Launch practical
About 100 minutes
- Set up the bench with the 16 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 29 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.
What it shows
rate = 1 ÷ time
Decay is the breakdown of dead material by microorganisms and their enzymes. In the lipase test, added lipase breaks fat into fatty acids and glycerol; the fatty acids lower the pH. In souring milk, bacteria ferment lactose to lactic acid, lowering the pH. Temperature affects these processes.
The graph to expect. The rate rises to a peak at the optimum (about 40–45 °C), then falls as the lipase is denatured: at 60 °C there is no end point.
Recall: learn it. Time is how long until the indicator changes colour.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Safety goggles | Protect your eyes from sodium carbonate and phenolphthalein. | — |
| Fresh full-fat milk (not UHT) | Its fat is broken down by added lipase in the tube test; its lactose can be fermented by bacteria in the souring test. | — |
| 0.05 mol/dm³ sodium carbonate solution | Makes the milk alkaline (about pH 10) at the start. | — |
| Phenolphthalein in a dropping bottle | Pink in alkali; colourless below about pH 8.3. | — |
| 5 % lipase solution | The enzyme that breaks down fat into fatty acids and glycerol. | — |
| Three 10 cm³ syringes, one labelled for each liquid | Measure the same volumes for every tube. | 0.2 cm³ |
| Test tubes, labelled, with glass rods | Hold the pink milk mixture and the lipase; the rods stir them. | — |
| Six 250 cm³ beakers as water baths, a kettle and ice | Keep each tube at its temperature: the independent variable. | — |
| Thermometers | Check each bath, and the milk in the tube, are at the right temperature. | 1 °C |
| White tile | Hold the tube against it to see when the last of the pink has gone. | — |
| Stopclock | Times each tube from the moment the lipase goes in. | 1 s |
| Three beakers of 25 cm³ of milk | The souring milk, one at each storage temperature. | — |
| Cling film | Covers the souring milk. | — |
| A fridge (5 °C) and two incubators (15 °C and 25 °C) | Keep the souring milk at a constant temperature for four days. | — |
| pH meter with a glass electrode | Measures the pH of the souring milk each day. | 0.1 |
| pH 7 buffer and a wash bottle of distilled water | Calibrate the meter and rinse the electrode between beakers. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Calibrate the pH meter
Rinse the electrode with distilled water, stand it in the pH 7 buffer and press CAL until the meter reads 7.0.
Why: An electrode drifts between lessons. Calibrating it in a buffer of known pH corrects that offset, so the readings are more accurate. It cannot remove every error: each reading still varies a little.
Put on eye protectionSafety
Put on safety goggles before you open the sodium carbonate or the phenolphthalein.
Why: Sodium carbonate solution irritates the eyes, and phenolphthalein is dissolved in ethanol, which stings.
Make up the pink tubes
Label a "milk" tube and a "lipase" tube for each temperature. In each milk tube: 5 drops of phenolphthalein, 5 cm³ of milk and 7 cm³ of 0.05 mol/dm³ sodium carbonate solution: it turns pink. In each lipase tube: 1 cm³ of 5 % lipase solution. Use a separate labelled syringe for each liquid, 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 sodium carbonate makes the milk alkaline (about pH 10), so the phenolphthalein is pink. The same volumes in every tube make it a fair test, and a separate syringe stops lipase getting into the milk before the clock starts.
Set up the water baths
Half fill six 250 cm³ beakers: ice and water for 10 °C, cold tap water for 20 °C, and water from the kettle mixed with cold for 30, 40, 50 and 60 °C. Check each with a thermometer, and add a little hot water or ice to keep it there.
Why: Temperature is the independent variable. Each bath must stay at its temperature for the whole run.
Leave the tubes in the baths first
Stand each milk tube and its lipase tube in their bath, with a thermometer in the milk tube. Wait until the thermometer reads the bath temperature before you add the lipase.
Do: Stand both tubes in the bath and wait until the thermometer in the milk reads the bath temperature, then add the lipase.
Why: The milk mixture and the lipase come from the bench at room temperature. They must reach the bath temperature before they meet, or the reaction happens at the wrong temperature.
