Launch practical
About 100 minutes
- Set up the bench with the 13 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 18 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Well 1 was tested at 0 s, then one well every 30 s. A small dark-orange drop is iodine alone, not tested yet. Find the first tested drop that stayed orange-brown.
- blue-black
- purple-brown
- orange-brown
What it shows
mean rate of reaction = amount of starch broken down ÷ time taken
Amylase is an enzyme that breaks starch down into the sugar maltose. Iodine solution turns blue-black while starch is there and stays orange-brown once it has all gone. Each enzyme works fastest at its optimum pH; away from it the shape of the active site changes, so starch fits less well and the reaction slows.
The graph to expect. The rate rises to a peak at the optimum pH (about pH 7) and falls on either side, more steeply on the acid side below pH 5.
Key equation for the exam: rate = 1 ÷ time.
Recall: learn it. Time is how long until the iodine stops turning blue-black.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Spotting tiles (two for each pH) | Hold the drops of iodine solution that each sample is tested in. | — |
| Iodine solution in a dropping bottle | Turns blue-black when starch is present; stays orange-brown when it is not. | — |
| 1 % starch solution | The substrate. | — |
| 1 % amylase solution | The enzyme. | — |
| Buffer solutions: pH 4, 5, 6, 7, 8 and 9 | Keep each mixture at one pH: the independent variable. | — |
| Three syringes: 5 cm³ for starch, 1 cm³ for buffer, 1 cm³ for amylase | Measure the same volumes for every run. | 0.2 cm³ and 0.01 cm³ |
| Test tubes in a rack | Hold the starch with buffer, and the amylase. | — |
| Water bath at 37 °C | Keeps the temperature the same for every run. | — |
| Thermometer | Checks the water bath temperature. | 1 °C |
| Glass rod | Stirs the mixture and carries each drop to a well. | — |
| Beaker of water | For rinsing the glass rod between drops. | — |
| Stopclock | Times each run from the moment you mix. | 1 s |
| Safety goggles | Protect your eyes from iodine solution. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical slightly differently. Your course setting picks one; you can change it before you start.
Why: The spotting-tile method is the same; what you change, and the exam questions, follow your board.
Put on eye protectionSafety
Put on safety goggles before you open the iodine solution.
Why: Iodine solution irritates the eyes. A splash from the dropper or the rod could reach your eyes.
Put iodine in the spotting tiles
Put one drop of iodine solution in every well of two spotting tiles for each pH: 24 wells in all.
Why: Each well tests one drop of the mixture for starch. Two tiles give enough wells for 11 minutes of testing.
Measure the solutions
For each run: 5 cm³ of starch solution and 1 cm³ of the pH buffer into one test tube, and 1 cm³ of amylase solution into another. Use a separate labelled syringe for each solution, with no air bubble in it, and read the volume at the edge of the plunger's seal with the syringe level with your eye.
Why: The same volumes in every run make it a fair test. A separate syringe for each solution stops amylase getting into the starch before you start the clock.
Choose how often to test a drop
Test a drop of the mixture at regular intervals, starting the moment you mix.
Do: Every 30 seconds.
Why: You only see the end point when you test a drop, so the time you record is the first test after the starch has gone.
Warm them in the water bath first
Stand both tubes in the water bath at 37 °C for 5 minutes before you mix them. Check with the thermometer that the bath reads 37 °C.
Why: Temperature changes the rate of an enzyme reaction. Both solutions must already be at the bath temperature when they meet, so the only thing changing between runs is the pH.
Take the readings
Six buffers (pH 4, 5, 6, 7, 8, 9), each run three times: a drop tested every 30 s until it stays orange-brown. 50 mg of starch in each run.
Repeats: Each pH is done three times and a mean time calculated. A mean reduces the effect of random errors (judging the colour, the sampling interval, an unstirred mixture) and lets you spot an anomaly. It does not remove a systematic error, such as never warming the solutions first.
Make safe and clear upSafety
Rinse the spotting tiles and glass rod under the tap. Pour the mixtures down the sink with plenty of water and rinse the test tubes. Wipe up any iodine on the bench and switch off the water bath. Put the syringes and buffers back on the tray, and wash your hands.
While you take readings
Mix the amylase in
Pour the amylase into the starch and buffer, and stir with the glass rod for a few seconds as you start the clock.
Why: The amylase must be spread evenly through the starch, so every drop you test is the same mixture.
Rinse the rod between drops
After each drop, rinse the glass rod in the beaker of water and shake it, then take the next drop.
Why: Mixture left on the rod from the last drop still contains starch. Carried into the next well it can turn that drop blue-black.
