States of Matter
Cambridge O Level Chemistry 5070 Topic 1 revision chapter covering states of matter and diffusion for examination in 2026, 2027 and 2028. The chapter teaches all seven atomic syllabus outcomes of Topic 1. Topic 1.1 begins with the 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, which are particle separation, particle arrangement and particle motion, and uses that model to explain each observed property. All five required changes of state are described and explained: melting, boiling, evaporating, freezing and condensing, with a precise separation of boiling from evaporation and a repeated warning that a change of state overcomes attractive forces between particles rather than breaking covalent bonds inside molecules. Heating and cooling curves are read before they are explained, 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 qualitative effects of temperature and pressure on the volume of a gas are explained 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 as net movement produced by random particle motion, then compares rates of diffusion of gases using relative molecular mass, 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.Show moreShow less
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What is States of Matter about?
States of matter are the three physical forms in which a substance can exist: solid, liquid and gas. Kinetic particle theory explains all three with one idea: matter is made of enormous numbers of tiny particles, and everything you observe about a substance follows from how far apart those particles are, how they are arranged and how they move. In a solid the particles are very close together in a regular arrangement and vibrate about fixed positions; in a liquid they are still very close together but irregular and able to move past one another; in a gas they are far apart and move rapidly and randomly in all directions.
A change of state is a physical change in which energy transferred to or from a substance lets its particles overcome, or be pulled back together by, the attractive forces between them. Melting, boiling and evaporating take energy in; condensing and freezing give energy out. The covalent bonds inside molecules are not broken, so steam and ice are both still \(\mathrm{H_2O}\). On a heating curve the flat plateaus mark these changes: the temperature stays constant because the energy being transferred is used to overcome the attractions between particles rather than to increase their average kinetic energy.
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.
- 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
- 1.1.1 I can state the distinguishing properties of solids, liquids and gases — shape, volume, flow, compressibility and whether the substance fills its container. Go to the lesson
- 1.1.2 I can describe the structures of solids, liquids and gases in terms of particle separation, arrangement and motion. Go to the lesson
- 1.1.3 I can describe and explain the changes of state — melting, boiling, evaporating, freezing and condensing — in terms of kinetic particle theory. Go to the lesson
- 1.1.4 I can interpret and explain heating curves and cooling curves in terms of kinetic particle theory. Go to the lesson
- 1.1.5 I can describe and explain, in terms of kinetic particle theory, the effects of temperature and of pressure on the volume of a gas. Go to the lesson
- 1.2.1 I can describe and explain diffusion in terms of kinetic particle theory. Go to the lesson
- 1.2.2 I can 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
- 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.
- 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.
- Gas pressure
- The effect produced by the particles of a gas colliding with the walls of its container. Each collision exerts a small force on the wall, and the pressure depends on how often those collisions happen and how forcefully. Raising the temperature of a gas makes its particles move faster, so they collide with the walls more often and more forcefully. If the container can change size the boundary is pushed outward until the pressures balance and the volume increases; if the container is rigid the volume cannot change and the pressure of the gas rises instead.
- 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.
- 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.
- 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. Energy transferred to the substance increases the movement of its particles and allows them to overcome the attractive forces holding them together; energy transferred away allows those attractions to pull the particles back together. 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.
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 1.1B.
- 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 1.1B.
- 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 1.1B.
- 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 1.1C.
- 5. “The graph is flat, so nothing is being heated during that time.” 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 1.1D.
- 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 1.2A and Lesson 1.2B.
Examiner tips
- The single most valuable sentence in this chapter: 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.
- 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.
- 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.
Frequently asked questions
What is the difference between a solid, a liquid and a gas?
The particles are identical; only their separation, arrangement and motion differ. In a solid the particles are very close together in a regular arrangement and vibrate about fixed positions, so the solid has a fixed shape and volume. In a liquid the particles are still very close together but irregularly arranged and move past one another, so a liquid flows and keeps a fixed volume. In a gas the particles are far apart and move rapidly and randomly, so a gas fills its container and can be compressed easily.
Why does the temperature stay constant while a substance melts or boils?
The plateau on a heating curve does not mean heating has stopped: energy is still being transferred the whole time. During a change of state that energy is used to overcome the attractive forces holding the particles together and move them further apart, rather than to increase the average kinetic energy of the particles. Because temperature depends on the average kinetic energy, the temperature does not rise until the change of state is complete. The first plateau is the melting point and the second is the boiling point.
What is the difference between boiling and evaporation?
Boiling happens throughout the liquid, at the boiling point, with bubbles of gas forming inside the liquid. Evaporation happens from the surface only and can take place at temperatures below the boiling point, which is why a puddle disappears on a cool day. Both are changes from liquid to gas that take energy in, and in both the particles overcome the attractive forces between them — but they are not the same process and should never be used as if they were.
Do the particles get bigger when a substance is heated and expands?
No. Particles never change size. Heating increases the average kinetic energy of the particles, so they move or vibrate faster and, in a substance that is free to expand, end up further apart. It is the spacing between the particles that grows, not the particles themselves. Writing that the atoms expand contradicts the particle model the question is built on, so say instead that the particles vibrate more strongly and the average distance between them increases.
Why does a gas exert pressure, and why does the pressure rise when the gas is heated?
Gas pressure is caused by the gas particles colliding with the walls of their container; each collision exerts a small force on the wall. Heating the gas makes its particles move faster, so they collide with the walls more often and more forcefully. If the container is rigid, the volume cannot change and the pressure rises. If the container can expand, such as a balloon or a free syringe plunger, the volume increases instead. Always state what the container is doing before you write the answer.
Why do gases with a lower relative molecular mass diffuse faster?
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. In an exam, use the words relative molecular mass rather than lighter or less dense, and always add at the same temperature, because a hotter heavy gas can easily out-diffuse a colder light one. A heavier gas diffuses more slowly, not downwards: all gases spread through one another because their particles move randomly in all directions.
How do you write a full-mark particle explanation in the exam?
Every explanation in this chapter has two levels, and the examiner wants both: the observation you could see or measure, and what the particles are doing in terms of separation, arrangement and motion. Join them with because and therefore: because the particles gained kinetic energy and moved further apart, therefore the gas expanded. An answer with no because and no therefore is usually a description, not an explanation. For a graph question, read the axes and the flat sections first, then give the particle explanation second.
Syllabus reference and sources
Written against: Cambridge O Level Chemistry (5070) 2026–2028 Syllabus (Subject Content, Topic 1: States of matter).
Written by: Academiq Edu Instructor Panel
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