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Amylase breaks down starch into smaller carbohydrates. Use appropriate buffers to compare pH while controlling mixed enzyme and starch concentrations, total volume and temperature. Allow the relevant components to reach the selected temperature before mixing and timing. Start each comparison using the same timing and mixing rule.
Video by Malmesbury Education. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Design a controlled pH comparison
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which variable should change in a pH investigation?
The buffered reaction pH. Keep other relevant conditions comparable.
After watching 2: Why sample onto separate iodine portions?
To test remaining starch without changing the whole reaction with indicator. The sample reports the reaction state at an interval.
What you'll be able to do
Amylase breaks down starch into smaller carbohydrates. Use appropriate buffers to compare pH while controlling mixed enzyme and starch concentrations, total volume and temperature. Allow the relevant components to reach the selected temperature before mixing and timing. Start each comparison using the same timing and mixing rule.
An active site must have a suitable shape for its substrate. Conditions away from an enzyme’s suitable pH range can reduce activity, but a single slow reading does not prove permanent denaturation. Check preparation, temperature, concentrations and observation uncertainty as alternative explanations.
Take small samples from the reaction at the specified regular intervals and test them on separate iodine portions. A blue-black result shows detectable starch remains in that sample; an iodine colour that does not become blue-black supports the defined endpoint. Do not add all the indicator to the main reaction vessel and then treat that altered mixture as the original method.
Keep sampling and indicator quantities comparable. The actual starch endpoint lies between the last positive sample and the first negative sample, so sampling interval limits time resolution. Retain the raw sequence rather than claiming the reaction ended at an exact unobserved instant.
AQA RP5 specifies iodine testing of starch samples every 30 seconds, with temperature controlled by a water bath or electric heater. Explain the timing limitation of that sampling interval.
For the same initial substrate amount and endpoint rule, 1/time can be used as a relative rate proxy. A shorter digestion time gives a larger reciprocal-time value. The unit is s⁻¹ when time is in seconds; it is not automatically a measured mass of starch digested per second.
Use repeated fresh reactions at each pH, calculate appropriate summaries and show the variation. Choose a suitable graph and avoid assuming a perfectly symmetrical pH response. An anomalous point needs investigation or a stated treatment rule rather than silent deletion to produce an expected curve.
| Reaction pH | First negative starch sample / s | Reciprocal-time proxy / s⁻¹ |
|---|---|---|
| 5 | 120 | 0.00833 |
| 7 | 60 | 0.0167 |
| 9 | 150 | 0.00667 |
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The starch test is positive at 60 s and first negative at 90 s. What time information is justified?
An endpoint bracket between 60 and 90 s, not an exact observed finish at 75 s.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.