State and explain why salt dissolved in water is classified as a mixture rather than a compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sea water contains sodium chloride (common salt) dissolved in water. A student claims that salt water is a compound because it contains more than one substance.
Model answer (4 marks)
Salt dissolved in water is a mixture because the ratio of salt to water can vary, so the composition is not fixed.
The salt and water are not chemically combined; no chemical bonds are formed between the two substances.
They can be separated by a physical method such as evaporation or filtration, and the individual properties of salt and water are retained.
A compound, by contrast, has a fixed composition and its components are chemically bonded, which is not the case here.
The salt and water are not chemically combined; no chemical bonds are formed between the two substances.
They can be separated by a physical method such as evaporation or filtration, and the individual properties of salt and water are retained.
A compound, by contrast, has a fixed composition and its components are chemically bonded, which is not the case here.
Examiner tips
- Use the word ‘mixture’ first, then give two reasons: variable composition and lack of chemical bonding.
- Show that separation is possible by a physical method, e.g. evaporation.
- Mention that a compound has a fixed composition and chemical bonds.
Common mistakes
- Confusing ‘mixture’ with ‘solution’ and not explaining variable composition.
- Claiming that salt and water are chemically bonded or that the mixture has a fixed ratio.
- Failing to state that the substances can be separated physically.
Mark scheme (4 marks)
- The composition (ratio of salt to water) can vary / the proportions of the substances are not fixed
- The substances are not chemically combined / no chemical bonds form between salt and water
- The salt and water can be separated by a physical method / properties of the individual substances are retained
- A compound has a fixed composition / substances are chemically combined in a compound, which is not the case here
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "State and explain" question →
- Decode the mark scheme abbreviations →
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