Chapter 01
States of Matter
Syllabus 0620
Core Revision Module
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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
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 Instructor Panel
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