When do you stop?
Keep testing a drop at every interval until the starch has gone.
Do: At the first drop that stays orange-brown: the iodine does not turn blue-black at all.
Why: Iodine solution turns blue-black with starch. When all the starch has been broken down, the iodine stays orange-brown.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Iodine solution | Irritates the eyes; stains the skin | Wear safety goggles, use the dropping bottle, and wipe up spills straight away. |
| Amylase (an enzyme) | Enzyme powder can cause an allergic reaction if breathed in | Use the solution the technician made up; never handle the powder. Wash your hands afterwards. |
| Water near the electric water bath | Electric shock | Keep your hands dry when you touch the controls, and mop up spills at once. |
| Glass test tubes and rod | Cuts if one breaks | Keep the tubes in a rack and tell your teacher about any breakage; do not pick up broken glass. |
Variables
- Independent: you change
- pH of the buffer solution
- Dependent: you measure
- Time taken for the starch to be broken down in s
- Control: you keep the same
- Temperature. Water bath at 37 °C; both solutions warmed in it before mixing.
- Volume of starch solution. 5 cm³ every run.
- Volume of amylase solution. 1 cm³ every run.
- Volume of buffer. 1 cm³ every run.
- Concentration of starch and of amylase. The same 1 % solutions for every run.
- Drops of iodine in each well. One drop in every well.
- Time between tests. The same interval throughout.
Results from a specimen run
| pH | Time, trial 1 in s | Time, trial 2 in s | Time, trial 3 in s | Mean time in s | Mean rate in mg/min |
|---|---|---|---|---|---|
| 4 | 420 | 450 | 420 | 430 | 7.0 |
| 5 | 210 | 180 | 180 | 190 | 15.8 |
| 6 | 90 | 90 | 90 | 90 | 33.3 |
| 7 | 90 | 90 | 90 | 90 | 33.3 |
| 8 | 120 | 150 | 140 | 21.4 | |
| 9 | 300 | 300 | 300 | 300 | 10.0 |
Anomaly spotted: Time at pH 8, trial 2, retaken.
Processing the data
- Mean rate of reaction
- 33.3 mg/min
- Shape of the graph
- It rises to a highest rate (the optimum pH) and then falls on the other side.
- The optimum pH
- 6.5
- What your testing interval means
- Some time between 60 s and 90 s.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Optimum pH (your class’s amylase) | 6.5 | 6.8 |
| Mean time at pH 7 (careful technique, tested every 30 s) | 90 s | 90 s |
Across 120 simulated careful groups, optimum pH of amylase (pH) came out between 6.50 and 6.92 (median 6.80). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Every minute
Each time can be up to a minute after the real end point, and every time is a whole number of minutes. At pH 6 and pH 7 both runs often read 1 or 2 minutes, so you cannot tell which is faster and the optimum is less certain.
The fix: Choose how often to test a drop. You only see the end point when you test a drop, so the time you record is the first test after the starch has gone.
Method: Mix them straight away
The solutions start at room temperature and take a minute or two to warm up, so the amylase works slowly at first. The fast runs (pH 6 and 7) take about 1.5 to 2 times as long; slow ones about 1.2 times. The curve is flatter and the temperature is not controlled (a systematic error).
The fix: Warm them in the water bath first. Temperature changes the rate of an enzyme reaction. Both solutions must already be at the bath temperature when they meet, so the only thing changing between runs is the pH.
Method: Just pour it in
The amylase is not spread evenly, so how fast the starch goes differs from run to run: repeats scatter by up to about ±30 % (a random error) and some stand out as anomalies.
The fix: Mix the amylase in. The amylase must be spread evenly through the starch, so every drop you test is the same mixture.
Time at pH 4, trial 1: Typed 450 s
The fix: That is the drop you tested to check it stayed orange-brown. Record the first drop that stayed orange-brown.
Time at pH 4, trial 1: Typed 390 s
The fix: That drop still had starch in it: it was not orange-brown. Record the next one.
Exam answer: Leaving out: pH (of the buffer / solution / mixture) (ignore acidity or alkalinity)
The fix: Name what the student changed: the pH.
Exam answer: Leaving out: "Amount" of starch, amylase or buffer (allow "amount" once only)
The fix: Write "volume of …" rather than "amount of …": AQA allows "amount" only once.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Independent variable · Applying knowledge[1 mark]
A student investigated the effect of pH on the time amylase takes to break down starch. They mixed starch solution, a pH buffer and amylase solution in a water bath at 37 °C. Every 30 seconds they put a drop of the mixture into iodine solution on a spotting tile.
