Chemistry 0620Chapter 1 of 12

States of Matter

Cambridge IGCSE Chemistry 0620 Topic 1 revision chapter covering states of matter and diffusion for examination in 2026, 2027 and 2028. The chapter teaches all eight atomic syllabus outcomes of Topic 1 across both routes: five Core statements studied by every candidate, and three Supplement statements studied in addition by candidates on the Extended route. Every Supplement block is labelled and placed after the Core idea it extends, so a Core candidate can revise the whole of their required route without opening one. Topic 1.1 begins with the Core distinguishing properties of solids, liquids and gases: whether shape and volume are fixed, whether the substance flows, whether it can be compressed by a noticeable amount and whether it fills its container. It then builds the kinetic particle model in the three dimensions the syllabus names for Core, which are particle separation, particle arrangement and particle motion, and uses that model to account for each observed property. All five changes of state are described at Core level: melting, boiling, evaporating, freezing and condensing, with a precise separation of boiling from evaporation. The Supplement route then explains those changes of state in terms of kinetic particle theory and reads heating and cooling curves, so that a student can identify sloping regions, constant-temperature plateaus, the state present in each region, and the melting and boiling points of an unfamiliar pure substance rather than assuming the substance is water. The effects of temperature and pressure on the volume of a gas are described at Core and explained at Supplement through collisions with container walls, using a flexible boundary for the temperature story, a sealed syringe for the pressure story and a rigid container as a counterexample in which pressure rises but volume cannot change. Topic 1.2 develops diffusion at Core as net movement produced by random particle motion, explanation included, and then compares rates of diffusion of gases using relative molecular mass at Supplement, including the ammonia and hydrogen chloride tube demonstration. Original exam-style questions, worked examples, misconception repairs, retrieval practice and a mixed challenge with indicative marking points are provided throughout, each tagged with its route.Show more

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Interactive notes with exam tips and worked examples.

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Chapter overview

A summary of this Chemistry chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.

Key ideas to remember

Matter is made of enormous numbers of tiny particles. Everything you can see or measure about a substance — whether it keeps its shape, whether it can be squashed, whether it spreads through a room — follows from three things about those particles: how far apart they are, how they are arranged, and how they move.
Gas pressure is produced by the particles of a gas colliding with the walls of the container. Every collision pushes on the wall a little; the pressure depends on how often those collisions happen and how forcefully. Both required explanations are consequences of that one sentence.
At the same temperature, a gas with a lower \(M_r\) diffuses faster than a gas with a higher \(M_r\), so in the same time it travels further.
The shape of a full-mark explanation: “Because [what changed about the particles], they [collide / are attracted] … therefore [what is observed].” If your answer has no because and no therefore, it is probably a description.

What you need to be able to do

Core 1.1.1 State the distinguishing properties of solids, liquids and gases. Go to the lesson
Core 1.1.2 Describe the structures of solids, liquids and gases in terms of particle separation, arrangement and motion. Go to the lesson
Core 1.1.3 Describe changes of state in terms of melting, boiling, evaporating, freezing and condensing. Go to the lesson
Core 1.1.4 Describe the effects of temperature and pressure on the volume of a gas. Go to the lesson
Core 1.2.1 Describe and explain diffusion in terms of kinetic particle theory. Go to the lesson
Supplement 1.1.5 Explain changes of state in terms of kinetic particle theory, including the interpretation of heating and cooling curves. Extension in Lesson C and the whole of Lesson D
Supplement 1.1.6 Explain, in terms of kinetic particle theory, the effects of temperature and pressure on the volume of a gas. Extension in Lesson E
Supplement 1.2.2 Describe and explain the effect of relative molecular mass on the rate of diffusion of gases. Go to the lesson

