States of matter and the particle model
Connect what you can see—shape, flow and changes of state—to a model of particles too small to see individually.
Core: properties, diagrams, state changes, temperature and absolute zero. Supplement / Extended: explanations using forces, spacing and motion. This is Cambridge IGCSE, not Egyptian Baccalaureate / Thanaweya Amma or Year 3.
Start with what you can observe
A solid keeps its own shape and approximately its volume. A liquid keeps approximately its volume but takes the shape of the space it occupies. A gas has no fixed volume or shape: it spreads throughout an available container. A liquid does not automatically fill the whole container.
Worked example
Pour 100 cm³ of water from a narrow vessel into a wide one without spilling. Its depth and shape change, but its volume remains about 100 cm³. Release that same volume of gas into a larger closed container and it spreads through the available space.
Think first: Does “can flow” identify only a liquid?
No. Both liquids and gases flow. Combine shape, volume and compressibility observations rather than using just one clue.
Describe first, then explain
- Identify whether the question asks for a visible property, particle description or explanation.
- For a diagram, give separation, arrangement and motion. Keep individual particle size unchanged between states.
- For a state change, identify the start and end states before choosing its name.
- For temperature data, compare with both transition temperatures; do not force a single-state answer at a boundary.
- Supplement explanations connect spacing, interactions and motion to the observed property.
Core worked example: two changes in order
A hypothetical pure material melts at 10°C and boils at 80°C at the pressure used. At −10°C it is solid because −10 is below 10. Heating it to 100°C passes first through melting (solid → liquid) and then boiling (liquid → gas). The interval between the two transition temperatures is 80 − 10 = 70°C. This interval is not an amount of energy.
Supplement worked explanation: compressing gas, not particles
A sealed gas sample contains particles with large gaps [1]. Reducing its volume mainly reduces those gaps; the number and individual size of the particles need not change [1]. A liquid is much harder to compress because its particles are already close together [1]. “The particles get smaller” does not explain the observation correctly.
Particle model laboratory
Describe the particle model
Core activity. Select a state, inspect the diagram, then choose its separation, arrangement and motion. All three diagrams use 16 equal-sized circles: changing state does not create new particles or make individual particles melt. These are schematic 2-D snapshots, not scale drawings or a live molecular simulation.
State prediction laboratory
Predict the state from temperature
Use the melting and boiling data for a hypothetical pure substance at a fixed pressure. Assume equilibrium. At a boundary, “solid + liquid” or “liquid + gas” means the two states can coexist; their proportions are not specified.
| Substance | Melting point / °C | Boiling point / °C |
|---|---|---|
| A | -30 | 90 |
| B | 40 | 180 |
| C | -150 | -70 |
Try A at −30°C and 90°C. Then compare all three substances at 20°C. Changing a control clears the prediction. This is a data model, not an experiment to perform at extreme temperatures.
Practise, then explain
Four multiple-choice questions, two numerical responses and six written tasks. Describing states and particle motion is Core. Explaining properties using forces between particles is Supplement / Extended. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
A sample keeps its volume when poured between two differently shaped containers, but changes its shape. Which state is it?
Optional hint
Distinguish keeping a volume from keeping a shape.
Source clarifications and limits of the model
The original PDF is unchanged. Its final retrieval answer says particles become slower during condensation. Do not apply that as a universal rule: a pure substance can condense at constant temperature and fixed pressure. Distinguish cooling before the change from the change itself.
Use “very difficult to compress”, not literally impossible. A regular lattice is a useful simple solid model, not a description of every solid. Temperature refers to average particle motion, and absolute zero is approximately −273°C with least kinetic energy; we do not claim all motion must stop. Properties depend on the type of interactions as well as spacing and motion, not a universal “closer means more rigid” rule.
Figures 20.1–20.2 informed the particle diagrams and state-change sequence. Our liquid diagram avoids neat rows, and every state has the same 16 equal-sized model particles. Phase-change boundaries are treated explicitly rather than assigning a single state without enough information.
Source alignment: supplied Physics-0625 Volume A, PDF pp.142–148 / printed pp.134–140, Figures 20.1–20.2; official Cambridge 2026–2028 syllabus, sections 2.1.1–2.1.2. Original learning text, activities and questions. Existing source-download access is unchanged.