What was the independent variable in this investigation?
Worked answer and how the marks are given
A full-mark answer
The pH of the buffer solution.
Mark scheme
- pH (of the buffer / solution / mixture) (ignore acidity or alkalinity) Do not accept “time taken”, “rate of reaction”, “colour”.
Where marks are usually lost
- Missing: pH (of the buffer / solution / mixture) (ignore acidity or alkalinity). Name what the student changed: the pH.
- Question 2Mean without the anomaly · Applying knowledge[2 marks]
Look again at Table 1.
Calculate the mean time at pH 8.
Do not use the anomalous result.
Mean time = s
Worked answer and how the marks are given
A full-mark answer
mean = (120 + 150) ÷ 2
= 135 sMark scheme
- (120 + 150) ÷ 2 (the anomaly left out) (method)
- 135 (s)
- Allow answers from 134.4 to 135.6 s (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as s, seconds, sec, secs, second.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 260 s (no marks). You included the anomalous trial. Leave trial 2 out: (120 + 150) ÷ 2 = 135 s.
- Answer 270 s (gains 1 mark). That is the total of the two times. Divide by 2: 135 s.
- Answer 2.25 s (gains 1 mark). That is the mean in minutes. The table is in seconds: 135 s.
- Question 3Planning the enzymes investigation · Knowledge[6 marks]
A student wants to investigate the effect of pH on the time taken for amylase to break down starch.
The student has: starch solution, amylase solution, buffer solutions of different pH, iodine solution, a spotting tile, syringes, test tubes, a glass rod, a water bath and a stopclock.
Describe a method the student could use.
Worked answer and how the marks are given
A full-mark answer
1. Put one drop of iodine solution into each well of a spotting tile.
2. Measure 5 cm³ of starch solution and 1 cm³ of pH 5 buffer into a test tube with a syringe, and 1 cm³ of amylase into another tube.
3. Stand both tubes in a water bath at 37 °C for 5 minutes so the temperature is controlled.
4. Add the amylase to the starch and buffer, stir, and start the stopclock.
5. Every 30 seconds, use a glass rod to put a drop of the mixture into the next well of iodine.
6. Blue-black means starch is still there; record the time when the iodine stays orange-brown.
7. Repeat with buffers of pH 6, 7, 8 and 9, using the same volumes and concentrations.
8. Repeat each pH three times and calculate a mean time.Mark scheme
Indicative content:
- Put (drops of) iodine solution into the wells of a spotting tile
- Measure volumes of starch, amylase and buffer (with a syringe or measuring cylinder)
- Mix the amylase with the starch and the buffer (pH solution) and start the timer
- Put a drop of the mixture into the iodine at regular intervals (every 30 seconds)
- Blue-black shows starch is present; record the time when the iodine stays orange-brown
- Repeat with buffers of different pH (at least three)
- Control the temperature with a water bath (warm both solutions first)
- Keep the volumes and concentrations of starch and amylase the same
- Repeat each pH and calculate a mean
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Testing a drop only at intervals | Systematic | You only see the end point at the next test, so every time is recorded up to one interval after the starch has gone (about half an interval on average). | Test a drop more often, for example every 10 or 15 seconds. |
| Not warming the solutions in the water bath first | Systematic | The amylase is cooler than the bath for the first minute or two, so every time is too long, the fast ones most. | Leave both tubes in the water bath for 5 minutes, and check the thermometer reads the bath temperature, before mixing. |
| Calling the end point at the first drop that looks lighter | Systematic | Some starch is still there, so every time is too short. | Stop only when a drop stays orange-brown and does not turn blue-black at all. |
| Not rinsing the glass rod between drops | Random | Starch carried on the rod darkens some drops, so some times are one interval too long. | Rinse the rod in water between every drop. |
| Not stirring the amylase into the starch | Random | Some drops have more amylase than others, so repeats disagree. | Stir the mixture with the rod as the amylase goes in. |
| Judging when the colour has changed | Random | Deciding whether a drop is orange-brown is subjective: two people can call the end point at different drops. | Compare each drop with a well of iodine and water, or use a colorimeter. |
| Only six pH values, 1 pH unit apart | Random | The optimum is read between two pH values far apart, so it is uncertain. | Test more pH values between the two either side of the fastest (for example pH 6.2, 6.4 … 7.8). |
Repeatable. Your three runs at each pH, done by you with the same method, give similar times.
Reproducible. Another group, with a different water bath and their own tiles, finds the fastest rate at about the same pH.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 5 | This page | — |
| Edexcel | 1.10 | Same activity | — |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.