AQA GCSE Biology · EcologyTriple only
Decay required practical
Investigate the effect of temperature on the rate of decay of fresh milk by measuring pH change.
Lipase breaks down the fat in milk into fatty acids, so the pH falls. Phenolphthalein turns from pink to colourless when that happens, so you can time it at different temperatures.
This one is only on the separate Biology GCSE, so it won’t come up in Combined Science.
Equipment
- Full-fat milk and lipase solution
- Sodium carbonate solution (makes the milk alkaline)
- Phenolphthalein indicator
- Water baths at 20, 30, 40, 50 and 60 °C
- Test tubes, measuring cylinders and pipettes
- Thermometer and stopwatch
Variables
- Independent you change
- Temperature
- Dependent you measure
- Time for the phenolphthalein to turn colourless
- Control you keep the same
- Same volume of milk
- Same volume and concentration of lipase
- Same volume of sodium carbonate and indicator
Method
- 1
Put 5 drops of phenolphthalein into a test tube with 5 cm³ of milk and 7 cm³ of sodium carbonate solution. It turns pink.
- 2
Put this tube and a separate tube of 1 cm³ lipase in a water bath at 20 °C.
- 3
When both reach 20 °C (check with a thermometer), add the lipase to the milk and start the stopwatch.
- 4
Stir and watch until the solution turns from pink to colourless. Stop the clock.
- 5
Repeat at 30, 40, 50 and 60 °C.
- 6
Calculate the rate as 1000 ÷ time for each temperature.
Worked example
Time for the pink colour to disappear at each temperature.
| Temperature (°C) | Time (s) | Rate (1000 ÷ time, s⁻¹) |
|---|---|---|
| 20 | 480 | 2.1 |
| 30 | 250 | 4.0 |
| 40 | 140 | 7.1 |
| 50 | 220 | 4.5 |
| 60 | 600 | 1.7 |
Working
Rate = 1000 ÷ time
40 °C: 1000 ÷ 140 = 7.1 s⁻¹
60 °C: 1000 ÷ 600 = 1.7 s⁻¹
Conclusion
Decay is fastest at about 40 °C. Up to there, enzymes and particles move faster and collide more often. Above it, the lipase starts to denature, so the rate falls. This is why food is kept cold, and why compost heaps work best when warm.
Risks
- Phenolphthalein is flammable and irritates eyesWear eye protection and keep it away from flames.
- Sodium carbonate is an irritantAvoid skin contact and rinse off splashes.
- Hot water bathsTake care with the 60 °C bath.
Common mistakes
- Adding the lipase before the tubes reach temperature. Both must be warmed in the water bath first.
- Judging the end point differently each time. Put a white tile behind the tube and compare with a colourless reference.
- Saying the bacteria die at high temperature. Here, lipase is an enzyme; it is denatured.
Better results
- Use a pH meter or pH probe to measure the change more precisely than a colour change.
- Repeat each temperature three times and calculate a mean.
- Test temperatures closer together around 40 °C to find the optimum more precisely.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Ecology flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.