Set up the souring milk
Measure 25 cm³ of the same fresh milk into each of three beakers, labelled 5 °C, 15 °C, 25 °C. Measure the pH of the fresh milk. They will go in the fridge (5 °C) and two incubators (15 °C and 25 °C) for 4 days.
Why: Bacteria in the milk ferment lactose to lactic acid, so the pH falls as the milk sours. The same volume of the same milk in each beaker makes it a fair test.
Cover the beakers
Cover each beaker with cling film.
Why: Microorganisms from the air would get into the milk and make it decay faster. A cover also stops the milk drying out and keeps the smell in.
Take the readings
Five water baths (10, 20, 30, 40, 50 °C), three trials each: the time for the pink colour to disappear after 1 cm³ of lipase goes in. Then the souring milk at 5, 15, 25 °C: its pH once a day for 4 days.
Repeats: The lipase test is done three times at each temperature and a mean time calculated. A mean reduces the effect of random errors (judging the colour, an unstirred tube) and lets you spot an anomaly. It does not remove a systematic error, such as stopping the clock at the first paler pink every time. The souring milk has one beaker at each temperature; repeating it with several beakers and taking a mean would make it more repeatable.
Make safe and clear upSafety
Switch off and unplug the kettle; let the hot baths cool before you pour them away. Pour the milk mixtures down the sink with plenty of water and rinse the tubes and syringes. After day 4, pour the sour milk down the sink with plenty of water, rinse the beakers and put the electrode back in its storage solution. Wipe the bench and wash your hands.
While you take readings
Stir until the pink has gone
Add the lipase and stir each tube with its glass rod, gently and all the time, until the pink colour has gone.
Do: Stir each tube gently and all the time with its glass rod until the pink has gone.
Why: Stirring spreads the lipase through the milk, so all the fat is broken down at the same rate.
When do you stop the clock?
The pink fades slowly from bright pink to pale pink to the white of milk. Decide when you will note the time.
Do: When no pink is left at all: hold the tube against the white tile and stop the clock.
Why: Phenolphthalein is colourless below about pH 8.3. The end point is when no pink is left.
When will you read the pH each day?
Take each beaker out, rinse the electrode, stand it in the milk, wait for the reading to settle and record it. Put the beaker back.
Why: The milk decays all the time. Readings exactly a day apart show how much the pH changes in a day.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Sodium carbonate solution | Irritates the eyes if it splashes | Wear safety goggles; wash any splashes off the skin with water. |
| Phenolphthalein (dissolved in ethanol) | The ethanol is highly flammable; harmful if swallowed | No naked flames near it: heat the water in a kettle, not with a Bunsen burner. Use the dropping bottle. |
| Hot water from the kettle (up to 60 °C) | Scalds the skin | Fill the beakers at the bench, never carry a full kettle around, and keep the baths at 60 °C or below. |
| Decaying milk in the beakers | Harmful microorganisms can grow in it | Keep the beakers covered, incubate at 25 °C or below, never taste it, pour it down the sink with plenty of water and wash your hands. |
| Lipase (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 kettle, incubators and pH meter | Electric shock | Keep your hands dry when you touch the plugs and switches, and mop up spills at once. |
Variables
- Independent: you change
- Temperature in °C
- Dependent: you measure
- Time for the pink colour to disappear in s
- Control: you keep the same
- Volume of milk. 5 cm³ in every tube; 25 cm³ in every beaker.
- Type of milk. The same fresh full-fat milk for everything.
- Volume and concentration of sodium carbonate. 7 cm³ of 0.05 mol/dm³ every time.
- Volume and concentration of lipase. 1 cm³ of 5 % every time.
- Drops of phenolphthalein. 5 drops in every tube.
- How the end point is judged. No pink left, against the white tile, every time.
- Covering the beakers; time of day the pH is read. All three covered; read at the same time each day.
