AQA GCSE Combined Science: Trilogy Biology Paper 1 (Foundation), 2021: Question 6

Mark scheme

Show the mark scheme

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

Testing Food for Nutrients

What this question tests

This question assesses your knowledge of Required Practical 3 (Food Tests). You need to recall the specific chemical reagents used for starch, sugar, and protein, the experimental steps (including safety and heating), and the distinct colour changes that indicate a positive result.

Question 06 (6 Marks)

The Full Method for Food Testing

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.

💡 Key Knowledge: Reagents

Starch: Iodine solution.
Protein: Biuret reagent (or Copper Sulfate + Sodium Hydroxide).
Sugar: Benedict's solution.

🧠 Exam Technique: 6-Markers

This is a Levelled Response question. To get 5-6 marks (Level 3), you must:

Include all three tests.
Provide a logical sequence (e.g., preparing the sample first).
Mention specific conditions (like heating for sugar).

✅ Model Answer Structure

1. Sample Preparation: For all tests, first grind the food sample using a mortar and pestle and mix with distilled water to create a solution (filter if necessary, though not for the starch test).

2. Test for Starch: Add a few drops of Iodine solution to the food sample.
➔ Positive result: Colour changes from orange/brown to blue-black.

3. Test for Protein: Add Biuret reagent to the food solution and shake gently.
➔ Positive result: Colour changes from blue to purple or lilac.

4. Test for Sugar: Add Benedict's solution to the food solution. Place the test tube in a water bath heated to at least 65°C for 5 minutes.
➔ Positive result: Colour changes from blue to green, yellow, or brick-red (depending on the concentration of sugar).

❌ Common Errors

Forgetting Heat: Many students lose marks by not mentioning that Benedict's solution must be heated in a water bath.
Vague Colours: Avoid saying "it changes colour." You must state the specific starting and ending colours.
Mixing up "B"s: Don't confuse Biuret (Protein) with Benedict's (Sugar).

📢 Examiner Commentary

Top-level students noted that for the sugar test, the colour change is a scale. Green indicates a little sugar, while brick-red indicates a high concentration. Examiners also look for the mention of a "water bath" rather than using a direct Bunsen flame for safety and even heating.

Visual Summary

🎨 Colour Change Cheat Sheet

Nutrient	Reagent	Positive Result
Starch	Iodine	Blue-Black
Protein	Biuret	Purple / Lilac
Sugar	Benedict's (+ Heat)	Brick-Red

Topics

Biology · B2: Organisation