Food tests

1 Plan and risk assessment

Each reagent detects one kind of nutrient. Iodine solution turns blue-black with starch. Biuret reagent turns lilac with protein. Sudan III stains lipid red, and the stained oil floats as a layer. Benedict’s turns from blue through green, yellow and orange to brick red with reducing sugars, but only when heated: more sugar, further along.

Reducing sugar + Benedict’s (blue) → brick-red precipitate, when heated

Prediction

2 Apparatus

• ✓Safety goggles Protect your eyes from the reagents and hot liquid.
• ✓Three foods to test The class’s foods, labelled A, B and C.
• ✓Pestle and mortar, distilled water and a stirring rod Grind each solid food and stir it with water, so some of it dissolves.
• ✓Filter funnel, filter paper and small beakers Filter the ground food to get as clear a solution as possible.
• ✓Known solutions: starch, egg albumin, cooking oil, glucose standards (0.1–1.0 %) and sucrose Positive controls, to see what each positive result looks like, and the glucose colour series.
• ✓Iodine solution, biuret reagent, Sudan III stain and Benedict’s solution, in dropping bottles One reagent for each nutrient.
• ✓Teat pipettes, labelled Move each solution and reagent without mixing them.
• ✓Spotting tile Small wells for the iodine test.
• ✓Test tubes in a rack For the biuret and Sudan III tests.
• ✓Test tubes in a second rack, labelled For the Benedict’s tests.
• ✓Water bath: a 1 dm³ beaker of water on a tripod and gauze, with a thermometer (resolution 1 °C) Heats every Benedict’s tube to the same temperature, about 80 °C; the thermometer checks it.
• ✓Bunsen burner, tripod, gauze, heatproof mat and test-tube holder Heat the water bath; the holder lifts hot tubes.
• ✓White tile Held behind the tubes so their colours are easy to judge.

3 Method: your choices

• Which version are you doing?: AQA required practical 4: food testsLipid with Sudan III stain: a red-stained oil layer floats on top.
• Make a solution of each solid food: Filter it through filter paper to get a clear solution (AQA’s method)A clear solution: every colour change is easy to see. Some starch grains and fat stay on the filter paper.
• Make the bench safeSafety: DoneBiuret reagent contains sodium hydroxide, which damages eyes. Ethanol is highly flammable. Hot water scalds.
• Choose how to heat the Benedict’s tubes: A water bath at about 80 °CEvery tube is heated to the same temperature for the same time, so the colours can be compared. The tubes cannot spit.
• Pipettes for the solutions: A clean, labelled pipette for each solutionNothing is carried from one solution to the next, so each result belongs to its own food.
• How much reagent to add: The amounts in the methodEach colour change shows clearly.
• Judge each colour against a white tile: DoneA pale colour is hard to judge against the dark bench. White behind the tube shows its true colour.
• Let the Sudan III tubes stand: DoneStained fat is less dense than water. Given time, it floats up and collects as a red layer on top.
• How long to leave the tubes in the water bath: 5 minutesEvery tube has time to heat up and change colour fully.

Anomaly: No anomalous reading.

5 Graph

This practical has no graph: the results are in the table above.

6 Analysis

1. Which nutrient tests gave a positive result?

   12 of 12 match a careful test Right first time

2. How much reducing sugar? (semi-quantitative)

   Your potato solution turned green with Benedict’s. Compare it with your glucose standards: about how much reducing sugar is in it?

   About 0.1 % Right first time

3. Why sucrose stays blue

   Sucrose (table sugar) is a sugar. Your sucrose tube stayed blue with Benedict’s. Why does sucrose not give a positive Benedict’s test?

   Sucrose is a non-reducing sugar, and Benedict’s only reacts with reducing sugars. Right first time

4. Why the water controls

   You tested distilled water with every reagent. Why?

   It shows what a negative result looks like, to compare every other tube with. Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.

1. 01.1[2 marks]

   A student wants to find out if a food contains protein.

   Name the reagent the student should use. Give the colour change if protein is present.

   Your answer

   Biuret reagent. The solution turns from blue to lilac.

   2/2 self-marked

   • Biuret (reagent) (allow copper sulfate and sodium hydroxide)
   • (From blue) to lilac / purple / mauve (allow pink)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Biuret reagent. It turns from blue to lilac (purple).

2. 01.2[3 marks]

   A student wants to show that a sports drink contains glucose.

   Describe how the student could test the drink for glucose. Give the result if glucose is present.

