit in a regular pattern. They vibrate, but each one stays in its own fixed position. Warm it up and watch them vibrate harder.

An example substance that melts at 40 °C and boils at 120 °C. Drag the temperature up, then back down, and watch the same particles all the way.

What's really melting?

An ice cube melts into a puddle

An ice cube sits on a warm plate and slowly melts into a puddle of water.

Which is closest to what you think happens to the particles?

The particles themselves melt. Each one goes soft and runny.The particles get bigger as they warm up, so they push each other apart and the ice falls apart.The particles stay the same. They move more and break out of their fixed positions, so they can move around each other.Each ice particle is cold and hard. When it melts, each particle becomes warm and wet.

How sure are you?

I'm sureI think soI'm guessing

The particles themselves melt. Each one goes soft and runny.

This is a common misconception

Tempting, because the whole ice cube goes runny. But 'runny' isn't something one particle can be. A liquid flows because huge numbers of particles can move around each other. Each particle stays exactly the same; only their arrangement and movement change.

The particles get bigger as they warm up, so they push each other apart and the ice falls apart.

This is a common misconception

The particles don't grow. In the model at the top of the page, the same particles kept the same size the whole way from solid to gas. What changed was how they were arranged and how they moved.

The particles stay the same. They move more and break out of their fixed positions, so they can move around each other.

This matches the evidence

Exactly. Nothing happens to the particles themselves. Melting is a change in arrangement and movement: the same particles, now free to move around each other instead of staying in fixed positions.

Each ice particle is cold and hard. When it melts, each particle becomes warm and wet.

This is a common misconception

Cold, hard and wet describe the whole lump of ice or the puddle — lots of particles together. A single particle isn't hard or wet the way the ice cube is. Those are bulk properties, and they come from how huge numbers of particles are arranged and move.

Atoms themselves don't have the bulk properties of the material they make. Hardness, runniness and 'being a liquid' belong to huge numbers of particles together — how they're arranged and how they move.

Different substances, different temperatures

Reason it through

Why does one substance melt or boil at a much higher temperature than another?

Link 1 of 4

First link · your turn

Start with the particles. What decides how strongly they hold on to each other?

Locked — reveal the link above first

Higher

Where the simple model breaks down

The simple particle modelvsReal particles

Start with the missing forces. It's the limitation that matters most for changes of state.

Focus

Forces between particles

The simple particle model

None. The spheres sit next to each other or move about, and nothing pulls them together.

Real particles

There are forces between the particles, and how strong they are depends on the substance.

The insight

With no forces, the model can't explain why energy is needed to melt or boil a substance, or why different substances melt and boil at different temperatures.

Shape

The simple particle model

Every particle is drawn as a sphere.

Real particles

Real particles are not all spheres.

Solid, inelastic spheres

The simple particle model

Each sphere is solid and inelastic — a tiny hard ball that can't be squashed or change shape.

Real particles

Real particles are not tiny solid balls.

Predict the state

Solid, liquid or gas?

Pick a substance, then choose the state it is in at this temperature.

What state is each substance in at 20 °C?

Still to sort

Solid (0)

The temperature is below the melting point.

Where the line is: The melting point is the line between solid and liquid.

Liquid (0)

The temperature is between the melting point and the boiling point.

Where the line is: A liquid sits between two lines: above its melting point, below its boiling point.

Gas (0)

The temperature is above the boiling point.

Where the line is: The boiling point is the line between liquid and gas.

5 of 5 still to sort.

You don't need to see a substance to know its state. Compare the temperature with its melting point and boiling point.

Watch out: Watch the minus signs. −150 °C is much colder than 20 °C, so a substance that boils at −150 °C is already a gas at 20 °C.

GCSE exam practice

Explain it in your own words

Substance P melts at 650 °C. Substance Q melts at −20 °C. Explain why P has a much higher melting point than Q. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The forces between the particles in P are stronger than the forces between the particles in Q. (1 mark)
More energy is needed to overcome stronger forces. (1 mark)
So more energy has to be transferred to P, meaning it must be heated to a higher temperature before its particles can leave their fixed positions. (1 mark)

Model answer

The forces between the particles in P are much stronger than the forces between the particles in Q. Stronger forces need more energy to overcome, so more energy has to be transferred to P before its particles can break out of their fixed positions. That means P has to be heated to a much higher temperature before it melts.

