8.1 Pure substances and formulations
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which statement about a pure substance is correct?
Choose one answer.
Correct Not quite: the answer is It has a sharp melting point
Mark scheme
B
Model answer
B: a pure substance melts at one sharp temperature.
Explain the difference between the meaning of "pure" in chemistry and in everyday language.
Mark scheme
- chemistry: a single element or compound (1)
- everyday: natural / nothing added, though it may be a mixture (1)
Model answer
In chemistry a pure substance is a single element or compound; in everyday language "pure" just means nothing has been added, even if it is a mixture.
A sample of aspirin melts between 128 °C and 133 °C. Pure aspirin melts at 136 °C.
Is the sample pure? Explain your answer.
Mark scheme
- no / impure (1)
- melts over a range / below the true melting point (1)
Model answer
It is impure: it melts over a range of temperatures, below the melting point of pure aspirin.
What is a formulation? Give one example.
Mark scheme
- a mixture designed as a useful product, with components in measured quantities (1)
- e.g. paint, medicine, fertiliser, fuel, alloy, cleaning agent (1)
Model answer
A formulation is a mixture designed as a useful product, with each component in a measured amount, such as a paint.
Suggest why the amounts of each component in a medicine are measured very carefully.
Mark scheme
- so the medicine works properly / gives the correct dose / is safe (1)
Model answer
So that each tablet has exactly the right dose and the medicine is effective and safe.
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