The Rate of Decomposition

1. 1 State [2 marks]

   State two conditions that make decomposition faster.

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   Model answer

   Any two of: warm temperature (up to the optimum), moisture, plenty of oxygen.

   Mark scheme

   • Condition 1 — 1 mark
   • Condition 2 — 1 mark
2. 2 Explain [3 marks]

   Explain why food lasts longer in a freezer than on a kitchen worktop.

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   Model answer

   The low temperature in the freezer slows the microorganisms that cause decay. The enzymes in the microorganisms work very slowly, and the microorganisms respire and grow very slowly, so the food decays more slowly.

   Mark scheme

   • The temperature is low — 1 mark
   • The microorganisms work or respire more slowly — 1 mark
   • So the food decays more slowly — 1 mark
3. 3 Explain [3 marks]

   Use Figure 1 to explain why the rate of decay falls when the temperature rises above 40 °C.

   

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   Model answer

   The enzymes in the decomposers are denatured at higher temperatures. The microorganisms stop working, or die. So the rate of decay falls.

   Mark scheme

   • Enzymes are denatured — 1 mark
   • Decomposers stop working or die — 1 mark
   • Rate of decay falls — 1 mark
4. 4 Explain [3 marks]

   Explain why the rate of decay of dead leaves is very slow in waterlogged soil.

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   Model answer

   The water fills the spaces between the soil particles and pushes out the air. There is little oxygen, so the decomposers can only respire anaerobically. This releases less energy, so decay is slow.

   Mark scheme

   • Water pushes out the air, so there is little oxygen — 1 mark
   • Decomposers respire anaerobically — 1 mark
   • Less energy is released, so decay is slow — 1 mark
5. 5 Describe [4 marks]

   A student wants to find the best temperature for composting grass clippings. Describe how the student could do this.

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   Model answer

   Put the same mass of grass clippings into several identical containers, and keep each at a different temperature, for example in water baths. Keep the moisture and the amount of air the same. Measure the mass of the grass clippings at the start and after the same time. Repeat and calculate a mean, and find the temperature with the biggest change in mass.

   Mark scheme

   • Same mass in identical containers at different temperatures — 1 mark
   • Keep the other factors, such as moisture and air, the same — 1 mark
   • Measure the mass at the start and after a set time — 1 mark
   • Repeat and calculate a mean — 1 mark
6. 6 Calculate [3 marks]

   A student measures 80 g of dead leaves in a compost bin. After 20 days the mass of the leaves is 62 g. Calculate the mean rate of decay in g per day, and the percentage decrease in mass.

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   Model answer

   The mean rate is 0.9 g per day. The percentage decrease is 22.5%.

   Mark scheme

   • \(80 - 62 = 18\) g — 1 mark
   • \(18 \div 20 = 0.9\) g per day — 1 mark
   • \(\frac{18}{80} \times 100 = 22.5\%\) — 1 mark
7. 7 Explain [3 marks]

   Explain why turning a compost heap makes the material decay faster.

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   Model answer

   Turning lets air into the heap, so that there is more oxygen. The decomposers can then respire aerobically, which releases more energy. They grow and work faster, so the material decays faster.

   Mark scheme

   • Air gets in, so more oxygen — 1 mark
   • Decomposers respire aerobically, releasing more energy — 1 mark
   • They work faster, so the decay is faster — 1 mark
8. 8 Suggest [2 marks]

   Dead plants in a cold, waterlogged peat bog do not decay completely. Suggest two reasons for this.

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   Model answer

   The low temperature slows the decomposers. The waterlogging means there is little oxygen, so aerobic decay cannot happen.

   Mark scheme

   • Low temperature slows the decomposers — 1 mark
   • Waterlogged, so little oxygen — 1 mark

Quick check

1. 1

   At which temperature is decay fastest in a compost heap?

   1. A0 °C
   2. B20 °C
   3. C40 °C
   4. D80 °C
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   C: 40 °C

   The optimum temperature for the microorganisms in compost is about 40 °C.

2. 2

   Why does decay slow down above the optimum temperature?

   1. AThe water freezes
   2. BThe enzymes are denatured
   3. COxygen increases
   4. DThe material dries out
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   B: The enzymes are denatured

   The enzymes of the decomposers are denatured.

3. 3

   What does aerobic mean?

   1. AWith oxygen
   2. BWithout oxygen
   3. CWith water
   4. DWithout water
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   A: With oxygen

   Aerobic means with oxygen.

4. 4

   Which gas can be made by anaerobic decay?

   1. AOxygen
   2. BNitrogen
   3. CHydrogen
   4. DMethane
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   D: Methane

   Anaerobic decay can produce methane.

5. 5

   Why is aerobic decay faster than anaerobic decay?

   1. AIt uses less oxygen
   2. BIt makes methane
   3. CIt releases more energy
   4. DIt needs no water
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   C: It releases more energy

   Aerobic respiration releases more energy, so the decomposers work faster.

6. 6

   Why does waterlogging slow down decay?

   1. AIt makes the soil too hot
   2. BIt pushes out the air
   3. CIt kills the plants
   4. DIt adds salt
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   B: It pushes out the air

   Water pushes out the air, so oxygen runs out.

7. 7

   In the investigation, what should be kept the same?

   1. AThe temperature in every container
   2. BThe factor being tested
   3. CThe result
   4. DThe conditions other than the one being tested
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   D: The conditions other than the one being tested

   Only one factor is changed, so everything else is kept the same.

8. 8

   A leaf loses 6 g in 12 days. What is the mean rate of decay?

   1. A0.5 g per day
   2. B2 g per day
   3. C6 g per day
   4. D72 g per day
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   A: 0.5 g per day

   6 g divided by 12 days is 0.5 g per day.