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Practical Skills, Apparatus and Techniques

Qualitative reagents for biological molecules

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on the iodine test for starch, the Benedict's test for reducing sugars and its colour sequence, testing for non-reducing sugars such as sucrose, the biuret test for protein, the emulsion test for lipids, DCPIP for vitamin C, and negative controls.

What you need to know

Answer each one before you reveal it.

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Key term

Which reagent tests for starch, and what colour change shows a positive result?

Process

State the sequence of colours seen in a positive Benedict's test as the concentration of reducing sugar increases.

Key term

Name two reducing sugars that give a positive Benedict's test.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Which reagent tests for starch, and what colour change shows a positive result?

    Model answer

    Iodine in potassium iodide solution, which changes from orange-brown to blue-black.

  2. 2

    Describe how the Benedict's test for a reducing sugar is carried out.

    Model answer

    An approximately equal volume of Benedict's solution is added to the sample and the mixture is heated in a boiling water bath for a few minutes.

  3. 3

    State the sequence of colours seen in a positive Benedict's test as the concentration of reducing sugar increases.

    Model answer

    The blue solution turns green, then yellow, then orange, and finally forms a brick-red precipitate.

  4. 4

    Name two reducing sugars that give a positive Benedict's test.

    Model answer

    Glucose and maltose.

  5. 5

    Which common disaccharide does not give a positive result with Benedict's solution directly?

    Model answer

    Sucrose.

  6. 6

    Describe how a sample is tested for a non-reducing sugar.

    Model answer

    It is boiled with dilute hydrochloric acid to hydrolyse the sugar, neutralised with sodium hydrogencarbonate, and then tested with Benedict's solution.

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