States of Matter and Kinetic Particle Theory
The particle model underpins the whole of Chemistry. Questions here are marked on precise wording — and one word in particular, vibrate, is worth a mark on its own.
1. The three states
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | regular, closely packed | close together, random | far apart, random |
| Movement | VIBRATE about fixed positions | slide past each other | move rapidly in all directions |
| Forces of attraction | strong | moderate | very weak |
| Kinetic energy | lowest | moderate | highest |
| Shape | fixed | takes the container’s shape | fills the container |
| Volume | fixed | fixed | not fixed |
| Compressible? | no | almost no | yes |
Use the word “VIBRATE” for solid particles. Tutors flagged this as a specific mark — solid particles are not stationary; they vibrate about fixed positions.
Gas particles are FAR APART, not closely packed. A recorded error had this backwards. It is solids that are closely packed.
Gases have the MOST kinetic energy; solids the least (at the same temperature). Confusion over which state has most was recorded.
Solids have a fixed shape and CANNOT be compressed. A recorded error claimed the opposite. Liquids also can’t be meaningfully compressed — only gases can, because only they have large gaps between particles.
Both liquids and gases flow, but only gases have no fixed volume. Confusion about fixed volume in liquids and gases was recorded — a liquid keeps its volume and changes shape; a gas does neither.
Particles cannot be seen with the naked eye or an ordinary microscope. Recorded as an error.
2. The six changes of state
Melting — solid → liquid Freezing (solidifying) — liquid → solid Boiling / evaporating — liquid → gas Condensing — gas → liquid SUBLIMATION — solid → gas directly DEPOSITION — gas → solid directly
Sublimation goes solid → gas, skipping the liquid entirely. It was confused with condensation, with freezing, and with deposition in separate lessons — this is the most-muddled vocabulary in the topic.
Examples of sublimation: solid carbon dioxide (dry ice), iodine, and ammonium chloride.
Iodine is a grey-black solid that sublimes to a PURPLE vapour. The colour change was a recorded error — grey-black solid, purple gas.
Energy:
Melting, boiling and sublimation ABSORB energy (particles gain energy and separate). Freezing, condensing and deposition RELEASE energy.
3. Explaining a change of state
This is a standard extended question, and tutors gave the exact structure:
Four points are needed: (1) heat energy is supplied, (2) the particles gain KINETIC ENERGY, (3) they vibrate/move faster and further apart, (4) the FORCES OF ATTRACTION between them are overcome.
Reversing it for cooling: energy is released, kinetic energy falls, movement slows, and forces of attraction pull the particles together.
Include all four points. Tutors flagged this as a checklist — missing “overcoming the forces of attraction” is the commonest omission.
Talk about kinetic energy when the temperature changes. A recorded error confused a fall in particle speed with a fall in mass; what decreases is the kinetic energy, and therefore the temperature.
4. Heating and cooling curves
A graph of temperature against time as a substance is heated (or cooled) steadily.
On a heating curve:
- Sloping sections — the substance is warming; kinetic energy and temperature rise
- Flat sections (plateaus) — a change of state is occurring; temperature is constant
The FIRST plateau is at the melting point; the SECOND is at the boiling point.
A flat line does not mean “nothing is happening”. A recorded misconception read the plateaus simply as melting and boiling points without understanding why they are flat.
Why the plateaus are flat — the mark-scheme answer:
The energy supplied is used to overcome the forces of attraction between the particles, not to increase their kinetic energy. Since temperature measures average kinetic energy, the temperature stays constant.
Explain flat lines “by discussing energy use” — tutors’ own phrasing. State where the energy goes.
Cooling curves are the mirror image, sloping downwards with plateaus where the substance freezes and condenses.
Check the direction of the graph before reading it. Confusion about which way the lines run, and misidentifying the freezing point on a cooling curve, were both recorded. On a cooling curve the first plateau is condensing, the second freezing.