Results from a specimen run
| Temperature of the milk in °C | Time, trial 1 in s | Time, trial 2 in s | Time, trial 3 in s | Mean time in s | Rate (1 ÷ mean time) in /s |
|---|---|---|---|---|---|
| 10 | 662 | 637 | 657 | 652 | 0.0015 |
| 20 | 300 | 284 | 299 | 294 | 0.0034 |
| 30 | 166 | 171 | 174 | 170 | 0.0059 |
| 40 | 85 | 90 | 96 | 90 | 0.0111 |
| 50 | 192 | 185 | 182 | 186 | 0.0054 |
| 60 | Still pink at 600 s | — | — | — | about 0 |
| Time in days | pH at 5 °C | pH at 15 °C | pH at 25 °C |
|---|---|---|---|
| 0 | 6.7 | 6.7 | 6.7 |
| 1 | 6.5 | 6.0 | 5.3 |
| 2 | 6.2 | 5.5 | 4.9 |
| 3 | 5.9 | 5.2 | 4.8 |
| 4 | 5.7 | 5.0 | 4.7 |
- 5 °C
- 15 °C
- 25 °C
Processing the data
- Rate from the time
- 0.011 /s
- Shape of the graph
- It rises to a highest rate (the optimum temperature) and then falls.
- The optimum temperature
- 40 °C
- Inside the tube
- 3 of 3 right first time
- The tube at 60 °C
- The lipase was denatured: its active site changed shape, so fat no longer fitted and no fatty acids were made.
- Shape of the graph
- It fell quickly at first, then more slowly: a curve.
- Gradient of your tangent at day 2 (15 °C)
- -0.400 pH units per day
- Mean rate at 5 °C
- 0.25 pH units per day
- How many times faster
- 1.6
| Quantity | Specimen run | Accepted |
|---|---|---|
| Optimum temperature of your class’s lipase | 40 °C | 43 °C |
| Mean time at 40 °C (careful technique) | 90 s | 90 s |
Across 120 simulated careful groups, optimum temperature of lipase (T) came out between 39.5 and 41.4 °C (median 40.2). 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: Use it as it is
Every pH reading is 0.2 to 0.4 too high or too low (a systematic error). The change in pH from one day to the next is still right, because the same error is in both readings.
The fix: Calibrate the pH meter. An electrode drifts between lessons. Calibrating it in a buffer of known pH corrects that offset, so the readings are more accurate. It cannot remove every error: each reading still varies a little.
Method: Add the lipase straight away
Each tube starts at room temperature and takes a minute or two to reach its bath. At 30 and 40 °C the times are too long; at 10 °C too short; at 50 °C the lipase is not denatured at first, so that time is too short too. Your curve is flatter and its peak moves (a systematic error).
The fix: Leave the tubes in the baths first. The milk mixture and the lipase come from the bench at room temperature. They must reach the bath temperature before they meet, or the reaction happens at the wrong temperature.
Method: Leave them open
Microorganisms from the air get into the milk, so each beaker decays faster and by a different amount: the pH falls faster and less evenly (a random error). Open beakers of decaying milk are also a hazard.
The fix: Cover the beakers. Microorganisms from the air would get into the milk and make it decay faster. A cover also stops the milk drying out and keeps the smell in.
Time for the pink colour to disappear at 10 °C, trial 1: Typed 11.02 s
The fix: 11:02 means 11 minutes and 2 seconds. Multiply the minutes by 60 and add the seconds: 662 s.
Time for the pink colour to disappear at 10 °C, trial 1: Typed 1102 s
The fix: 11:02 is not 1102 s: a minute is 60 seconds, so it is 11 × 60 + 2 = 662 s.
Exam answer: Leaving out: Temperature (of the water bath / of the milk)
The fix: Name what the student changed: the temperature.
Exam answer: Leaving out: Lipase breaks down (digests) fat / lipids
The fix: Start with what lipase does: it breaks down the fat in the milk.
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 temperature on the decay of milk. Each tube held milk, sodium carbonate solution and phenolphthalein, which made it pink. The student added lipase to each tube in a water bath and timed how long the pink colour took to disappear.
What was the independent variable in this investigation?
Worked answer and how the marks are given
A full-mark answer
The temperature of the water bath.
Mark scheme
- Temperature (of the water bath / of the milk) Do not accept “time taken / time for”, “colour”, “ph”.
