Changes of state

SPEC 4.3.1.2 PARTICLE MODEL — CHANGES OF STATE

Six processes between three states. The defining principle: mass is conserved. Particles aren't created or destroyed — only rearranged.

Why This Lesson Earns Marks

Changes of state are tested as short-answer name-the-process questions, as multiple-choice on particle motion, and as PEE questions on conservation of mass. Each appears at least once across the paper series.

3

States Of Matter

6

Named Processes

m

Mass Conserved

By The End

Recognise the three states from particle diagrams. Name the spec-required processes (melt, freeze, boil, evaporate, condense, sublimate). Distinguish boiling from evaporation. Explain mass conservation using the particle model. Compare particle arrangement and motion across two states in 6-mark style.

What A Change Of State Is

When ice melts, the substance is still water — just in a different form. Nothing new has been made. That's the essence of a change of state.

Definition [DEF]

A change of state is a physical change in which a substance changes between the solid, liquid and gas states. The particles themselves are unchanged — only their arrangement and motion change.

Physical, Not Chemical — Spec Wording

Changes of state are physical changes that differ from chemical changes because "the material recovers its original properties if the change is reversed" (AQA spec phrasing). No new substance is made — freezing the water gives back the exact same substance.

The Three States — Recap

You met these in 3.01. The arrangement of particles is what makes each state different.

Mark-Scheme Phrasing

Solid: particles vibrate about fixed positions in a regular arrangement. Liquid: particles can slide past each other but stay close. Gas: particles move in random directions at a range of speeds, far apart.

The Named Processes

AQA spec 4.3.1.2 names six verbs: melt, freeze, boil, evaporate, condense, sublimate. All are spec-required. Deposition (gas→solid) is included here as enrichment but is not in the AQA spec.

Melting

SOLID → LIQUID

Freezing

LIQUID → SOLID

Boiling / Evaporation

LIQUID → GAS

Condensing

GAS → LIQUID

Sublimation

SOLID → GAS

Deposition Beyond Spec

GAS → SOLID

Examples

Sublimation: dry ice (solid CO₂) turns straight to gas at room temperature. Deposition (extra context only): frost forming on a cold window is water vapour going directly to solid ice without becoming liquid first.

State Change Flow

The complete map. Solid lines = AQA-required processes. The dashed deposition arc is included for completeness but is not in the AQA spec.

Boiling vs Evaporation — Subtle But Tested

Both are liquid → gas. But they happen under different conditions. Mixing these up costs marks on short-answer questions.

BoilingEvaporation

Where?Throughout the liquid (bubbles form inside)Only at the surface of the liquid

Temperature?At a fixed boiling point (100°C for water at atmospheric pressure)At any temperature below boiling point

Speed?Fast — all the liquid can vaporiseSlow — only the fastest particles escape

Why Evaporation Cools

Only the fastest-moving particles have enough energy to escape from the surface. When they leave, the average kinetic energy of the remaining particles is lower — so the liquid cools down. That's why sweat works.

Mass Is Conserved

This is the headline rule of changes of state — and the most common exam question on this topic.

Key Principle

Mass is conserved during a change of state. The total mass of substance before equals the total mass after. 100 g of ice melts to give exactly 100 g of water.

Why It's True (Particle Model)

The number of particles stays the same — particles aren't created or destroyed in a state change. Each particle has the same mass before and after. So total mass (= particle count × mass per particle) is unchanged.

Common Misconception

"The water boiled away" suggests the water vanished. It didn't. It became water vapour — same mass of substance, just dispersed into the air. If you could capture every escaping molecule, the total mass would be exactly what you started with.

PEE Worked Example

A 50 g ice cube is left in a sealed container. After it fully melts, the container is reweighed. Using the particle model, explain why the mass of water in the container is also 50 g. 3 marks

P

Point: The mass stays at 50 g because mass is conserved during a change of state.

E

Evidence: The same water molecules that made up the ice are now in the liquid water — no particles have been created or destroyed.

E

Explain: Only the arrangement and motion of the particles have changed; the number of particles, and so the total mass, stays the same. (The container being sealed means no particles escape to the surroundings.)

Recap From 3.01 — Density Changes

Synoptic recap (not new 4.3.1.2 content): mass stays the same during a change of state. But the volume changes — and that means the density changes too. This is the bridge from 3.01.

ChangeWhat Happens to V?What Happens to ?

Solid → Liquid (most substances)V slightly increasesslightly decreases

Liquid → GasV increases massively (~1000×)drops massively

Water — the exceptionIce has larger V than liquid waterIce floats on water

Why Ice Floats

Water is the unusual substance whose solid state is less dense than its liquid state. When water freezes, the molecules arrange into an open hexagonal structure that takes up more space — the same mass occupies a larger volume, so density falls. That's why icebergs float and frozen pipes burst.

