AQA GCSE Biology Biology Paper 1 (Higher), 2024: Question 3

💡 Key knowledge

• Iodine solution tests for starch.
• Iodine changes from brown/orange to blue-black if starch is present.
• Benedict’s reagent tests for sugar/glucose.
• For Benedict’s, you must heat the sample, usually in a hot water bath.
• Benedict’s changes from blue to green/yellow/orange/brick red if sugar is present.

✅ Correct answer structure

To test for starch: add iodine solution to the cake sample. If starch is present, the iodine turns blue-black.

To test for sugar/glucose: add Benedict’s reagent to the cake sample and heat it in a water bath. If sugar is present, it changes from blue to green/yellow/orange/brick red.

Risk assessment: use a water bath and care with hot equipment to avoid burns; wear eye protection and point test tubes away from the face when heating; clean up spills because reagents may irritate skin/eyes.

🧠 Exam technique

• Name the reagent and the food tested.
• Always include the positive result colour.
• For Benedict’s, mention heating — without heating, the method is incomplete.
• For a top-level answer, write the tests in a logical sequence: starch test, then sugar test, then risk.

❌ Common errors

• Saying only “test for carbohydrates” is too vague.
• Mixing up the colour change: iodine goes blue-black, not Benedict’s.
• Forgetting to heat Benedict’s reagent.
• Calling the sugar test “Benedict’s test for starch” — this loses the mark.
• Giving a risk such as “be careful” without saying what the hazard is or how to reduce it.

✅ Correct answer

The time taken for the bread to taste sweet.

💡 Key knowledge

The dependent variable is what is measured in the investigation. Here, the student measured how long it took before the bread tasted sweet.

❌ Common errors

Don’t write bread type — that is the independent variable. Also, “time” alone is usually acceptable only if clearly linked to becoming sweet.

✅ Correct answers

• Size or mass of bread
• Surface area of bread
• Location of bread on the tongue
• Cleaning the mouth between tests

🧠 Exam technique

Give one specific factor that should be kept the same for each bread type. The mark scheme allows several answers, so choose the one easiest to explain clearly.

❌ Common errors

Do not give the same student or the temperature of the bread — these are ignored by the mark scheme.

✅ Correct answer

Bread contains starch.
Starch is broken down into sugar by amylase in saliva.
The sugar then reaches the taste receptors on the tongue, so the bread tastes sweet.

Alternative acceptable route: bread contains sugar/glucose; it dissolves in saliva; the sugar reaches the taste receptors.

💡 Key knowledge

• Saliva contains the enzyme amylase.
• Amylase breaks down starch into sugars such as maltose/glucose.
• Taste is detected when dissolved substances stimulate taste receptors on the tongue.

❌ Common errors

• Saying simply “the bread is sweet” without explaining why.
• Forgetting the enzyme name amylase.
• Confusing saliva with stomach acid or digestion in the stomach.

✅ Correct answers

• The investigation was not repeated by the same or different students.
• The results rely on the student’s perception of taste, so taste is subjective.
• The amount of saliva/amylase produced may vary.

🧠 Exam technique

This is a validity question, not a control-variable question. Give one reason the results may not be trustworthy or repeatable.

❌ Common errors

The mark scheme says to ignore control-variable answers here, so do not write about bread size or surface area.

💡 Carbohydrate tests

• Iodine → starch → blue-black
• Benedict’s + heat → sugar/glucose → green to brick red

🧠 What examiners want

• Correct reagent
• Correct positive colour change
• Risk assessment for heating and chemicals
• Clear explanation of the sweet taste in bread

📐 Calculation-style thinking

No numerical calculation is needed here, but you should still answer in a step-by-step way:

1. State the test.
2. State what it tests for.
3. State the positive result.
4. Add the safety point.