Where marks are usually lost
- Missing: Temperature (of the water bath / of the milk). Name what the student changed: the temperature.
- Question 2Mean without the anomaly · Applying knowledge[2 marks]
Look again at Table 1.
Calculate the mean time at 20 °C.
Do not use the anomalous result.
Mean time = s
Worked answer and how the marks are given
A full-mark answer
mean = (289 + 285) ÷ 2
= 287 sMark scheme
- (289 + 285) ÷ 2 (the anomaly left out) (method)
- 287 (s)
- Allow answers from 286.4 to 287.6 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 391 s (no marks). You included the anomalous trial. Leave trial 2 out: (289 + 285) ÷ 2 = 287 s.
- Answer 574 s (gains 1 mark). That is the total of the two times. Divide by 2: 287 s.
- Missing: (289 + 285) ÷ 2 (the anomaly left out). Add the two times that agree, then divide by 2.
- Question 3Planning the decay investigation · Knowledge[6 marks]
A student wants to investigate the effect of temperature on the rate of decay of fresh milk.
The student has: fresh milk, beakers, a measuring cylinder, cling film, a fridge, incubators and a pH meter.
Describe a method the student could use.
Worked answer and how the marks are given
A full-mark answer
1. Use a measuring cylinder to put 25 cm³ of the same fresh milk into each of nine beakers (three for each temperature).
2. Measure the starting pH of the milk with a pH meter.
3. Keep three beakers in a fridge at 5 °C, three in an incubator at 15 °C and three in an incubator at 25 °C.
4. Cover every beaker with cling film.
5. Measure the pH of each beaker with the pH meter at the same time every day for 4 days.
6. Calculate the mean pH of the three beakers at each temperature each day.
7. Plot pH against time for each temperature and compare the rate of pH change.Mark scheme
Indicative content:
- Measure the same volume of (the same) milk into each beaker (with a measuring cylinder)
- Keep the beakers at different temperatures (at least three), in a fridge, water bath or incubator
- Measure the pH at the start (day 0)
- Measure the pH with a pH meter / probe or universal indicator
- Measure the pH every day (at the same time) for several days
- Cover all the beakers (control exposure to air)
- Keep the volume and type of milk the same
- Plot pH against time / calculate the rate of pH change / compare the time to reach a set pH
- Repeat (several beakers at each temperature) 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 |
|---|---|---|---|
| Judging by eye when the pink has gone | Random | The pink fades slowly, so deciding when it has gone is subjective: two people stop the clock at different times. | Compare each tube with a tube of milk and water with no indicator, or use a pH probe and data logger. |
| Stopping the clock when the tube first looks paler | Systematic | Some pink is still there, so every time is too short. | Stop only when no pink is left, holding the tube against a white tile. |
| Adding the lipase before the tubes reach the bath temperature | Systematic | The reaction starts at room temperature, so the times at the warm baths are too long and the 10 °C time too short. | Leave both tubes in the bath until the thermometer in the milk reads the bath temperature. |
| Not stirring the lipase in | Random | Some tubes mix faster than others, so repeats disagree. | Stir every tube gently and all the time until the pink has gone. |
| Beakers of water as water baths | Random | Hot water cools and iced water warms during a run, so the temperature is not quite constant. | Use thermostatically controlled electric water baths. |
| A pH meter that was not calibrated | Systematic | Every pH reading is too high (or every one too low) by the same amount. The change in pH is still right. | Calibrate the meter in pH 7 buffer before the first reading each day. |
| Reading the pH at different times of day | Random | Some readings are taken after more or less than a day, so the points scatter about the curve. | Read every beaker at the same time each day, or leave a pH probe in each with a data logger. |
| One beaker at each temperature, read once a day | Random | One beaker with more bacteria than usual would change the result, and the curve between readings is a guess. | Use three beakers at each temperature and calculate a mean; take readings more often or use a data logger. |
Repeatable. Your three tubes at each temperature, done by you with the same method, give similar times.
Reproducible. Another group, with their own baths and lipase, finds the shortest time at about the same temperature.
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 10 | This page | — |
| Edexcel | — | Not on its list | — |
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.