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Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
All matter is classified as elements, compounds, or mixtures. Cambridge tests the definitions and the differences between compounds and mixtures in particular.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
The fundamental classification of all matter — know definitions and distinguishing features precisely.
| Type | Definition | Separation | Example |
|---|---|---|---|
| Element | One type of atom only | Cannot be broken down chemically | Fe, O₂, Cu, N₂ |
| Compound | Two+ elements chemically bonded in fixed ratio | Chemical methods only | H₂O, NaCl, CO₂, CuSO₄ |
| Mixture | Two+ substances physically combined (no fixed ratio) | Physical methods (filtration, distillation) | Salt water, air, crude oil, alloys |
Compound vs Mixture — key distinctions:
| Feature | Compound | Mixture |
|---|---|---|
| Ratio of components | Fixed (by chemical formula) | Variable |
| Formation | Requires chemical reaction | Physical mixing only |
| Properties | Different from elements | Components retain own properties |
| Separation | Only by chemical methods | Physical methods (distillation, filtration, etc.) |
| Energy change | Yes (bonds formed/broken) | No (usually) |
Example: Iron + sulfur mixture vs iron sulfide compound.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 2 MCQ: classify substances as element/compound/mixture from formulas or descriptions. Paper 4: 'Explain why air is a mixture and not a compound' (3 marks — variable composition, components keep properties, separated by physical methods, no energy change on mixing).
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on elements, compounds and mixtures, written exactly the way a tutor would explain them at the board.
Question
Classify each of the following as an element, compound, or mixture: copper, salt water, carbon dioxide, air.
Step-by-step solution
Step 1
Copper (Cu): one type of atom only → element.
Step 2
Salt water: sodium chloride dissolved in water; two substances not chemically combined, variable composition → mixture.
Step 3
Carbon dioxide (CO₂): carbon and oxygen chemically combined in fixed ratio 1:2 → compound.
Step 4
Air: mixture of N₂, O₂, Ar, CO₂, etc.; not chemically combined, composition varies → mixture.
Answer
Copper: element; salt water: mixture; carbon dioxide: compound; air: mixture.
Question
State TWO differences between a compound and a mixture.
Step-by-step solution
Step 1
Composition: A compound has a fixed composition (elements combined in a definite ratio by mass); a mixture has variable composition.
Step 2
Properties: A compound has new properties different from its constituent elements; a mixture retains the properties of its components.
Step 3
Separation: A compound can only be separated by chemical reactions; a mixture can be separated by physical methods (filtration, distillation, etc.).
Answer
Compound: fixed composition, new properties, separated by chemical means. Mixture: variable composition, component properties retained, separated physically.
Examiner tip
Any two clear differences with both compound and mixture stated earn full marks.
Question
Explain why water is a compound and not a mixture of hydrogen and oxygen.
Step-by-step solution
Step 1
In water (H₂O), hydrogen and oxygen are chemically combined in a fixed ratio of 2:1 by number of atoms.
Step 2
Water has completely different properties from hydrogen (flammable gas) or oxygen (supports combustion); water does not burn and does not support combustion.
Step 3
Water can only be separated into H₂ and O₂ by a chemical method (electrolysis), not a physical method.
Answer
Water has a fixed composition (H₂O), new properties different from H₂ and O₂, and can only be separated by chemical means (electrolysis) — all features of a compound, not a mixture.
Definitions to memorise and the exact keywords mark schemes credit for elements, compounds and mixtures answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A pure substance made of only one type of atom; cannot be broken down into simpler substances by chemical means.
A substance formed when two or more elements are chemically combined in a fixed ratio; has properties different from its constituent elements; can only be separated by chemical reactions.
Two or more substances that are not chemically combined; variable composition; retains the properties of its components; separated by physical methods.
The smallest particle of an element that can take part in a chemical reaction; cannot be broken down further by chemical means.
Two or more atoms covalently bonded together; may be of the same element (e.g. O₂) or different elements (e.g. H₂O).
The traps other students keep falling into on elements, compounds and mixtures questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Students associate multiple types of atoms with 'compound' without checking whether they are chemically combined.
How to avoid it
Air is a mixture: N₂ and O₂ are not chemically joined; their ratio varies; properties of each gas are retained.
Why it happens
Elements like O₂ and P₄ have formulae but are not compounds — they are composed of one type of atom.
How to avoid it
A compound must contain two or more DIFFERENT elements chemically combined (e.g. CO₂ = C + O; NaCl = Na + Cl).
Why it happens
Students confuse mixture separation (physical) with compound decomposition (chemical).
How to avoid it
Compounds are separated by chemical reactions (e.g. electrolysis, thermal decomposition); mixtures by physical methods (filtration, distillation).
The things students keep getting wrong in this sub-topic, answered.