Three links: stronger forces → more energy needed to overcome them → higher temperature before it melts. Name the substance with the stronger forces — 'it has strong forces' on its own doesn't compare the two.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why things melt, boil, freeze and condense — and why every substance does it at its own temperature.

What you need to know

Describe what happens to the particles when a substance melts, boils, freezes or condenses.
Say where each change of state happens: at the melting point or the boiling point.
Explain why stronger forces between particles mean higher melting and boiling points.
Predict the state of a substance at a given temperature from its melting and boiling points.
Recognise that single atoms don't have the bulk properties of a material.
Higher tier: explain the limitations of the simple particle model.

The big picture

Solids, liquids and gases are made of particles that are arranged and move differently. Melting and freezing happen at the melting point; boiling and condensing happen at the boiling point. The stronger the forces between a substance's particles, the more energy is needed to overcome them, so the higher its melting and boiling points.

Key points

1The three states of matter are solid, liquid and gas.

2In the simple particle model, particles are shown as small solid spheres.

3Melting and freezing happen at the melting point. Boiling and condensing happen at the boiling point.

4Energy is transferred to a substance to melt or boil it, and from a substance when it freezes or condenses.

5The energy needed to change state depends on the strength of the forces between the particles, which depends on the type of bonding and structure.

6The stronger the forces between the particles, the higher the melting point and boiling point.

7Atoms themselves don't have the bulk properties of a material, such as being hard or runny.

8Higher tier: the simple model has no forces, shows every particle as a sphere, and treats the spheres as solid and inelastic.

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Worked example

Problem

Three substances have these melting points: F melts at 1500 °C, G melts at −30 °C and H melts at 210 °C. Put them in order of the strength of the forces between their particles, weakest first.

⚠ Watch out

Thinking freezing and condensing happen at their own separate temperatures. A substance freezes at its melting point and condenses at its boiling point — the same temperatures where it melts and boils, just in the other direction.

🧠

Memory hook

Same particles, new arrangement. Melt and freeze meet at the melting point; boil and condense meet at the boiling point. Stronger grip, higher temperature.

✓

Check yourself

Without looking back: a gas is cooled until it turns into a liquid. What is this change of state called, at which temperature does it happen, and is energy transferred to or from the substance?

Flashcards

(12)

Name the three states of matter.

Solid, liquid and gas.

How are the particles arranged and moving in a solid, a liquid and a gas?

Solid: regular pattern, vibrating in fixed positions. Liquid: close together and touching, moving around each other. Gas: far apart, moving quickly in all directions.

In the simple particle model, how are particles represented?

As small solid spheres.

Which changes of state happen at the melting point, and which at the boiling point?

Melting point: melting and freezing. Boiling point: boiling and condensing.

Which changes of state need energy transferred to the substance, and which transfer energy from it?

To it: melting and boiling. From it: freezing and condensing.

What does the amount of energy needed to melt or boil a substance depend on?

The strength of the forces between its particles.

What decides how strong the forces between a substance's particles are?

The type of bonding and the structure of the substance.

Why does a substance with stronger forces between its particles have a higher boiling point?

More energy is needed to overcome the stronger forces, so it has to be heated to a higher temperature before it boils.

How do you use a melting point and boiling point to predict a substance's state?

Below the melting point: solid. Between the melting and boiling points: liquid. Above the boiling point: gas.

Is a single atom of a metal hard and shiny like the lump of metal?

No. Bulk properties like hardness come from huge numbers of particles together — how they're arranged and move — not from single atoms.

Higher tier: What is the most important limitation of the simple particle model for explaining changes of state?

It has no forces between the particles, so it can't explain why energy is needed to change state, or why substances change state at different temperatures.

Higher tier: Apart from having no forces, what else is unrealistic about the particles in the simple model?

Every particle is shown as a sphere, and the spheres are solid and inelastic — real particles are not like that.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Turn up the heat — then turn it back down

An ice cube melts into a puddle

Solid, liquid or gas?

Explain it in your own words

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