AQA GCSE Biology Biology Paper 2 (Foundation), November 2021: Question 9
8 marks · Standard Demand difficulty · Short Answer
Describe control measures and plot graph data from an investigation into the effect of temperature on the rate of milk decay.
Practise this questionQuestion
Question text
09 A student investigated the effect of temperature on the decay of milk.
Figure 15 shows the apparatus the student used.
Figure 15
This is the method used.
1. Set up the apparatus as shown in Figure 15 with the milk at 20 °C.
2. Record the pH over 5 days using the computer.
3. Repeat with another batch of fresh milk at 25 °C.
09.1 How could the student keep the milk at a constant temperature for 5 days?
[1 mark]
09.2 Give one variable the student should keep constant.
Do not refer to temperature in your answer.
[1 mark]
Table 2 shows the student’s results for the milk at 20 °C.
Table 2
*31* Time in days 0 1 2 3 4 5
pH 6.7 6.7 6.3 5.3 4.6 4.4
09.3 Complete Figure 16.
[4 marks]
You should:
• label the x-axis
• use a suitable scale for the x-axis
• plot the data from Table 2
• draw a line of best fit.
Figure 16
09.4 The data you plotted in Question 09.3 were obtained at 20 °C.
Sketch a line on Figure 16 to show the results you would expect at 25 °C.
Label this line ‘25 °C’.
[2 marks]
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 (put beaker in a) water bath allow (put beaker in an) 1 AO1
incubator 4.7.2.3
RPA10
09.2 volume of the milk allow amount of milk 1 AO1
or 4.7.2.3
type of milk allow named type of milk, eg RPA10
cows’ or semi-skimmed
09.3 correct scale and axis labelled scale must be at least 1 cm for 1 1 AO2
day 4.7.2.3
RPA10
all points plotted correctly allow a tolerance of ± ½ small 2
square
allow 4 or 5 correct plots for 1
mark
suitable curved line of best fit ignore line joined point to point 1
with straight lines
09.4 similar shaped line drawn to left 1 AO2
of 20°C line on Figure 4 4.7.2.3
RPA10
same start pH allow a tolerance of ± ½ small 1
square
allow from student’s line of best
fit or student’s plot for 0 days
Total 8
How to answer it
Investigating the Rate of Milk Decay Using pH
This question assesses practical investigation skills and biological knowledge from Topic 4.7 (Ecology - Decay & Required Practical 10):
- Apparatus & Techniques: Methods for maintaining a stable temperature over long periods (5 days).
- Variables: Identifying controlled variables to ensure valid experimental data.
- Graphing Skills: Correct axis scaling, labelling with units, accurate plotting within tolerances, and drawing smooth lines of best fit.
- Biological Reasoning: Predicting and sketching how an increase in temperature affects the rate of decay by microorganisms.
Maintaining Constant Temperature
How could the student keep the milk at a constant temperature for 5 days?
✅ Acceptable Answers
- (Place the beaker in a) water bath (thermostatically controlled)
- (Place the beaker in an) incubator
🧠 Exam Technique
The experiment runs continuously over 5 days. Room temperature fluctuates significantly between day and night, so passive methods fail. Always specify temperature-regulating laboratory equipment.
❌ Common Misconceptions & Errors
- "Keep it in a warm room" or "Leave on the bench": Inadequate because ambient air temperature changes throughout the 24-hour cycle.
- "Use a Bunsen burner": A Bunsen burner cannot be left unattended for 5 days and would boil the milk rather than maintaining 20 °C.
Control Variables
Give one variable the student should keep constant. (Do not refer to temperature).
✅ Acceptable Answers
- Volume of milk (e.g. 100 cm³) / amount of milk
- Type of milk (e.g. cow's milk, semi-skimmed, whole milk, pasteurised)
- Freshness / age of the starting milk (e.g. opened on the same day)
🧠 Exam Technique: Read the Negative Constraint
The prompt explicitly commanded: "Do not refer to temperature in your answer." Always circle or underline negative constraints in exam papers so you don't throw away an easy mark.
❌ Common Errors
- Writing "temperature" or "starting temperature" (strictly ignored/disallowed by the question stem).
- Vague answers like "the beaker" (beaker size affects surface area, but state "surface area of milk" or "volume of milk" instead).
Graph Construction (pH against Time)
Plotting data from Table 2 and drawing a line of best fit
📐 Step-by-Step Graph Construction
- X-axis Scale & Label (1 mark):
Label the horizontal axis: Time in days (or Time / days ).
Choose an even linear scale starting at 0 up to at least 5. The grid has plenty of space—make sure 1 day takes up at least 2 large squares (≥ 1 cm per day) so the plot occupies more than 50% of the grid. - Plotting Points (2 marks):
Plot each coordinate with a neat cross ( × ):
• Day 0 = 6.7
• Day 1 = 6.7
• Day 2 = 6.3
• Day 3 = 5.3
• Day 4 = 4.6
• Day 5 = 4.4
Tolerance is within ± 0.5 small square. (All 6 correct = 2 marks; 4 or 5 correct = 1 mark). - Line of Best Fit (1 mark):
Draw a single, smooth, continuous curved line that follows the trend of the plotted points.
❌ Plotting Pitfalls
- Point-to-point ruler joins: Do not play "join-the-dots" with straight line segments. Decay processes produce curves!
- Fuzzy/thick lines: Never sketch with repeated hairy strokes or a blunt pencil.
- Missing unit on axis: Writing just "Time" instead of "Time in days" loses the scale/axis mark.
💡 Why Does pH Decrease?
Bacteria in the milk respire and digest lactose (sugar) using enzymes, producing lactic acid. Bacterial lipases also break down lipids (fats) into glycerol and fatty acids. Both acids lower the pH.
[2] All 6 points plotted correctly within ± 1/2 small square (1 mark for 4 or 5 points).
[1] Smooth curved line of best fit.
Predicting Rates at Higher Temperatures
Sketch a line on Figure 16 to show expected results at 25 °C. Label it '25 °C'.
💡 Biological Mechanism
- 25 °C is warmer than 20 °C: Microorganisms (bacteria) multiply faster and their digestive enzymes have greater kinetic energy.
- More successful enzyme-substrate collisions mean acidic products form more rapidly.
- Therefore, the pH will drop faster (earlier in time).
✅ What Your Line Must Show
- Identical Start Point: Starts at exactly the same pH at Day 0 ( pH 6.7 , or matching your line).
- Drawn to the Left: Follows a similar sigmoidal/decay shape, but sits entirely to the left / below the 20 °C line because the drop occurs faster.
- Label: Clearly written text: 25 °C .
❌ Common Misconceptions
- Starting at a lower pH: Fresh milk starts at the same initial pH regardless of which incubator temperature it is placed into at Day 0.
- Drawing the curve to the right: A curve to the right implies a slower rate of decay, which would occur at lower temperatures (e.g. 5 °C in a fridge), not at 25 °C.
[1] Similar shaped line drawn to the left of the 20 °C curve.
Topics
Biology · Required Practicals · B7: Ecology · Required Practicals
Question and mark scheme from the AQA GCSE Biology examination, Biology Paper 2 (Foundation), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.