Key terms in States of Matter

State of matter: One of the three physical forms in which a substance can exist under ordinary laboratory conditions: solid, liquid or gas. The three states are distinguished by observable properties - whether shape and volume are fixed, whether the substance flows, whether it can be compressed by a noticeable amount, and whether it fills its container - and a substance can be changed from one state to another by transferring energy to or from it, without changing the substance itself.
Change of state: A physical change in which a substance goes from one state of matter to another - solid, liquid or gas - because energy is transferred to or from it. The five changes of state named by the syllabus are melting (solid to liquid), boiling and evaporating (liquid to gas), condensing (gas to liquid) and freezing (liquid to solid). Melting, boiling and evaporating require energy to be transferred to the substance; condensing and freezing require energy to be transferred away from it. The substance itself is unchanged: no new substance is made, the particles keep their size and chemical identity, and the change can be reversed by reversing the energy transfer.
Relative molecular mass: A number, written Mr, defined as the sum of the relative atomic masses of the atoms shown in the formula of a substance. It is found by adding together the relative atomic mass of every atom in that formula, and it has no units because each relative atomic mass is itself a comparison of one mass with another. In the study of diffusion it is used to compare gases: at the same temperature, a gas with a lower relative molecular mass diffuses faster than a gas with a higher relative molecular mass, so it travels further in the same time.
Heating curve: A graph of temperature against time for a substance that is being heated at a steady rate. For a pure substance at constant pressure it consists of sloping sections, where the temperature rises because the average kinetic energy of the particles is increasing, separated by horizontal sections called plateaus, where the temperature stays constant while a change of state takes place because the energy transferred is used to overcome the attractive forces between particles instead of increasing their average kinetic energy. The temperature of the first plateau is the melting point and the temperature of the second is the boiling point. A cooling curve is the same graph for a substance losing energy, with plateaus at the freezing point and the condensing temperature.
Kinetic particle theory: The model that all matter is made of very large numbers of extremely small particles that are in constant motion, and that the observable properties of a substance follow from how far apart those particles are, how they are arranged, and how they move. Increasing the temperature of a substance increases the average kinetic energy of its particles, so on average they move faster. The model explains the differences between the solid, liquid and gas states, the changes between those states, gas pressure and diffusion.
Volume of a gas: The space occupied by a gas, which is always the space available inside its container rather than a fixed property of the gas itself. Two things change it. Raising the temperature of a gas at constant pressure increases its volume, and lowering the temperature decreases it, provided the container can change size. Increasing the pressure on a gas at constant temperature decreases its volume, and reducing the pressure increases it. If the gas is sealed in a rigid container whose walls cannot move, the volume cannot change at all, and heating the gas raises its pressure instead. In every case the number of particles and the size of each particle stay the same; only the space between them changes.
Diffusion: The net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration, brought about by the random motion of the particles. Individual particles move in all directions and change direction whenever they collide; because there are more particles on one side to begin with, more happen to cross away from that side than towards it, so over time the substance becomes evenly spread. Diffusion needs no stirring and no external push, it happens faster at higher temperatures, and it is fastest in gases because their particles are far apart and move rapidly.

Common mistakes to avoid

1. “When you heat something, its particles expand.” Repair Particles never change size — heating makes them move faster and, in a substance that is free to expand, move further apart, so it is the spacing that grows, not the particles. Taught in Lesson B. Core 1.1.2
2. “The particles in a solid do not move.” Repair Solid particles are held in fixed positions but they are not still: they vibrate about those positions, and they vibrate faster when the solid is heated. Taught in Lesson B. Core 1.1.2
3. “Liquid particles are spread out, somewhere between a solid and a gas.” Repair Liquid particles are as close together as solid particles — what changes is the arrangement, which becomes irregular, and the motion, which lets them move past one another. Taught in Lesson B. Core 1.1.2
4. “Evaporation is just boiling by another name.” Repair Boiling happens throughout the liquid, at the boiling point, with bubbles of gas forming inside it; evaporation happens at the surface only and can happen at temperatures below the boiling point. Taught in Lesson C. Core 1.1.3
5. “The graph is flat, so nothing is being heated during that time.” (Supplement) Repair Energy is still being transferred to the substance the whole way along a plateau — it is being used to overcome the attractions between the particles and separate them, which changes the state without changing the average kinetic energy, and therefore without changing the temperature. Taught in Lesson D. Supplement 1.1.5
6. “The heavier gas sinks to the bottom and the lighter gas floats to the top.” Repair Gas particles move rapidly and randomly in all directions, so two gases released into the same space spread through one another and mix — a higher relative molecular mass makes a gas diffuse more slowly, not downwards. Taught in Lesson F Core 1.2.1 and Lesson G Supplement 1.2.2

