Decomposition

Many farmers keep animals such as cows and chickens.

The animals produce a lot of manure (faeces).

The farmer can put the manure into a biogas generator.

In the biogas generator, microorganisms produce biogas.

Figure 11 shows the percentages of the gases found in a sample of biogas.

Calculate the percentage of carbon dioxide in the biogas in Figure 11.

Scientists investigated biogas production from manure produced by cows and by chickens.

The scientists measured the production of biogas for 6 weeks.

Figure 12 shows the results.

The scientists made three conclusions from the results.

Give evidence from Figure 12 to support each conclusion.

Conclusion 1: ‘It is better to use chicken manure than cow manure for making biogas.’

Conclusion 2: ‘Biogas production is fastest in the first week.’

Conclusion 3: ‘After 4 weeks it is a waste of money to run the biogas generator.’

Another group of scientists investigated the effect of adding fish fat to the manure used for biogas production.

Fish fat is a waste product from the sea fishing industry.

Table 4 shows the results.

Complete the sentences.

Choose answers from the box.

Each answer may be used once, more than once or not at all.

When fish fat is added, the volume of biogas

When fish fat is added, the volume of methane

produced.

When fish fat is added, the percentage of methane in

the biogas.

A farmer has a farm 250 kilometres from the sea.

The farmer has a biogas generator on the farm.

The farmer adds manure to the biogas generator.

The farmer decided not to add fish fat to the manure in the biogas generator.

Suggest one reason why.

There is some manure left in the biogas generator.

Explain why the farmer spreads the manure on fields where crops will be grown.

Peat bogs are estimated to contain twice as much carbon as all the world’s forests.

Figure 13 shows a section through part of a peat bog.

Layer A contains a lot of air.

Layer B:

• contains the dead remains of plants

• has a low pH

• contains very little oxygen

• contains carbon dioxide and methane.

Explain why most of the dead remains of plants in layer B do not decay.

The peat bog in Figure 13 is a stable community.

The moss produces biomass at a rate of 340 g/m²/year.

What is the approximate biomass of the moss that becomes biomass in primary

Tick (✓) one box.

The sundew plant shown in Figure 13 on page 38 has leaves with sticky hairs that trap and digest insects.

Digestion of the insects releases phosphates and simple compounds of nitrogen that are used by the sundew plant.

What substance can the sundew plant make using the phosphates ?

Tick (✓) one box.

What substance can the sundew plant make using the nitrogen ?

Tick (✓) one box.

Humans have destroyed large areas of peat bog to collect peat.

The peat provides fuel and provides compost for gardeners to use.

The peat comes from layer B in Figure 13 on page 38.

Layer B:

• contains the dead remains of plants

• has a low pH

• contains very little oxygen

• contains carbon dioxide and methane.

Figure 14 shows the removal of peat from a peat bog.

Explain how the destruction of peat bogs and the use of peat affects the temperature of the Earth’s atmosphere.

Peat bogs are estimated to contain twice as much carbon as all the world’s forests.

Figure 2 shows a section through part of a peat bog.

Layer A contains a lot of air.

Layer B:

• contains the dead remains of plants

• has a low pH

• contains very little oxygen

• contains carbon dioxide and methane.

Explain why most of the dead remains of plants in layer B do not decay.

The peat bog in Figure 2 is a stable community.

The moss produces biomass at a rate of 340 g/m²/year.

What is the approximate biomass of the moss that becomes biomass in primary

Tick () one box.

The sundew plant shown in Figure 2 on page 4 has leaves with sticky hairs that

trap and digest insects.

Digestion of the insects releases phosphates and simple compounds of nitrogen that are used by the sundew plant.

What substance can the sundew plant make using the phosphates ?

Tick () one box.

What substance can the sundew plant make using the nitrogen ?

Tick () one box.

Humans have destroyed large areas of peat bog to collect peat.

The peat provides fuel and provides compost for gardeners to use.

The peat comes from layer B in Figure 2 on page 4.

Layer B:

• contains the dead remains of plants

• has a low pH

• contains very little oxygen

• contains carbon dioxide and methane.

Figure 3 shows the removal of peat from a peat bog.

Explain how the destruction of peat bogs and the use of peat affects the temperature of the Earth’s atmosphere.

Bacteria are one type of organism that cause decay.

Which other type of organism causes decay?

Tick (✓) one box.

Students investigated the effect of temperature on the decay of milk.

The decay was caused by bacteria in the milk.

Figure 8 shows the apparatus used.

This is the method used.

1. Set up the apparatus as shown in Figure 8 with the milk at 20 C.

2. Record the pH each day for 7 days.

3. Repeat with more samples of milk at 5 C and at 30 C.

What was the dependent variable in the investigation?

Tick (✓) one box.

How could the students keep the milk at 30 C for 7 days?

Tick (✓) one box.

As the milk decays, the bacteria digest fats in the milk.

What type of acid is produced by digestion of fats in the milk?

Tick (✓) one box.

Figure 9 shows the results.

Why did the pH not fall below pH 4.5 at 20 C?

Tick (✓) one box.

The digestion of fat was fastest at 30C and slowest at 5 C.

Give one reason why the rate of digestion was faster at 30 C than at 5 C.

