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CIE AS Biology (9700) 2019-2021 Revision Notes

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2. Biological Molecules

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The Benedict’s test for reducing sugars

Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample

solution in a test tube

Heat the test tube in a water bath or beaker of water that has been brought to a boil for a

few minutes

If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced

to copper (I) oxide which is insoluble in water

A positive test result is, therefore, a colour change somewhere along a colour scale from blue

(no reducing sugar) to brown/brick-red (a high concentration of reducing sugar)

This test is semi-quantitative as the degree of the colour change can give an

indication of how much (the concentration of) reducing sugar present

The Benedict’s test for reducing sugars produces a colour change from blue towards red