Lesson 3.1.2.3

3.1.2.3 Biochemical tests for sugars and starch Quiz: AQA Biology, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.2.3, Biochemical tests for sugars and starch: 20 multiple choice questions for the AQA Biology (7402), Unit 1: Biological molecules, written with Revision Ninja.

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The 20 questions

  1. Which reagent is used to test for reducing sugars?

    • Ethanol emulsion
    • Biuret reagent
    • Benedict's solution
    • Iodine in potassium iodide solution

  2. Which reagent is used to test for starch?

    • Biuret reagent
    • Iodine in potassium iodide solution
    • Benedict's solution
    • Sodium hydroxide and copper(II) sulfate

  3. What colour is a positive starch test with iodine in potassium iodide solution?

    • Blue-black
    • Brick-red
    • Green
    • Purple

  4. What is a positive result for a reducing sugar with Benedict's solution when the concentration is high?

    • A purple solution
    • A blue-black colour
    • A white precipitate that clears
    • A brick-red precipitate

  5. What colour does Benedict's solution stay if no reducing sugar is present?

    • Brick-red
    • Blue
    • Green
    • Yellow-orange

  6. Which sugar is a non-reducing sugar?

    • Glucose
    • Lactose
    • Maltose
    • Sucrose

  7. Why must a Benedict's test be heated?

    • To break the peptide bonds in the sample so that the released amino acids can be detected by the test
    • To dissolve the starch in the sample so that the iodine solution is able to react with it fully
    • To remove the water from the sample so that the concentration of sugar in the solution increases
    • To provide the energy needed for the reducing sugar to reduce the copper(II) ions

  8. A student wants to test a sample of sucrose for the presence of sugar. What must be done before the Benedict's test?

    • The sample must be mixed with iodine solution before testing, so that any sucrose turns blue-black
    • The sample must be dissolved in ethanol and then filtered, to separate the sucrose from the water
    • The sample must be heated above 100 C for a long time to dehydrate the sucrose before testing it
    • The sucrose must be hydrolysed with dilute hydrochloric acid and then neutralised before testing

  9. Why is the hydrolysed sucrose sample neutralised with sodium hydrogencarbonate before the Benedict's test?

    • Benedict's test needs alkaline conditions, so the acid must be removed
    • Sodium hydrogencarbonate makes sucrose more soluble in water
    • Sodium hydrogencarbonate is needed to make the iodine react with glucose
    • Acid would destroy the glucose, so it must be replaced by sodium ions

  10. A colorimeter reading of a standard glucose solution of 0.5 g/dm3 gives an absorbance of 0.20, and 1.0 g/dm3 gives 0.40. An unknown gives 0.30. What is its concentration?

    • 1.50 g/dm3
    • 0.50 g/dm3
    • 0.60 g/dm3
    • 0.75 g/dm3

  11. A 10 cm3 sample of a 2.0 mol/dm3 sugar solution is diluted with water to a final volume of 50 cm3. What is the new concentration?

    • 10.0 mol/dm3
    • 0.4 mol/dm3
    • 0.2 mol/dm3
    • 1.0 mol/dm3

  12. A solution turns orange-brown when iodine in potassium iodide solution is added. What conclusion can be drawn?

    • Starch is present in high concentration
    • Glucose is present in a high concentration
    • Starch is absent from the solution
    • The solution contains a reducing sugar

  13. A sample gives a brick-red precipitate with Benedict's solution and stays orange-brown with iodine. What can be concluded?

    • Neither starch nor a reducing sugar is present in the sample
    • Both starch and a reducing sugar are present in equal amounts
    • A reducing sugar is present and starch is absent
    • Starch is present and no reducing sugar is present

  14. A Benedict's test gives a green precipitate, and a second sample gives a brick-red precipitate. What does this suggest?

    • The first sample contains starch, while the second contains glucose
    • The first sample contains a lower concentration of reducing sugar than the second
    • The first sample contains a non-reducing sugar, while the second contains a reducing sugar
    • The first sample has been heated for too long, so the copper ions have been destroyed

  15. Which piece of apparatus is used to measure the colour intensity in a quantitative Benedict's test?

    • A Bunsen burner
    • A pipette filler
    • A water bath
    • A colorimeter

  16. A student uses distilled water as a control in a Benedict's test. What is the purpose of this control?

    • To provide a second source of glucose so that the result for the sample can be compared with it
    • To neutralise any acid that is present in the sample so that the reagent is able to work correctly
    • To increase the temperature of the mixture so that the Benedict's reaction proceeds more quickly
    • To show that the colour change is caused by the sugar and not by the reagent or the apparatus

  17. A student uses a colorimeter to measure the absorbance of a diluted sugar solution. The diluted solution was prepared by 1 in 5 dilution. Its calibrated concentration is 0.6 mg/cm3. What was the concentration of the original sample?

    • 0.12 mg/cm3
    • 0.6 mg/cm3
    • 3.0 mg/cm3
    • 5.6 mg/cm3

  18. Evaluate a student's claim that a green Benedict's test proves the sample has a lower sugar concentration than a sample giving an orange result.

    • The claim is correct, and an exact concentration can be read directly from the colour of the solution by eye
    • The comparison is reasonable as a relative judgement, but colour alone does not give an exact concentration without calibration
    • The claim is wrong because colour can never indicate the concentration of a reducing sugar at any level of accuracy
    • The claim is wrong because orange results always come from non-reducing sugars that do not react with the reagent

  19. Why does the iodine test give a blue-black colour with starch?

    • Iodine molecules fit into the coiled helix of amylose and form a coloured complex
    • Iodine is reduced by starch, forming a blue copper compound
    • Iodine reacts with the peptide bonds in starch to form a dye
    • Iodine breaks the glycosidic bonds, releasing glucose that is coloured

  20. A fruit juice is tested for sucrose. Which experimental design would best show whether sucrose is present?

    • Heat the juice with Benedict's solution for much longer to see whether the colour eventually becomes brick-red with no exceptions
    • Test one sample with Benedict's directly, then hydrolyse and neutralise a second sample and repeat the Benedict's test
    • Boil the juice with plain water for several minutes and then check whether the solution becomes cloudy
    • Test the juice with iodine solution only, because sucrose is known to turn the iodine solution blue-black

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