AQA AS Level Biology Paper 2, June 2024: Question 6

7 marks · Medium difficulty · Practical Techniques & Data Analysis

Describe how to prepare a 60% dilution of disinfectant, determine the absolute and percentage uncertainty of a pipette measurement, and evaluate the disinfectant's effectiveness from absorbance data.

Practise this question

Question

Question 06 shows an investigation into the effect of disinfectant X concentrations on the growth of Bacillus subtilis. Table 2 provides data for percentage concentration of disinfectant X (0, 20, 40, 60, 80, 100) and percentage light absorbance (100, 100, 87, 52, 10, 10). Part 06.1 asks how to prepare 5 cm³ of 60% concentration disinfectant using distilled water. Part 06.2 asks for the uncertainty and percentage uncertainty of measuring 2 cm³ using a pipette with 0.1 cm³ graduations. Part 06.3 asks to evaluate the effectiveness of disinfectant X at killing microorganisms on non-living surfaces using Table 2.
Question text

06.1 Disinfectants are used to kill microorganisms on non-living surfaces. A student

investigated the effect of different concentrations of disinfectant X on the growth of

Bacillus subtilis.

The student:

• added 5 cm3 of a different concentration of disinfectant X to 5 different test tubes

• added 5 cm3 of distilled water to another test tube

• added 2 cm3 of a culture of B. subtilis to all 6 test tubes

• incubated the test tubes at 25 °C for 24 hours

• used a colorimeter to record the percentage of light absorbed by the contents of

each tube.

Table 2 shows the student’s results.

Table 2

Percentage concentration

0 20 40 60 80 100

of disinfectant X

Percentage light absorbance 100 100 87 52 10 10

The student prepared the different concentrations of disinfectant X.

Describe how the student made 5 cm3 of the 60% concentration using distilled water

and undiluted disinfectant.

[1 mark]

06.2 The student used a sterile pipette with 0.1 cm3 graduations to transfer 2 cm3 of

B. subtilis into each test tube.

What is the uncertainty in measuring 2 cm3 with this pipette?

Calculate the percentage uncertainty of this 2 cm3 measurement.

[2 marks]

Uncertainty cm3

Percentage uncertainty

06.3 Use Table 2 to evaluate the effectiveness of disinfectant X at killing microorganisms

*14* on non-living surfaces.

[4 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 06: 06.1 awards 1 mark for 3 cm³ of disinfectant and 2 cm³ of water. 06.2 awards 2 marks: 1 for uncertainty of 0.05 cm³, 1 for percentage uncertainty of 2.5%. 06.3 awards up to 4 marks for evaluation points including: equally effective at 80% and 100%; not effective at 20%; light absorbance may not indicate live bacteria/cells; 100% still leaves 10% absorbance; only tested on one species; only tested in test tube/laboratory; only tested at 25 °C.

Question Marking Guidance Mark Comments

3 cm3 of (undiluted) disinfectant and 2 cm3 of 1

06.1 (distilled) water; (1 x

AO2)

1. (Uncertainty =) 0.05 (cm3);

2. (Percentage uncertainty =) 2.5; 2 2. Allow correct

06.2 percentage

(2 x

uncertainty from

AO2)

student’s incorrect

answer to MP1.

1. Equally effective at 80% as at 100%;

2. Not effective at 20%

OR

Need higher than 20% concentration to be

effective;

3. Light absorbance may not indicate number of

living bacteria/cells

OR

Dead cells/bacteria absorb light

OR 4 max

06.3 (4 x

Don’t know if cells alive/dead at 80/100 %;

AO3)

4. 100% not totally effective as still 10%/some 6. Accept any

absorbance; description which

indicates not tested

5. Only (tested on) one species;

on non-living

OR surfaces.

Only tested on B. subtilis; Ignore statistical test,

sample size and

6. Only tested in laboratory/test tube

reference to repeats.

OR

May not be as effective on other surfaces;

7. Only (tested) at 25oC;

How to answer it

Evaluating Disinfectant Effectiveness on Microbial Growth

📌 What this question tests

This question assesses practical experimental skills, quantitative data handling, and scientific evaluation in microbiology:

  • Dilution calculations: Preparing a specific volume and concentration from a stock solution using simple proportional dilution.
  • Measurement uncertainty: Determining absolute uncertainty from instrument graduations and calculating percentage uncertainty.
  • Critical data evaluation (AO3): Interpreting colorimeter absorbance data, recognising biological limitations (viable vs non-viable counts), and identifying flaws in experimental design when generalizing conclusions.

Question 06.1

Dilution Preparation

"Describe how the student made 5 cm³ of the 60% concentration using distilled water and undiluted disinfectant." [1 mark]

📐 Step-by-Step Calculation

  1. Identify target parameters: Total volume = 5 cm³, Desired concentration = 60%.
  2. Calculate stock disinfectant volume:
    Volume = 0.60 × 5 cm³ = 3.0 cm³
  3. Calculate volume of distilled water:
    Water = 5.0 cm³ - 3.0 cm³ = 2.0 cm³

✅ Correct Answer

3 cm³ of (undiluted) disinfectant and 2 cm³ of (distilled) water.