   Your answer

   Add Benedict’s solution to a sample of the drink. Heat it in a water bath at 80 °C for 5 minutes. The colour changes from blue to brick red.

   3/3 self-marked

   • Add Benedict’s (solution / reagent) to the drink (allow Fehling’s)
   • Heat it (boiling, or a hot water bath at 65 °C or more; ignore "warm"; a water bath with no temperature is not enough)
   • (Blue) changes to (brick) red (allow orange / yellow / green / brown)

   Self-marked: 3 of 3 marks. This is your own judgement against the mark scheme.

   Model answer: Add Benedict’s solution to the drink. Heat it in a water bath at about 80 °C. The blue solution changes to brick red.

3. 01.3[2 marks]

   A student wants to find out if a potato contains starch.

   Describe a test for starch. Give the result if starch is present.

   Your answer

   Add a few drops of iodine solution. It changes from orange-brown to blue-black.

   2/2 self-marked

   • Add iodine solution, or a drop of iodine (the word “iodine” on its own, with no adding, is not enough)
   • (From orange / brown / yellow) to blue-black (allow black or dark blue)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Add iodine solution. It changes from orange-brown to blue-black.

4. 01.4[1 mark]

   A student added Sudan III stain to a food solution in a test tube, shook it gently and left it to stand.

   Which result shows that the food contains lipid?

   Tick (✓) one box.

   • The solution turns blue-black.
   • ✓ A red-stained oily layer floats on top. (your choice)
   • The solution turns lilac.
   • A brick-red precipitate forms.

   Your answer

   A red-stained oily layer floats on top.

   1/1

   • A red-stained oily layer floats on top.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: A red-stained oily layer floats on top.

5. 02.1[1 mark]

   You heated glucose solutions of 0 %, 0.1 %, 0.25 %, 0.5 % and 1.0 % with Benedict’s solution and recorded the colour of each.

   Give one control variable in this investigation.

   Your answer

   The temperature of the water bath.

   1/1 self-marked

   • Volume of (glucose) solution / volume of Benedict’s / temperature of the water bath / time in the water bath (ignore amount unqualified)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The temperature of the water bath (or the volume of Benedict’s solution, or the time in the water bath).

6. 02.2[1 mark]

   Figure 1 shows part of the thermometer in your water bath.

   What is the resolution of the thermometer?

   Your answer

   1

   1/1

   • The size of one small division of the scale

   Full marks. Well done.

   Model answer: 1 °C

7. 02.3[1 mark]

   Table 1 shows another student’s Benedict’s results for the glucose standards.

   Which result is anomalous? Select one row.

   Table 1

   Concentration of glucose in %	Colour after heating
   0	Stays blue
   0.1	Green
   0.25	Yellow
   0.5	Green
   1	Brick-red precipitate

   Your answer

   the 0.5 % tube

   1/1

   • The tube whose colour is behind the weaker solution before it

   Full marks. That is exactly what the mark scheme is looking for. The colour should move further along the series as the concentration goes up.

   Model answer: The 0.5 % tube (green): it is behind the 0.25 % tube.

8. 02.4[1 mark]

   Table 1 shows another student’s Benedict’s results for the glucose standards.

   Table 1

   Concentration of glucose in %	Colour after heating
   0	Stays blue
   0.1	Green
   0.25	Yellow
   0.5	Green
   1	Brick-red precipitate

   Suggest one reason for the anomalous result in Table 1.

   Your answer

   The tube was put into the water bath later, so it was heated for less time than the others.

   1/1 self-marked

   • The tube was not heated enough / for less time / not fully in the water bath (allow the solution was made up too dilute; ignore human error unqualified)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The tube was not heated for as long as the others (it went into the water bath late, or came out early).

9. 02.5[1 mark]

   Table 2 shows your glucose standards. Your potato solution turned green, heated in the same way.

   Table 2

   Concentration of glucose in %	Colour after heating
   0	Stays blue
   0.1	Green
   0.25	Yellow
   0.5	Orange
   1	Brick-red precipitate

   Estimate the concentration of reducing sugar in the potato solution.

   Tick (✓) one box.

   • Less than 0.1 %
   • ✓ About 0.1 % (your choice)
   • About 0.25 %
   • About 0.5 %
   • About 1 % or more

   Your answer

   About 0.1 %

   1/1

   • About 0.1 %

   Full marks. Well done.

   Model answer: About 0.1 %: the standard with the same colour.

10. 02.6[2 marks]

    Your 1 % sucrose solution stayed blue when it was heated with Benedict’s solution.

