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CAIE A Level Biology 9700 · Biological molecules

Biological molecules: explain question

Explain4 marksStandard

A student is given a sample of a white powder. They dissolve a small amount in water and carry out the following tests:

  • Benedict's test: solution turns orange after 2 minutes.
  • Biuret test: solution turns purple.
  • Iodine test: solution remains yellow-brown. They then take a second sample, boil it with dilute hydrochloric acid for 5 minutes, neutralise the solution with sodium hydrogencarbonate, and then carry out Benedict's test. This time the solution turns deep red. Explain what the results show about the composition of the powder.
0 words · 4 marks

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How this question is marked

4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.

Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.

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