Particles and Changes of State

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 5.8 — Why heating a system changes its stored energy and raises temperature or changes state (bold P statement — Paper 2 only)
• 5.9 — The changes that occur when a solid melts and when a liquid evaporates or boils (bold P statement — Paper 2 only)
• 5.10 — The arrangement and motion of particles in solids, liquids and gases (bold P statement — Paper 2 only)
• 5.11 — Practical: obtain a temperature–time graph showing constant temperature during a change of state (bold P statement — Paper 2 only) (required practical)

Specification section 5(b) Density and pressure — also taught in Density, Pressure, and Pressure in Liquids, Specific Heat Capacity.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Everything on this page follows from what the particles are doing, so start there. In a SOLID the particles are packed closely in a regular arrangement, held by strong forces of attraction, and can only vibrate about fixed positions. In a LIQUID they are still close together but no longer in a fixed arrangement — they can slide past one another. In a GAS they are far apart, the forces between them are negligible, and they move quickly in all directions.

What follows from the arrangement — the properties explained

1. A SOLID has a FIXED SHAPE and volume, because its particles are held in position and cannot move past each other.
2. A LIQUID has a fixed VOLUME but takes the shape of its container, because the particles are still close together but free to slide.
3. A GAS has neither a fixed shape nor a fixed volume and fills its container, because the particles are far apart with almost no attraction and move freely.
4. SOLIDS AND LIQUIDS are hard to compress, because their particles are already touching; a GAS is easily compressed, because most of its volume is empty space.
5. That last difference also explains the densities: a gas is typically about a thousand times less dense than the same substance as a liquid.

Now heat a substance steadily and record its temperature against time. The graph does something that looks wrong the first time you see it: it rises, then goes FLAT while the substance melts, then rises again, then goes flat again while it boils. Energy is going in throughout — the heater has not been turned off — and yet for long stretches the temperature does not change at all.

The explanation requires separating two things that heating can do. TEMPERATURE is a measure of the average KINETIC energy of the particles — how fast they are moving or vibrating. When energy goes into increasing that, the temperature rises. But energy can also go into breaking the forces of attraction BETWEEN the particles, pulling them further apart, which increases their POTENTIAL energy instead. That does not make them move faster, so it does not raise the temperature.

Think of it like paying off a debt before you can start saving

Imagine money arriving at a steady rate into an account that is overdrawn. For a while the balance climbs steadily — that is the temperature rising. Then you hit a fixed charge that has to be cleared in full before anything else can happen, and for a period every penny that arrives goes into clearing it while the visible balance does not move at all. Money is still arriving at exactly the same rate; it is simply going somewhere that does not show on the balance. Once the charge is cleared, the balance starts climbing again. The flat sections of a heating curve are those charges: energy is going in at the same rate throughout, but during melting and boiling all of it is being spent separating particles rather than speeding them up.

Statement 5.9 — what actually happens at each change

1. MELTING: energy is supplied, the particles vibrate more and more strongly until they break free of their fixed positions. The regular arrangement is lost, the particles can now slide past one another, and the solid becomes a liquid. The temperature stays at the melting point throughout.
2. BOILING: energy is supplied until the particles have enough to overcome the attractions holding them in the liquid entirely, and they escape into the gas. This happens throughout the liquid, forming bubbles, and only at the boiling point. The temperature stays constant throughout.
3. EVAPORATION: this happens at ANY temperature and only at the SURFACE. The particles have a range of speeds, and the fastest ones at the surface can escape. Because the fastest leave, the average speed of those remaining falls — which is why evaporation COOLS the liquid, and why sweating works.
4. The reverse changes — freezing and condensing — release exactly the same energy, because the same attractions are being re-formed rather than broken.

The distinction between boiling and evaporation is examined more often than almost anything else here, and it comes down to three differences. Boiling happens at one fixed temperature; evaporation happens at any temperature. Boiling happens throughout the liquid; evaporation only at the surface. Boiling is rapid; evaporation is gradual. A puddle drying on a cold day is evaporating, and it never gets anywhere near 100 °C.

One consequence of statement 5.8 that is worth stating plainly: the energy put in during melting is not lost and is not hiding. It is stored in the arrangement of the particles, and you get every joule of it back when the substance freezes again. That is why ice is so effective at cooling a drink — it is not simply cold, it is absorbing a large quantity of energy at a constant temperature while it melts.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 5.10 — the three states, arrangement and motion

Boiling against evaporation — three differences

A. BOILING happens at one fixed temperature, throughout the whole liquid, and is rapid. Bubbles form within the liquid.

B. EVAPORATION happens at ANY temperature, only at the SURFACE, and is gradual. It cools the liquid, because the fastest particles are the ones that escape.

Statement 5.8 — explaining a flat section on a heating curve

1. State that energy is still being supplied at the same rate throughout.
2. Say that the energy is being used to BREAK the forces of attraction between the particles, separating them.
3. Say that this increases their POTENTIAL energy rather than their kinetic energy.
4. Temperature depends on the average KINETIC energy of the particles, which is not changing.
5. So the temperature stays constant until the change of state is complete, and then begins to rise again.

Required practical 5.11 — a temperature–time graph through a change of state

1. Place crushed ice in a boiling tube with a thermometer, and stand the tube in a beaker of water heated gently and steadily by a Bunsen burner or an electric heater.
2. Record the temperature every 30 seconds from before the ice begins to melt until well after all of it has melted.
3. Stir gently and continuously so that the temperature is the same throughout the tube.
4. Continue heating and recording until the water in the tube is boiling steadily.
5. Plot temperature on the vertical axis against time on the horizontal axis.
6. Identify the flat sections of the graph, which occur at the melting point and the boiling point, and note that the temperature is constant during each change of state.