    Sucrose is a sugar. Explain why the Benedict’s solution stayed blue.

    Your answer

    Sucrose is a non-reducing sugar. Benedict’s only reacts with reducing sugars such as glucose.

    2/2 self-marked

    • Sucrose is a non-reducing sugar
    • Benedict’s only reacts with / detects reducing sugars (such as glucose); so sucrose does not reduce the copper (ions) in Benedict’s / does not react with it

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: Sucrose is a non-reducing sugar. Benedict’s solution only reacts with reducing sugars, such as glucose.

11. 02.7[1 mark]

    Another student tested starch solution, then distilled water, with iodine solution on a spotting tile. The distilled water turned blue-black.

    Give one reason for testing distilled water with the iodine solution.

    Your answer

    It is a negative control to compare the other results with.

    1/1 self-marked

    • It is a (negative) control: to compare the other results with / to show what a negative result looks like / to show the iodine solution on its own does not turn blue-black

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: It is a control: it shows what a negative result looks like, to compare the foods with.

12. 02.8[1 mark]

    A student tested starch solution, then distilled water, with iodine solution on a spotting tile. The distilled water turned blue-black.

    Suggest why the distilled water turned blue-black.

    Your answer

    The same pipette was used for the starch and the water, so some starch contaminated the water.

    1/1 self-marked

    • The same / a dirty pipette (or well) carried starch into the water (contamination)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: The same pipette was used for the starch and the water, so starch was carried into the water (contamination).

13. 03.1[2 marks]

    A technician made 25 cm³ of 0.2 % glucose solution by diluting a 1.0 % glucose solution with distilled water.

    Calculate the volume of 1.0 % glucose solution the technician used.

    Your answer

    5

    Working: 0.2 / 1.0 x 25 = 5

    2/2

    • 0.2 ÷ 1.0 (= 0.2) × 25
    • 5 cm³

    Full marks. Clear and precise.

    Model answer: volume = 0.2 ÷ 1.0 × 25 = 5 cm³

14. 03.2[2 marks]

    A 1.0 % glucose solution contains 1.0 g of glucose in every 100 cm³.

    Calculate the mass of glucose in the volume of 1.0 % solution the technician used (your answer to Question 03.1). Give your answer in milligrams (mg).

    Your answer

    50

    Working: 5 x 1.0 / 100 = 0.05 g 0.05 x 1000

    2/2

    • mass = 5 × 1.0 ÷ 100 = 0.05 g (allow error carried forward from 03.1)
    • 50 mg (allow error carried forward)

    Full marks. Well done.

    Model answer: mass = 5 × 1.0 ÷ 100 = 0.05 g = 50 mg

15. 03.3[1 mark]

    Two students compare the Benedict’s colours of two fruit juices to decide which contains more sugar.

    Give one limitation of using the colour of the Benedict’s solution to compare the juices.

    Your answer

    The colour is judged by eye, which is subjective.

    1/1 self-marked

    • Judging a colour is subjective / based on opinion (ignore human error unqualified)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: Judging the colour is subjective: different people may see the same colour differently.

16. 04[2 marks]

    A student heated a tube of food solution and Benedict’s solution by holding it in a Bunsen flame.

    Give one risk of doing this, and one way to reduce the risk.

    Your answer

    The hot Benedict’s solution could spit into the student’s eyes. Use a water bath instead and wear goggles.

    2/2 self-marked

    • Burns from the flame / Benedict’s spitting (boiling over) into the face or eyes / scalds (ignore “it is hot”)
    • Use a water bath / wear eye protection (goggles) / point the tube away from people / use a test-tube holder

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: Risk: the boiling Benedict’s could spit into the student’s eyes (or the flame could burn the skin). Control: heat the tube in a water bath instead (and wear eye protection).

17. 05[6 marks]

    A student wants to find out which nutrients are in a sample of milk.

    Describe how the student could test the milk for protein, starch and sugar. Give the result of each test if the nutrient is present.

    Your answer

    Protein: 1. Put 2 cm³ of milk in a test tube. 2. Add 2 cm³ of biuret reagent and shake gently. 3. If it turns from blue to lilac (purple), the milk contains protein. Starch: 4. Put a separate sample of milk on a spotting tile and add a few drops of iodine solution. 5. If it turns from orange-brown to blue-black, starch is present. Sugar: 6. Put 2 cm³ of milk in a test tube and add 2 cm³ of Benedict’s solution. 7. Heat it in a water bath at about 80 °C for 5 minutes. 8. If the blue changes to green, yellow, orange or brick red, sugar is present. Compare each tube with a water control against a white tile.

