Decay

1 Plan and risk assessment

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.

rate = 1 ÷ time

Prediction

2 Apparatus

• ✓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 (resolution 0.2 cm³) Measure the same volumes for every tube.
• ✓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 (resolution 1 °C) Check each bath, and the milk in the tube, are at the right temperature.
• ✓White tile Hold the tube against it to see when the last of the pink has gone.
• ✓Stopclock (resolution 1 s) Times each tube from the moment the lipase goes in.
• ✓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 (resolution 0.1) Measures the pH of the souring milk each day.
• ✓pH 7 buffer and a wash bottle of distilled water Calibrate the meter and rinse the electrode between beakers.

3 Method: your choices

• Calibrate the pH meter: DoneAn 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.
• Leave the tubes in the baths first: Stand both tubes in the bath and wait until the thermometer in the milk reads the bath temperature, then add the lipaseThe milk mixture and the lipase are at the bath temperature when they meet, so each time belongs to the temperature you set.
• Cover the beakers: DoneMicroorganisms 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.
• Stir until the pink has gone: Stir each tube gently and all the time with its glass rod until the pink has goneThe lipase is spread through all the milk, so the fat is broken down evenly and your repeats agree.
• When do you stop the clock?: When no pink is left at all: hold the tube against the white tile and stop the clockYou record the time when the fatty acids have brought the pH down to about 8.3.
• When will you read the pH each day?: DoneThe milk decays all the time. Readings exactly a day apart show how much the pH changes in a day.

Anomaly: No anomalous reading.

6 Analysis

1. Rate from the time

   rate = 1 ÷ mean time

   The shorter the time, the faster the fat is broken down, so 1 ÷ time stands for the rate. Calculate the rate at 40 °C. Give your answer to 2 significant figures.

   0.011 /s Right first time

2. Shape of the graph

   Describe the shape of your curve.

   It rises to a highest rate (the optimum temperature) and then falls. Right first time

3. The optimum temperature

   Read the temperature below the highest point of your curve.

   Use your curve. At what temperature does the lipase work fastest? Read it to the nearest degree.

   40 °C Right first time

4. Inside the tube

   3 of 3 right first time Right first time

5. The tube at 60 °C

   Your 60 °C tube was still pink after 10 minutes. Why?

   The lipase was denatured: its active site changed shape, so fat no longer fitted and no fatty acids were made. Right first time

6. Shape of the graph

   Describe how the pH of the milk at 15 °C changed.

   It fell quickly at first, then more slowly: a curve. Right first time

7. Gradient of your tangent at day 2 (15 °C)

   The pH is falling, so the gradient is negative. Its size is the rate of pH change at day 2.

   -0.400 pH units per day Right first time

8. Mean rate at 5 °C

   mean rate of pH change = change in pH ÷ time

   Calculate the mean rate of pH change of the milk in the fridge (5 °C) over the 4 days, in pH units per day.

   0.25 pH units per day Right first time

9. How many times faster

   number of times faster = faster rate ÷ slower rate

   The rate at day 2 at 15 °C is the size of your tangent’s gradient (0.40 pH units per day). How many times faster is it than the mean rate at 5 °C? A ratio has no unit.

   1.6 Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.

1. 01.1[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?

   Your answer

   The temperature of the water bath

   1/1 self-marked

   • Temperature (of the water bath / of the milk)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The temperature of the water bath.

2. 01.2[2 marks]

   Explain why the pink colour disappeared after lipase was added.

   Your answer

   Lipase breaks down the fat in the milk into fatty acids and glycerol. The fatty acids lower the pH, and below about pH 8.3 the phenolphthalein becomes colourless.

   2/2 self-marked

   • Lipase breaks down (digests) fat / lipids
   • Fatty acids are produced, which lower the pH (do not accept amino acids)
   • The pH falls (below about 8.3) / it becomes less alkaline, so the phenolphthalein becomes colourless

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Lipase breaks down the fat in the milk into fatty acids and glycerol. The fatty acids lower the pH, and below about pH 8.3 phenolphthalein is colourless.