Variables

Independent (changed) — Time, in seconds
Dependent (measured) — Temperature, in degrees Celsius

Sources of error

Learn this definition · Temperature

A measure of the average kinetic energy of the particles of a substance. Energy that goes into separating particles rather than speeding them up does not change the temperature.

The four changes of state, and what each does to the particles

Melting

Solid to liquid: energy is transferred in, breaking the fixed arrangement so particles can slide past each other.

Freezing

Liquid to solid: the same energy is transferred out as the attractions re-form and the arrangement becomes fixed.

Boiling

Liquid to gas at the boiling point, throughout the liquid, as particles escape completely.

Condensing

Gas to liquid: energy is transferred out as the particles are drawn back together.

Model answer [4 marks]

Explain why the temperature of a substance stays constant while it is melting, even though it is still being heated. [4]

Energy is still being supplied to the substance at the same rate throughout. During melting, that energy is used to break the forces of attraction holding the particles in their fixed positions, separating them so that they can move past one another. This increases the potential energy of the particles rather than their kinetic energy. Temperature is a measure of the average kinetic energy of the particles, and since that is not increasing, the temperature does not change until all of the solid has melted.

Model answer [3 marks]

Explain why evaporation cools the liquid that is left behind. [3]

The particles in a liquid have a range of different speeds. Only the fastest-moving particles at the surface have enough energy to overcome the attractions holding them in the liquid and escape. Because it is the fastest particles that leave, the average kinetic energy of the particles remaining is lower than before, and since temperature is a measure of average kinetic energy, the temperature of the liquid falls.

Not this: During melting the energy supplied is not doing anything, because the temperature is not rising.

This: It is doing a great deal: it is breaking the forces of attraction between the particles and increasing their potential energy. You get every joule of it back when the substance freezes, which is why ice is so effective at cooling a drink.

Mark-losing trap. During a change of state, energy breaks the FORCES BETWEEN particles — it does not speed them up.

Mark-losing trap. Temperature measures average KINETIC energy, so it is unchanged while potential energy is rising.

Mark-losing trap. Boiling: fixed temperature, throughout the liquid. Evaporation: any temperature, surface only.

Mark-losing trap. Evaporation COOLS the liquid because the FASTEST particles are the ones that escape.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — In which state of matter do the particles vibrate about fixed positions in a regular arrangement?
2. Grade 7 · State [2 marks] — Which of these is a difference between boiling and evaporation?
3. Grade 8 · Explain [5 marks] — Select every statement that belongs in a full-mark explanation of why the temperature stays constant while a solid is melting, even though heating continues.
4. Grade 9 · Explain [5 marks] — Select every statement that belongs in a full-mark explanation of why sweating cools the body.
5. 9+ · Analyse [6 marks] — A student heats a beaker of water and finds the temperature rises steadily to 100 °C and then stops rising, although the Bunsen is unchanged. They conclude the burner must have stopped supplying energy. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who proved the particles are real and moving, and the one who established that heat is motion rather than a substance. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Albert Einstein — the one who proved the particles are real and moving

This whole page assumes particles exist and are in constant motion, and in 1905 that was still seriously disputed — respected physicists held that atoms were a useful fiction. Einstein's paper on Brownian motion changed it: he showed that the erratic jiggling of pollen grains suspended in water, visible under a microscope, is exactly what would be produced by bombardment from invisible molecules, and predicted quantitatively how far a grain should wander. Perrin measured it and the numbers matched. After that, treating particles as real was no longer a modelling choice.

• “How can the jiggling of a pollen grain prove molecules exist?”
• “Why did serious scientists doubt that atoms were real?”
• “What did you predict that could actually be measured?”
• “What is the water doing to the grain, exactly?”
• “Does everything in a liquid move like that, or only small things?”

James Prescott Joule — the one who established that heat is motion rather than a substance

The claim underlying this page — that temperature measures how fast particles are moving — replaced a rival account that most of his contemporaries held. Heat was widely believed to be caloric, a weightless fluid that flowed from hot bodies to cold ones. Joule's measurements destroyed it: he showed that a fixed amount of mechanical work always produces a fixed amount of heat, by stirring, by compressing gas and by passing currents, which makes no sense if heat is a substance being moved about but perfect sense if it is motion being transferred.

• “What did people believe heat was before your work?”
• “How does stirring water make it warmer?”
• “Why does the same amount of work always give the same amount of heat?”
• “What is happening to the particles when something gets hotter?”
• “How long did it take before the caloric theory was abandoned?”

Carry on through the course

• Previous lesson: Pressure, and Pressure in Liquids
• Next lesson: Specific Heat Capacity
• Density — Not how heavy something is, but how much of it is packed into the space it occupies.
• Pressure, and Pressure in Liquids — Why a drawing pin works, why pressure at a point pushes every way at once, and why depth is the only thing that matters underwater.
• Specific Heat Capacity — Why the sand burns your feet and the sea is freezing on the same afternoon.
• Gas Pressure and Absolute Zero — Where the pressure of a gas actually comes from, and why the temperature scale has to start somewhere else.
• The Kinetic Theory of Gases — One picture — fast random molecules bouncing off walls — that predicts everything a gas does.
• All of Physics · Physics key terms · required practicals · equations · Incandio Science · The 4PH1 specification