Explain the difference between a compound and a mixture, using the properties of each to support your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A compound is a pure substance made of two or more elements that are chemically bonded together, giving it a fixed composition and a definite ratio of elements. For example, water (H₂O) always contains two hydrogen atoms for every oxygen atom.
A mixture is a combination of two or more substances that are not chemically bonded; the components retain their own properties and the composition can vary. For example, air contains nitrogen, oxygen and other gases in varying proportions.
Because a compound has a fixed composition, it cannot be separated into its elements by physical means. In contrast, a mixture can be separated into its components by physical methods such as filtration, distillation, evaporation or chromatography.
A mixture is a combination of two or more substances that are not chemically bonded; the components retain their own properties and the composition can vary. For example, air contains nitrogen, oxygen and other gases in varying proportions.
Because a compound has a fixed composition, it cannot be separated into its elements by physical means. In contrast, a mixture can be separated into its components by physical methods such as filtration, distillation, evaporation or chromatography.
Examiner tips
- Use the terms 'fixed composition' and 'definite ratio' for compounds; mention 'variable composition' for mixtures.
- Show that compounds cannot be separated physically but mixtures can, citing at least one separation technique.
Common mistakes
- Confusing chemical bonds with physical mixing; stating that compounds can be separated by filtration.
- Using vague phrases like 'different substances' without specifying chemical vs physical separation.
Mark scheme (4 marks)
- A compound contains two or more elements chemically combined / joined together
- A compound has a fixed composition / definite ratio of elements
- A mixture contains two or more substances not chemically combined / no chemical bonds between the substances
- A mixture can be separated by physical methods (e.g. filtration / distillation / evaporation / chromatography) whereas a compound cannot be separated by physical methods
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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