3. 01.3[1 mark]

   Another student timed three tubes at 20 °C. Table 1 shows their results.

   Which trial gave an anomalous result?

   Select one row.

   Table 1

   Trial	Time for the pink colour to disappear at 20 °C in s
   1	289
   2	600
   3	285

   Your answer

   trial 2

   1/1

   • Trial 2: 600 s is far from the other two, which agree

   Full marks. That is exactly what the mark scheme is looking for. Trial 2 (600 s) is far from the other two times, which agree with each other.

   Model answer: Trial 2. Its time, 600 s, is much longer than the other two, which agree with each other.

4. 01.4[2 marks]

   Look again at Table 1.

   Calculate the mean time at 20 °C.

   Do not use the anomalous result.

   Your answer

   287

   Working: mean = (289 + 285) ÷ 2 = 287 s

   2/2

   • (289 + 285) ÷ 2 (the anomaly left out)
   • 287 (s)

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: mean = (289 + 285) ÷ 2 = 287 s

5. 01.5[2 marks]

   The rate of the reaction can be found from: rate = 1 ÷ mean time

   Use your answer to Question 01.4 to calculate the rate of the reaction at 20 °C.

   Give your answer to 2 significant figures.

   Your answer

   0.0035

   Working: rate = 1 ÷ 287 = 0.0035 /s

   2/2

   • 1 ÷ 287
   • 0.0035 (/s); allow error carried forward from 01.4

   Full marks. Well done.

   Model answer: rate = 1 ÷ 287 = 0.0035 /s

6. 01.6[1 mark]

   You stopped the clock when no pink was left, holding each tube against a white tile.

   The stopclock measures to 1 second. Suggest why the times may still not be accurate.

   Your answer

   The pink fades gradually, so deciding when it has gone is subjective; it is judged by eye.

   1/1 self-marked

   • The colour change is subjective / judged by eye (by opinion); the pink fades gradually so it is hard to tell exactly when it has gone (ignore "human error" unqualified; ignore "the stopclock")

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The pink fades gradually, so deciding exactly when it has gone is subjective: it is judged by eye.

7. 01.7[2 marks]

   A tube kept at 60 °C was still pink after 10 minutes.

   Explain why the tube at 60 °C stayed pink.

   Your answer

   The lipase is denatured: the shape of its active site changes, so fat no longer fits and no fatty acids are made, so the pH stays high.

   2/2 self-marked

   • The lipase is denatured (do not accept killed)
   • The shape of the active site changes
   • So fat no longer fits (is not complementary to) the active site / no fatty acids are made, so the pH does not fall

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: At 60 °C the lipase is denatured: the shape of its active site changes, so fat no longer fits. No fatty acids are made, so the pH does not fall below 8.3 and the tube stays pink.

8. 02.1[1 mark]

   The student also put 25 cm³ of fresh milk into each of three beakers and kept them at 5, 15, 25 °C for 4 days. Bacteria in the milk ferment lactose (milk sugar). The student measured the pH of the milk each day.

   Suggest how the student could keep the milk at 15 °C for 4 days.

   Your answer

   Put the beaker in an incubator set at 15 °C.

   1/1 self-marked

   • Use an incubator / a (thermostatically controlled) water bath set to 15 °C

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Put it in an incubator (or a water bath) set at 15 °C.

9. 02.2[1 mark]

   A student kept samples of fresh milk in beakers at different temperatures and measured the pH of the milk each day for several days.

   Give one variable the student should keep the same in the souring milk investigation.

   Do not refer to temperature or pH.

   Your answer

   The volume of milk in each beaker

   1/1 self-marked

   • Volume of milk; type (source, brand, fat content) of milk; freshness or age of the milk; whether the beakers are covered (exposure to air); time of day the pH is measured; size or type of beaker; same pH meter (do not accept temperature or pH)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The volume of milk in each beaker.