Examiner Alert

Describing Gas Particle Motion

"Particles move fast" or "particles bounce around" loses marks. The mark-scheme phrases are: particles move in random directions, at a range of (different) speeds, and collide with each other and the container walls. Avoid "predictable", "same speed", or "circular paths".

Mass Conservation Wording

Don't write "the ice disappears" or "the water turns into steam and goes away". Examiners want the particle-count chain: same particles, same number, same total mass — only the arrangement and motion differ.

Spec Bridge

This lesson set up the conservation principle. The next lesson (3.03) takes the next step: what happens to the energy during a state change? Spoiler: it goes into the particles' arrangement, not their motion — which is why temperature stops rising at a melting or boiling point.

6-Mark Frame — Compare Solid vs Liquid

A recurring 6-mark question asks you to compare particle arrangement AND motion in two states. Only 9% of students reach Level 3 (5–6 marks). The trick: cover both states on both arrangement and motion — that gives Level 2 minimum. Level 3 adds connection to mass & volume.

AspectSolidLiquid

ArrangementParticles in a regular pattern , very close together, touchingParticles still close together but with no fixed pattern (disordered)

MotionVibrate about fixed positions — cannot move past each otherParticles can slide past each other — able to flow

Macroscopic linkHas a fixed shape and volume ; high densityTakes the shape of its container ; volume fixed; density similar to solid

Writing The Answer

Examiners want a side-by-side comparison, not separate descriptions. Use linking words: "In contrast", "whereas", "while in a liquid". Cover both states for each point. Level 1: one state only. Level 2: both states compared on arrangement and motion. Level 3: also link to macroscopic properties or mass conservation.

Weak vs Strong — "Why Does Mass Stay The Same?"

Same 3-mark question. Same physics. Very different marks.

Weak — 1/3

"When the ice melts it just turns into water. Nothing is added or taken away, so the mass is the same."

Why it loses: Restates the result. No particle-model reasoning. "Nothing is added or taken away" describes the outcome, not the mechanism.

Strong — 3/3

P: Mass is conserved during a change of state. E: The same particles that made up the ice are now in the liquid water — no particles are created or destroyed. E: Only the arrangement and motion of the particles change, so the total number of particles — and therefore the total mass — stays the same.

Practice 1 — Name The Process

4 marks

1

Water droplets form on the outside of a cold glass.

2

A puddle on the pavement disappears overnight, even though it never reached 100°C.

3

A block of dry ice (frozen CO₂) shrinks at room temperature without leaving any liquid.

4

Molten iron poured into a mould cools into a solid casting.

1

Condensing — gas (water vapour in the air) → liquid (droplets on the glass).

2

Evaporation — liquid → gas, occurring at the surface and below the boiling point.

3

Sublimation — solid → gas directly, no liquid stage.

4

Freezing (or solidification) — liquid → solid.

Practice 2 — PEE Answer

A student boils 200 g of water in a kettle. After all the water has boiled away, the kettle is dry. Using the particle model, explain whether the 200 g of water has been destroyed. 3 marks

P

Point: The 200 g of water has not been destroyed — mass is conserved during a change of state.

E

Evidence: The water has changed state from liquid to gas (water vapour) by boiling. The same water molecules now exist in the air around the kettle.

E

Explain: No particles have been created or destroyed — only their arrangement has changed (close together in the liquid, spread far apart as a gas). If all the vapour could be collected and condensed, its mass would be exactly 200 g.

Bridge — What About The Energy?

You've now mastered which changes happen and the principle that mass is conserved. The next question is about energy.

The Puzzle

Heat a beaker of ice. The temperature rises — until it reaches 0°C. Then, as the ice melts, the temperature stops rising. Energy is still going in (you haven't turned off the heat), but the thermometer doesn't move. Where is the energy going?

Next Lesson Preview

The answer is internal energy — the total of all the particles' kinetic and potential energies. During a state change, the energy goes into rearranging the particles (breaking bonds), not into making them move faster. We meet this in 3.03.

Checkpoint — You Can Now

Knowledge

Define a change of state as a physical (reversible) change with unchanged particles.

Name the AQA spec processes: melt, freeze, boil, evaporate, condense, sublimate . (Deposition is enrichment, not spec-required.)

State that mass is conserved during any change of state.

Skills

Identify changes of state from everyday scenarios.

Distinguish boiling from evaporation .

Describe gas particle motion in mark-scheme language: random directions, range of speeds .

Write a PEE answer using the particle model to explain mass conservation.

Link state changes to density (m same, V changes → changes).