Examiner tips

The single most valuable sentence in this route: a change of state overcomes the attractive forces between particles. It does not break the covalent bonds inside a molecule. Steam is still \(\mathrm{H_2O}\); ice is still \(\mathrm{H_2O}\). Nothing chemical has happened.
Read the numbering carefully. Cambridge numbers the statements straight through each subtopic and only then splits them by route, so 1.1 runs Core 1–4 followed by Supplement 5–6. A higher number does not mean a later topic, and 1.1.5 is not a continuation of 1.1.4 — it is the Supplement partner of 1.1.3.
Be careful with “cannot”. Solids and liquids are not easily compressed — at the pressures you meet in a school laboratory the change is far too small to notice. Writing “a liquid cannot be compressed at all” goes further than the evidence, and the safer wording is the one the syllabus uses: a gas can be compressed, a liquid and a solid cannot be compressed easily.
Separation and arrangement are not the same question, and swapping them gives the wrong cause for almost every property in this topic. A liquid's particles are just as close together as a solid's — that is why a liquid has a fixed volume and cannot be compressed easily. What changes when a solid melts is the arrangement, from regular to irregular, and the motion, from vibrating in place to moving past one another.
Freezing point and melting point are the same temperature. Pure water melts at \(0\ ^\circ\mathrm{C}\) and freezes at \(0\ ^\circ\mathrm{C}\). Which name you use depends only on which direction the energy is going. The same is true of the boiling point and the temperature at which a gas condenses.
Read before you explain. A graph question is answered in two separate stages, and students who skip the first one lose marks they clearly knew the chemistry for. Stage one is pure reading: what are the axes, where is the line flat, at what value. Stage two is the particle explanation. Do them in that order and write them in that order.
Always say what the container is doing. “Heating a gas increases its volume” is only true if the container can get bigger. In a sealed steel cylinder it is simply false. Before you write a word, decide whether the boundary can move — a balloon and a free syringe plunger can, a sealed rigid vessel cannot. This is the first thing to check on both routes. Core candidates need it to describe the right effect; Extended candidates need it to choose the right explanation.
This is the exception in Topic 1, so read the statement carefully. Everywhere else in this chapter, Core describes and Supplement explains. Statement 1.2.1 asks every candidate to describe and explain diffusion in terms of kinetic particle theory, so the whole particle mechanism in this lesson — random motion, net movement, the counting argument — is Core material and can be asked on the Core papers. Only the effect of relative molecular mass on the rate of diffusion is Supplement, and that is the separate statement 1.2.2 in Lesson G.
Context, not an examinable claim The honest limits of these two examples. In a real room, a smell usually reaches you faster than diffusion alone would carry it, because warm air rises and cool air sinks and those currents carry the particles along. In a beaker, any vibration or difference in temperature sets up currents too. Diffusion is the part that would still happen in perfectly still surroundings — and it is that part, not the currents, that Topic 1.2 is about.
Two words that must be in the answer. Say relative molecular mass, not “it is lighter” or “it is less dense”. And say at the same temperature, because a hotter heavy gas can easily out-diffuse a colder light one — the comparison only holds when the temperature is the same for both.
One condition governs the whole of table 5. Every row assumes the two gases are at the same temperature. Leave that phrase out of an exam answer and the comparison is not properly justified.
Reading this page does not make you ready, and neither does ticking a box. The only evidence that counts is that you produced the answer before you saw it. If you could not, that is not a failure — it is the whole point of doing the check, and the repair link tells you exactly where to go.
Two rules that make the difference. First, always try to produce the answer before you look — a failed attempt followed by the answer is worth far more than reading the answer straight away. Second, when a session goes badly, do not restart the chapter; use the self-diagnosis table and repair only the one or two outcomes that actually slipped.

Syllabus reference and sources

Written against: Cambridge IGCSE Chemistry (0620) 2026–2028 Syllabus (Subject Content, Topic 1: States of matter), covering Core statements 1.1.1–1.1.4 and 1.2.1 and Supplement statements 1.1.5, 1.1.6 and 1.2.2.

Written by: Academiq Edu Instructor Panel

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Cambridge IGCSE Chemistry 0620 syllabus for examination in 2026, 2027 and 2028 (version 1)

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