“Describe” a graph trend without explaining it unless asked. Tutors flagged that students lose time — and sometimes marks — by explaining when only a description was wanted.
5. Melting and boiling points
Melting point — where solid becomes liquid. Boiling point — where liquid becomes gas.
For water: melting point 0 °C, boiling point 100 °C.
Don’t swap them. Confusing water’s boiling point with its melting point was recorded.
Impurities:
Impurities LOWER the melting point and RAISE the boiling point, and both then occur over a range rather than at a sharp temperature.
A sharp melting point is evidence that a substance is PURE — which is why melting point is used as a purity test.
6. Evaporation vs boiling
| Evaporation | Boiling | |
|---|---|---|
| Where | at the surface only | throughout the liquid |
| Temperature | at any temperature | at a fixed temperature |
| Speed | slow | rapid |
| Bubbles | none | throughout |
Boiling and evaporation are different processes. Confusing them was recorded, as was uncertainty about the temperature at which condensation occurs (it occurs at the boiling point when cooling, or at any temperature at a cold surface).
Evaporation causes cooling because the fastest particles escape, lowering the average kinetic energy of those left.
7. Effect of temperature and pressure on gases
Increasing temperature → particles move faster → collide with the walls more often and harder → pressure increases (at constant volume), or the gas expands (at constant pressure). Decreasing volume → particles collide with the walls more frequently → pressure increases.
Gas particles collide most frequently at high temperature and in a small volume. Confusion about the conditions for frequent collisions was recorded.
8. Solubility and saturated solutions
A saturated solution is one that contains as much dissolved solute as it can hold AT A GIVEN TEMPERATURE.
The phrase “at a given temperature” is part of the definition. Omitting it was recorded directly — without it the definition is incomplete and loses the mark.
Solubility of most solids INCREASES as temperature increases. A recorded error stated the opposite. (Gases behave the reverse way — they become less soluble as temperature rises.)
9. Mistakes that cost marks
Saying gas particles are closely packed.
Not using the word “vibrate” for solids.
Saying solids can be compressed.
Confusing sublimation with condensation, freezing or deposition.
Giving fewer than four points when explaining a change of state.
Omitting “overcoming forces of attraction”.
Misreading a cooling curve as a heating curve.
Explaining when only a description was asked for.
Swapping melting and boiling points.
Saying impurities raise the melting point.
Omitting “at a given temperature” from the saturated solution definition.
Saying solubility falls as temperature rises (for solids).
Frequently asked questions
How are particles arranged in the three states? Solid: regular, close, vibrating. Liquid: close, random, sliding. Gas: far apart, random, fast.
Which state has the most kinetic energy? Gas; solids have the least.
What is sublimation? Solid → gas directly, without becoming a liquid.
What is the reverse of sublimation? Deposition — gas → solid.
Why is the temperature constant during melting? The energy is used to overcome the forces of attraction, not to raise kinetic energy.
What four points explain a change of state? Heat energy supplied → kinetic energy increases → particles move faster and apart → forces of attraction overcome.
What does a flat line on a heating curve show? A change of state at constant temperature.
How do impurities affect melting point? They lower it, and it occurs over a range.
What is a saturated solution? One holding as much dissolved solute as possible at a given temperature.
Does solubility increase with temperature? Yes for most solids; no for gases.
Quick revision checklist
- I can describe arrangement, movement, forces and energy in all three states
- I use the word vibrate for solids
- I know which states are compressible and which have fixed volume
- I can name all six changes of state
- I know sublimation is solid → gas
- I know which changes absorb and which release energy
- I can give the four-point explanation of a change of state
- I can read a heating curve and a cooling curve
- I can explain why the plateaus are flat
- I know the effect of impurities on melting and boiling points
- I can distinguish evaporation from boiling
- I can explain gas pressure changes with particles
- I can define a saturated solution, including “at a given temperature”
- I know how solubility varies with temperature
These notes cover states of matter and kinetic particle theory in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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