Plan a controlled pH investigation with buffers
Use matched amylase and starch samples to compare pH fairly.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall protein building blocks and the distinction between a molecule and a living cell.
- Use the supplied observations, quantities and units rather than assuming an optimum for every enzyme.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why use a buffer in the pH comparison?
What you will learn
- Use matched amylase and starch samples to compare pH fairly.
- Apply the explanation to the stated evidence and recognise the limits of the model.
Change pH deliberately
Use teacher-prepared buffers to maintain different pH conditions while comparing amylase action on starch. Match the enzyme and substrate portions and the total volume. Keep reaction temperature constant rather than using a different temperature at every pH.
Start timing when the enzyme and substrate are mixed, using the same order and mixing method for each trial. Test reaction samples with separate iodine drops at regular intervals; record the first sample in which starch is not detected.
Make the comparison credible
Keep enzyme, starch and buffer volumes and concentrations matched as appropriate for the approved method. Use working starch-positive and target-free iodine comparisons, clean equipment and repeated trials. A matched no-active-enzyme control helps check changes unrelated to enzyme action.
A different buffer volume can dilute enzyme and substrate, so pH and dilution must not change together unnoticed. Use appropriate controls for indicator behaviour in the chosen pH range. The best tested pH is an estimate for that preparation; these written exercises do not certify supervised practical skill.
Sources for this lesson
Pause and explain
Which design changes pH without an avoidable dilution difference?
Worked example
A student adds twice as much buffer at pH 8 as at pH 6 without matching total volume. What other change could affect the result?
Show the worked solution
- The added buffer changes total volume.
- The enzyme and starch are diluted differently.
- pH is therefore not the only changed condition.
Answer Unequal dilution is a confounder; use an approved matched-volume comparison.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the changed variable and a suitable control for the pH investigation.
Give me a hint
Separate pH from temperature.
Compare my reasoning
- Change pH using buffers.
- Keep temperature constant.
pH is changed; temperature is controlled.
Look for these in your work
- Identify pH.
- Name temperature as a control.
A pH trial has 2 cm³ enzyme, 2 cm³ starch and 1 cm³ buffer. Another uses the same enzyme and starch but 3 cm³ buffer. Compare total volumes and explain the concern.
Give me a hint
Add volumes before interpreting pH.
Compare my reasoning
- First total: 2 + 2 + 1 = 5 cm³.
- Second total: 2 + 2 + 3 = 7 cm³.
- Reactants are diluted differently.
Totals are 5 and 7 cm³; unequal dilution confounds the pH comparison.
Look for these in your work
- Calculate both totals.
- Explain the dilution confounder.
At one tested pH, even the known starch-positive iodine comparison remains yellow-brown. Can the enzyme endpoint at that pH be trusted?
Give me a hint
Validate the indicator before interpreting loss of starch.
Compare my reasoning
- The known positive comparison failed.
- A negative-looking sample could reflect the test failure.
- Check the indicator procedure and pH interference with teacher guidance and repeat.
No; the failed starch-positive comparison prevents reliable endpoint interpretation.
Look for these in your work
- Identify the failed positive comparison.
- Avoid claiming starch breakdown from a faulty test.
Common mistakes
- pH must not be confounded with current temperature or dilution.
- A colour endpoint requires working indicator comparisons.
What can you explain now?
Which design changes pH without an avoidable dilution difference?
Compare with the explanation
Matched total volumes and reactant quantities with the approved buffers.
Control quantities while varying pH.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain the role of a buffer and two ways a pH comparison can become confounded.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.