AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2021: Question 2

6 marks · Standard Demand difficulty · Extended Answer

Describe how to test a food sample for protein, starch and sugar, including the positive colour changes for each test.

Practise this question

Question

The image shows question 02 asking students to describe how to test a sample of food for protein, starch and sugar, and to give the colours seen if the sample contained each substance. The question is worth 6 marks and is followed by a large blank lined space for a written response. There are no diagrams, tables, graphs or additional visual elements apart from the question number and answer lines.
Question text

02 Describe how to test a sample of food for protein, starch and sugar.

Give the colours that would be seen if the food sample contained

protein, starch and sugar.

[6 marks]

Mark scheme

Show the mark scheme The mark scheme is a level-of-response table for question 02 with Level 3 worth 5 to 6 marks, Level 2 worth 3 to 4 marks, Level 1 worth 1 to 2 marks, and 0 for no relevant content. Indicative content states that protein is tested using Biuret reagent or copper sulfate and sodium hydroxide, giving a purple or lilac colour; starch is tested with iodine solution, giving black, blue-black or dark blue; and sugar is tested by grinding food, mixing with water, adding Benedict's solution, heating to at least 65 degrees Celsius in a water bath, and observing red, orange, brown, green or yellow. The specification reference shown is AO1, 4.2.2.1 and RPA3, and it notes that Level 3 requires correct references to all three tests.

AO /

Question Answers Mark

Spec. Ref.

Level 3: The method would lead to the production of a valid 5‒6 AO1

outcome. All key steps are identified and logically sequenced. 4.2.2.1

RPA3

Level 2: The method would not necessarily lead to a valid 3‒4

outcome. Most steps are identified, but the method is not fully

logically sequenced.

Level 1: The method would not lead to a valid outcome. Some 1‒2

relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content

Protein

• grind up food

• add Biuret (reagent / solution)

or

add copper sulfate (solution) and sodium hydroxide (solution)

or

add Biuret 1 and Biuret 2

• turns purple / lilac

Starch

• add iodine (solution)

• turns black / blue-black / dark blue

- ignore blue / purple

Sugar

• grind up food

• mix with water

• add Benedict’s (reagent / solution)

• heat mixture (≥ 65 °C)

• in a water bath

• turns (brick) red / orange / brown / green / yellow

For Level 3 correct references to all three tests are needed.

Total 6

How to answer it

Food Tests for Protein, Starch and Sugar

What this question tests
You need to know the correct food tests, the reagents used, how to carry out each test in the right order, and the positive colour results for protein, starch and reducing sugar. Full marks depend on giving all three tests clearly and linking each test to its result.
Overall difficulty: Low Demand

Question overview

This is a standard GCSE recall question. The examiner wants a clear method for testing a food sample for protein, starch and sugar, plus the colour changes if each nutrient is present.

Mark scheme clue: Level 3 responses included all three tests, in a sensible sequence, with the correct reagents and colours.

Part (a): Testing the food sample for protein, starch and sugar

What a full-mark answer should include

✅ Correct answers

  • Protein: grind up the food, add Biuret reagent (or copper sulfate + sodium hydroxide), and a positive result is purple / lilac.
  • Starch: add iodine solution; a positive result is blue-black (or black / dark blue).
  • Sugar: grind up the food, mix with water, add Benedict’s solution, then heat in a water bath (about 65 °C or above); a positive result is green / yellow / orange / brown / brick red.

💡 Key knowledge

  • Biuret test is for protein.
  • Iodine test is for starch.
  • Benedict’s test is for reducing sugars such as glucose.
  • Benedict’s needs heat; iodine and Biuret do not.
  • Food often has to be crushed / ground up so the chemicals can react with it properly.

🧠 Exam technique

  • Write the tests in a logical order: protein, starch, sugar.
  • Always pair the reagent with the colour change.
  • For sugar, mention heating in a water bath to show you know the method.
  • Use precise science words: solution , reagent , positive result .

❌ Common errors

  • Saying glucose instead of reducing sugar is usually okay, but the exact test is for reducing sugar.
  • Writing blue for iodine is not enough — the positive starch result is blue-black.
  • For Benedict’s, forgetting heat loses marks.
  • Mixing up Biuret and Benedict’s is a very common mistake.

Part (a): Model answer

Protein: Grind up the food and add Biuret reagent. If protein is present, the sample turns purple / lilac.

Starch: Add iodine solution to the food sample. If starch is present, it turns blue-black.

Sugar: Grind up the food, mix it with water, add Benedict’s solution, and heat it in a water bath at about 65 °C or above. If sugar is present, it turns green, yellow, orange, brown or brick red.

Why this scores full marks: It names all three tests, gives the correct reagents, includes the heating step for Benedict’s, and states the correct colours.

How the marks are awarded

Mark breakdown

  • 5–6 marks: All three tests are correct, clear, and logically sequenced.
  • 3–4 marks: Most tests are correct, but one part may be missing or not clearly linked.
  • 1–2 marks: Some relevant points, but the answer is incomplete or unclear.
  • 0 marks: No relevant content.

What made top answers stand out

  • They included all three food tests.
  • They used the correct reagent names.
  • They gave the positive colour change for each test.
  • They described the sugar test as a heated test, often in a water bath.

Where students lost marks

  • Only describing one or two tests.
  • Giving the reagent but not the colour.
  • Forgetting that Benedict’s needs heat.
  • Using vague wording like “it changes colour” without saying which colour.

Memory aid

  • Protein → Biuret → purple
  • Starch → iodine → blue-black
  • Sugar → Benedict’s + heat → green to brick red

Examiner-style checklist before you finish

  • Did I name the reagent for each test?
  • Did I say the colour result for each one?
  • Did I include heating for Benedict’s?
  • Did I mention grinding up the food sample?
  • Did I keep the answer clear and in the right order?

Topics

Biology · Required Practicals · B4: Bioenergetics · Biology Required Practicals

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 1 (Higher), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.