    6/6 self-marked

    • Add biuret reagent (allow copper sulfate and sodium hydroxide)
    • Biuret turns from blue to lilac / purple if protein is present
    • Add iodine solution (to a sample)
    • Iodine turns from orange-brown to blue-black if starch is present
    • Add Benedict’s solution (to a separate sample)
    • Heat with Benedict’s (in a water bath at about 80 °C, at least 65 °C) for a few minutes
    • Benedict’s turns from blue to green / yellow / orange / brick red if sugar is present
    • Put a separate (equal) sample in a test tube for each test
    • Compare with a control (water) / a white tile behind the tubes

    Self-marked: Level 3, 6 of 6 marks. You ticked 9 of 9 indicative points. This is your own judgement against the mark scheme.

    Model answer: Protein: 1. Put 2 cm³ of milk in a test tube. 2. Add 2 cm³ of biuret reagent and shake gently. 3. If it turns from blue to lilac (purple), the milk contains protein. Starch: 4. Put a separate sample of milk on a spotting tile and add a few drops of iodine solution. 5. If it turns from orange-brown to blue-black, starch is present. Sugar: 6. Put 2 cm³ of milk in a test tube and add 2 cm³ of Benedict’s solution. 7. Heat it in a water bath at about 80 °C for 5 minutes. 8. If the blue changes to green, yellow, orange or brick red, sugar is present. Compare each tube with a water control against a white tile.

Unseen data: the same skills, new apparatus

1. 01[2 marks]

   A separate investigation tests crisps. The positive and negative controls behaved as expected. Separate portions of the food were used for each test.

   New recorded observations

   Test	Result
   Iodine solution	Turns blue-black
   Biuret reagent	Turns lilac or purple

   Which two interpretations match the observations? Tick (✓) two boxes.

   • ✓ Starch was detected by the blue-black iodine reaction. (your choice)
   • ✓ Protein was detected by the purple biuret reaction. (your choice)
   • These two tests measure the exact amount of reducing sugar.
   • Any negative test proves complete absence of every nutrient from the original food.

   Your answer

   Starch was detected by the blue-black iodine reaction.; Protein was detected by the purple biuret reaction.

   2/2

   • Starch was detected by the blue-black iodine reaction.
   • Protein was detected by the purple biuret reaction.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Iodine tests for starch: positive. Biuret tests for protein: positive. These qualitative results do not provide exact concentrations or establish complete absence.

2. 02[1 mark]

   Another simulated student used one unrinsed pipette for the known starch and water controls. Evaluate the recorded results.

   Control observations

   Control solution	Iodine result
   Known starch	Turns blue-black
   Distilled water	Turns blue-black

   Which conclusion is justified? Tick (✓) one box.

   • The controls behaved as expected, supporting interpretation of this iodine test for starch.
   • ✓ Known starch and water both turned blue-black; investigate contamination and repeat with clean equipment and fresh controls. (your choice)
   • The positive control failed, or a control result was missing or unrecognised; check the method before interpreting the food.
   • A blue-black iodine result detects protein.

   Your answer

   Known starch and water both turned blue-black; investigate contamination and repeat with clean equipment and fresh controls.

   1/1

   • Known starch and water both turned blue-black; investigate contamination and repeat with clean equipment and fresh controls.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Distilled water should not produce a starch-positive result. Contamination is a possible cause to investigate, not a property of pure water.

3. 03[2 marks]

   Which two method statements are correct? Tick (✓) two boxes.

   • ✓ Use suitable controlled water-bath heating for Benedict’s tests, with a known glucose positive control and water negative control. (your choice)
   • ✓ Use separate food portions and clean labelled equipment for iodine, biuret and Benedict’s tests, and record each final colour. (your choice)
   • Add all three reagents to the same portion so only one colour has to be judged.
   • A blue-black iodine result is the positive test for protein.

   Your answer

   Use suitable controlled water-bath heating for Benedict’s tests, with a known glucose positive control and water negative control.; Use separate food portions and clean labelled equipment for iodine, biuret and Benedict’s tests, and record each final colour.

   2/2

   • Use suitable controlled water-bath heating for Benedict’s tests, with a known glucose positive control and water negative control.
   • Use separate food portions and clean labelled equipment for iodine, biuret and Benedict’s tests, and record each final colour.

   Full marks. Well done.

   Model answer: Benedict’s needs suitable heating and working controls. Test separate portions with the correct reagents and clean equipment to avoid interference and contamination.