10. 02.3[1 mark]

    The student used a pH meter. Give one other way to measure the pH of the milk each day.

    Your answer

    Universal indicator, compared with a colour chart

    1/1 self-marked

    • Universal indicator (paper or solution, with a colour chart) (ignore phenolphthalein, litmus; do not credit a pH meter or probe: the student used one)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: Add a few drops of universal indicator and compare the colour with a pH colour chart.

11. 02.4[1 mark]

    Name the acid produced when bacteria ferment lactose in the milk.

    Your answer

    Lactic acid

    1/1 self-marked

    • Lactic acid (do not accept fatty acids or amino acids)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: Lactic acid.

12. 02.5[2 marks]

    Table 2 shows your pH readings.

    Table 2

    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

    Calculate the mean rate of pH change at 25 °C over the first 2 days.

    Give your answer in pH units per day.

    Your answer

    0.9

    Working: change in pH = 6.7 − 4.9 = 1.8 rate = 1.8 ÷ 2 = 0.9 pH units per day

    2/2

    • 6.7 − 4.9 (= 1.8)
    • 0.9 (pH units per day)

    Full marks. Clear and precise.

    Model answer: change in pH = 6.7 − 4.9 = 1.8 rate = 1.8 ÷ 2 = 0.9 pH units per day

13. 02.6[1 mark]

    Look again at Table 2.

    How many times faster did the pH fall at 25 °C than at 5 °C over the first 2 days?

    Use your answer to Question 02.5.

    Your answer

    3.6

    Working: rate at 5 °C = (6.7 − 6.2) ÷ 2 = 0.25 0.90 ÷ 0.25 = 3.6

    1/1

    • 3.6 (no unit); allow error carried forward from 02.5

    Full marks. Well done.

    Model answer: rate at 5 °C = (6.7 − 6.2) ÷ 2 = 0.25 0.90 ÷ 0.25 = 3.6

14. 02.7[3 marks]

    Your milk at 25 °C fell from pH 6.7 to pH 4.7 in 4 days; at 5 °C it fell to pH 5.7.

    Explain why the pH of the milk fell faster at the higher temperature.

    Your answer

    At the higher temperature the bacteria divide faster, so there are more bacteria. Their enzymes have more kinetic energy so there are more collisions, lactose is fermented faster, and lactic acid is produced faster.

    3/3 self-marked

    • Bacteria divide (reproduce) faster, so there are more bacteria (ignore "bacteria grow faster")
    • Enzymes are more active / more kinetic energy, so more collisions between enzyme and substrate (ignore "enzymes work better")
    • Lactose (milk sugar) is fermented or broken down faster
    • So lactic acid is produced faster (do not accept fatty acids or amino acids)

    Self-marked: 3 of 3 marks. This is your own judgement against the mark scheme.

    Model answer: At a higher temperature the bacteria divide faster, so there are more of them. Their enzymes have more kinetic energy and are more active, so lactose is fermented faster and lactic acid is produced faster, which lowers the pH faster.

15. 02.8[1 mark]

    The student read the pH of each beaker once a day.

    Suggest one way the student could improve the souring milk investigation.

    Your answer

    Use three beakers at each temperature and calculate a mean pH for each day.

    1/1 self-marked

    • More temperatures (in between); readings more often / shorter intervals; a data logger with a pH probe; several beakers at each temperature (repeat) and calculate a mean / identify anomalies; read at the same time each day; cover all the beakers (ignore "repeat" alone; ignore "do it for longer")

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: Use three beakers at each temperature and calculate a mean pH for each day (or: use a pH probe and data logger to read the pH more often).

16. 03[2 marks]

    Another student stopped the clock as soon as each tube looked paler pink, instead of waiting until no pink was left.

    Explain how this would affect their times.

    Your answer

    The tube was still pale pink, so the pH had not fallen to 8.3 yet. The clock was stopped too early, so the times were too short.