Award 1 mark (AO2) for both correct volumes and their corresponding components. Units (cm³) must be clear.

🧠 Exam Technique

  • State both the solute (disinfectant) and solvent (distilled water) volumes clearly.
  • Always double-check that the sum equals the required total volume: 3 cm³ + 2 cm³ = 5 cm³ .

❌ Common Errors

  • Giving a ratio (e.g. 3:2) without specifying actual volumes.
  • Calculating 60% of 100 cm³ instead of 5 cm³.
  • Omitting units or confusing which volume refers to water vs. disinfectant.

Question 06.2

Apparatus Uncertainty & Percentage Uncertainty

"The student used a sterile pipette with 0.1 cm³ graduations to transfer 2 cm³ of B. subtilis into each test tube. What is the uncertainty in measuring 2 cm³ with this pipette? Calculate the percentage uncertainty of this 2 cm³ measurement." [2 marks]

📐 Step-by-Step Calculation

  1. Determine absolute uncertainty:
    For standard measuring instruments, absolute uncertainty is half the smallest division:
    Uncertainty = 0.1 cm³ ÷ 2 = 0.05 cm³
  2. Apply percentage uncertainty formula:
    % Uncertainty = (Absolute Uncertainty ÷ Measured Value) × 100
  3. Calculate:
    % Uncertainty = (0.05 ÷ 2.0) × 100 = 2.5%

✅ Correct Answer

  • Uncertainty: 0.05 cm³ [1 mark]
  • Percentage uncertainty: 2.5% [1 mark]
Allow Error Carried Forward (ECF) for the percentage calculation if an incorrect absolute uncertainty was derived in MP1 (e.g. if 0.1 was given, (0.1/2)×100 = 5% gains 1 mark).

💡 Key Knowledge

The margin of error (uncertainty) of a single reading on an analogue scale is taken as half the smallest graduation:

Smallest graduation = 0.1 cm³ → Uncertainty = ±0.05 cm³

❌ Common Errors

  • Stating the graduation itself (0.1 cm³) as the absolute uncertainty instead of dividing by 2.
  • Dividing by the total volume of the pipette rather than the measured value (2 cm³).
  • Forgetting to multiply by 100 to convert the decimal into a percentage.

Question 06.3

Data Evaluation: Disinfectant Effectiveness

"Use Table 2 to evaluate the effectiveness of disinfectant X at killing microorganisms on non-living surfaces." [4 marks]

Percentage concentration of disinfectant X (%) 0 20 40 60 80 100
Percentage light absorbance (%) 100 100 87 52 10 10

✅ Mark Scheme Points (Any 4 from):

  • Equally effective at 80% as at 100%: Absorbance levels off at 10% for both concentrations.
  • Not effective at 20%: Light absorbance remains at 100% (same as 0% control) / Need a concentration >20% to have any effect.
  • Turbidity/absorbance limitation: Absorbance measures turbidity and does not distinguish between living and dead bacteria (dead cells still scatter/absorb light).
  • Incomplete elimination: Disinfectant is not 100% effective even at maximum concentration because there is still 10% absorbance remaining.
  • Species limitation: Only tested on one bacterial species (Bacillus subtilis); other bacteria/viruses/fungi may respond differently.
  • Environmental validity limitation: Only tested in liquid culture inside a laboratory/test tube, not on actual dry non-living surfaces.
  • Temperature limitation: Only tested at one set temperature (25 °C).

🧠 Exam Technique: Structuring an "Evaluate" Question

Always balance your response with points supporting and points limiting the conclusion:

  • Evidence for effectiveness: Cite data trends (e.g. absorbance decreases sharply from 40% to 80%).
  • Evidence against effectiveness: Mention threshold effects (20% has zero effect) and plateau (10% residual absorbance).
  • Biological limitations: Explain what absorbance actually measures (turbidity ≠ living viable cells).
  • Ecological / experimental validity: Critique the experimental setup versus the real-world claim ("non-living surfaces" vs liquid test tubes).

❌ Common Errors & Examiner Pitfalls

  • Vague generic statements: Simply saying "they didn't do repeats" or "sample size is unknown" gains zero marks. The mark scheme explicitly states: Ignore statistical test, sample size and reference to repeats.
  • Confusing absorbance with cell death: Assuming 10% absorbance means "90% of bacteria died". A colorimeter detects turbidity, not viability.
  • Failing to compare to the control: Overlooking that 20% disinfectant has exactly the same absorbance (100%) as the 0% control.

Topics

Practical skills · Required Practicals · Experimental design · Data analysis · Uncertainty and evaluation · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.