Question 1 (1 marks)
Knowledge
What does iodine indicate when starch is present?
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Blue-black colour (1).
Required Practical 04: Enzymes | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Investigate how pH affects the rate at which amylase digests starch.
Introduction
We know that enzymes work best in their optimum conditions and this is your opportunity to investigate one. You are going to test how well amylase works at different pH levels. You will get to use a food test to measure how quickly the enzyme works. Remember to look at the three types of variables!
The science behind the practical
Amylase is an enzyme; changing pH changes its active site and therefore its reaction rate. Iodine turns blue-black if starch remains.
Key things to remember
Use buffer solutions; check aliquots with iodine at regular 30-second intervals; control temperature and volumes; use the time to disappearance of starch.
Equipment and apparatus
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Amylase solution
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starch solution
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pH buffer solutions
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iodine solution
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spotting tile
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pipettes
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test tubes
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water bath
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thermometer
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stopwatch
Method: How to carry out the experiment
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Into each well on a spotting tie, add a drop of iodine solution.
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add 2cm3 of amylase solution, 1cm3 of pH5 buffer solution and 2cm3 of starch solution to a test tube. Place this into a 35 ° C water bath.
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After 10 seconds, take a sample using a pipette and place 1 drop into the first well, note if starch is still present (yellow to black).
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After another 10 seconds, repeat the sampling. Continue to follow this until no starch is present. Record the time for this pH.
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Repeat using the different buffers provided.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | pH of buffer solution |
| Dependent variable | Time taken for starch to disappear, or rate calculated as 1/time |
| Control variables | Temperature; enzyme and starch concentrations; volumes; sampling interval |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Iodine solution |
Irritation and staining |
Use small amounts and follow local eye/skin protection guidance. |
Heated water bath and glassware |
Burns or breakages |
Use controlled temperatures and appropriate racks. |
Results, calculations and analysis
Recording, presenting and interpreting results
Table: pH, digestion time (s), calculated rate (s⁻¹). Graph pH against reaction rate; identify the optimum pH.
Calculations
Relative rate = 1 ÷ time in seconds, if comparing fixed endpoints.
Conclusions, accuracy and evaluation
Drawing a conclusion
Rate increases towards the optimum pH and decreases outside the optimum as enzyme activity is reduced.
Evaluating the investigation
Repeat trials; keep water bath at a constant temperature; use matching reagent volumes; 30 s sampling gives a timing uncertainty.
More to learn
How to measure the endpoint
Test one drop of reaction mixture on iodine every 30 seconds; record the first sample that stays iodine-brown rather than blue-black.
Common exam mistake
A shorter reaction time means a faster enzyme-controlled reaction; time and rate are not the same quantity.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Method
Why are pH buffer solutions used in the investigation?
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To maintain the chosen pH (1) so changes in digestion time can be attributed to pH rather than changing pH during the reaction (1).
Question 3 (2 marks)
Calculation
Starch disappears after 120 s. Calculate relative rate as 1/time.
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1 ÷ 120 = 0.00833 s⁻¹ (2).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 11th June 2026