    2/2 self-marked

    • The tube was still (pale) pink / the pH had not yet fallen to 8.3 / not enough fatty acid had been made: the end point had not been reached
    • So every time was too short (the rates too high)

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: The tube was still pale pink, so the pH had not yet fallen to 8.3. The clock was stopped before the end point, so their times were too short.

17. 04[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.

    Your 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.

    6/6 self-marked

    • 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

    Self-marked: Level 3, 6 of 6 marks. You ticked 9 of 9 indicative points. This is your own judgement against the mark scheme.

    Model 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.

Unseen data: the same skills, new apparatus

1. 01[3 marks]

   Ecologists put 20 g of dead leaves into each of five mesh bags and buried them in soil kept at different temperatures. They found the time for half the mass of leaves to decay.

   Time for half the mass of leaves to decay

   Soil temperature in °C	Time in days
   5	148
   10	108
   15	79
   20	57
   25	42

   Rate of decay = 1 ÷ time. Calculate how many times faster the leaves decayed at 20 °C than at 5 °C. Give your answer to 2 significant figures.

   Your answer

   2.6

   Working: 1 / 57 = 0.0175 1 / 148 = 0.00676 0.0175 / 0.00676

   3/3

   • Rates: 1 ÷ 57 and 1 ÷ 148.
   • The faster rate ÷ the slower rate (or 148 ÷ 57).
   • The ratio, to 2 significant figures; no unit.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Rate at 20 °C = 1 ÷ 57 = 0.0175 per day; at 5 °C = 1 ÷ 148 = 0.00676 per day. 0.0175 ÷ 0.00676 = 2.6 times faster (the same as 148 ÷ 57). A ratio has no unit.

2. 02[2 marks]

   A gardener makes compost from grass cuttings and vegetable peelings. In summer, the inside of the heap is about 30 °C and the compost is ready in about 3 months. In winter, it is about 7 °C and takes about 8 months. Turning the heap with a fork every week also makes it decay faster.

   Which TWO statements explain these observations? Tick (✓) two boxes.

   • ✓ In summer the microorganisms’ enzymes and molecules have more kinetic energy, and the microorganisms divide faster. (your choice)
   • ✓ Turning the heap lets in more oxygen, so the decomposers can respire aerobically and break down the waste faster. (your choice)
   • In winter the cold kills the microorganisms, so the heap stops decaying.
   • In summer enzymes work better.
   • Turning the heap dries it out completely, which speeds up decay.

   Your answer

   In summer the microorganisms’ enzymes and molecules have more kinetic energy, and the microorganisms divide faster.; Turning the heap lets in more oxygen, so the decomposers can respire aerobically and break down the waste faster.

   2/2

   • In summer the microorganisms’ enzymes and molecules have more kinetic energy, and the microorganisms divide faster.
   • Turning the heap lets in more oxygen, so the decomposers can respire aerobically and break down the waste faster.

   Full marks. Clear and precise.

   Model answer: Warmth: more kinetic energy, so more collisions between enzymes and substrate, and the bacteria and fungi divide faster. Turning adds oxygen for aerobic respiration by the decomposers. Cold slows them without killing them; “enzymes work better” is not enough; some water is needed, so drying out slows decay.

3. 03[2 marks]

   A student left 50 cm³ of single cream in a covered beaker at 20 °C and measured its pH with a pH probe at the same time each day.

   pH of the cream

   Time in days	pH
   0	6.5
   1	5.7
   2	5.3
   3	5.0
   4	4.8
   5	4.7

   Calculate the mean rate of pH change between day 2 and day 4.

   Your answer

   0.25

   Working: 5.3 - 4.8 = 0.5 0.5 / 2

   2/2

   • Change in pH = 5.3 − 4.8 = 0.5.
   • ÷ 2 days.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Change in pH = 5.3 − 4.8 = 0.5. Mean rate = 0.5 ÷ 2 = 0.25 pH units per day.