Calculate the rate of digestion at 30 °C from day 2 to day 3.

Complete the following calculation.

Use data from Figure 9.

The rate of digestion at 5°C from day 2 to day 3 is 0.1 pH units/day.

How many times faster is the rate of digestion at 30°C than the rate of digestion at 5°C from day 2 to day 3?

Use your answer to Question 05.7.

Organic substances decay into simpler substances.

The leaves fall off many trees in autumn. The dead leaves contain carbon compounds and nitrogen compounds. Describe how carbon and nitrogen in compounds in the leaves are recycled and used by living trees. You should include a description of:

• how the leaves are broken down

• how substances are taken in and used by the trees.

Students investigated the effect of temperature on the decay of milk. This is the method used.

1. Place 25 cm³ of fresh milk into each of three beakers.

2. Keep one beaker of milk at 5 C.

3. Keep one beaker of milk at 15 C.

4. Keep one beaker of milk at 25 C.

5. Record the pH of the milk in each beaker every day for 4 days.

Table 1 shows the results.

Suggest one improvement the students could have made to the method.

Figure 6 shows the results at 5 C and at 15 C.

Complete Figure 6. You should:

• plot the data for 25 C from Table 1

• draw a line of best fit.

Figure 7 shows the results for 5 C and at 15 C again.

The rate of pH change increases with an increase in temperature. The rate of pH change at 5 C is 0.3 pH units per day. Calculate how many times faster the rate of pH change is at 15 C than the rate of pH

You should draw a tangent on Figure 7.

Milk contains lipids. The lipids are broken down when the milk decays. Explain why the pH changes more quickly when the temperature is higher.

A student investigated the effect of temperature on the decay of milk. Figure 15 shows the apparatus the student used.

This is the method used.

1. Set up the apparatus as shown in Figure 15 with the milk at 20 C.

2. Record the pH over 5 days using the computer.

3. Repeat with another batch of fresh milk at 25 C.

How could the student keep the milk at a constant temperature for 5 days?

Give one variable the student should keep constant. Do not refer to temperature in your answer.

Table 2 shows the student’s results for the milk at 20 C.

Complete Figure 16.

You should:

• label the x-axis

• use a suitable scale for the x-axis

• plot the data from Table 2

• draw a line of best fit.

The data you plotted in Question 09.3 were obtained at 20 C. Sketch a line on Figure 16 to show the results you would expect at 25 C.

Decay occurs in a compost heap. Figure 19 shows a compost heap.

Describe:

• how microorganisms in the layers of soil help to recycle chemicals in the dead plants

• how the chemicals are used again by living plants.

A student investigated the effect of temperature on the decay of milk. Figure 3 shows the apparatus the student used.

This is the method used.

1. Set up the apparatus as shown in Figure 3 with the milk at 20 C.

2. Record the pH over 5 days using the computer.

3. Repeat with another batch of fresh milk at 25 C.

How could the student keep the milk at a constant temperature for 5 days?

Give one variable the student should keep constant. Do not refer to temperature in your answer.

Table 1 shows the student’s results for the milk at 20 C.

Complete Figure 4. You should:

• label the x-axis

• use a suitable scale for the x-axis

• plot the data from Table 1

• draw a line of best fit.

The data you plotted in Question 02.3 were obtained at 20 C. Sketch a line on Figure 4 to show the results you would expect at 25 C.

This question is about the decay of milk.

Name two types of microorganism that cause decay.

Cows’ milk is pH 6.6.

As milk decays, lipids in the milk are broken down.

One of the products of the breakdown of lipids causes the pH of milk to decrease.

Name the product that causes the pH to decrease.

A student investigated the effect of temperature on the time taken for different types of

milk to decay.

This is the method used.

1. Put cows’ milk in six test tubes.

2. Keep each test tube at a different temperature.

3. Measure the pH of the milk in each tube every day for 12 days.

4. Record the number of days taken to reach pH 5.

5. Repeat steps 1 to 4 with goats’ milk and with almond milk.

Give one way the pH can be measured.

Give two control variables the student should have used in this investigation.

The student improved the investigation to produce valid results.

Figure 11 shows the results.

Which type of milk stays fresh the longest at 10 ° C?

Describe the effect of temperature on the time taken for goats’ milk to reach pH 5.

Use data from Figure 11 in your answer.

The time taken for cows’ milk to reach pH 5 at 10 °C is less than the time taken for cows’ milk to reach pH 5 at 5 °C.

Suggest one reason why.

Suggest two reasons why the different types of milk took different lengths of time to reach pH 5.

The student said:

‘The temperature milk is stored at affects how likely the milk is to cause food poisoning.’

How can the investigation be developed to find out if the student is correct?

Tick (✓) one box.

Question 9 starts on page 36

This question is about the decay of milk.

Name two types of microorganism that cause decay.

Cows’ milk is pH 6.6. As milk decays, lipids in the milk are broken down. One of the products of the breakdown of lipids causes the pH of milk to decrease. Name the product that causes the pH to decrease.

A student investigated the effect of temperature on the time taken for different types of

milk to decay. This is the method used.

1. Put cows’ milk in six test tubes.

2. Keep each test tube at a different temperature.

3. Measure the pH of the milk in each tube every day for 12 days.

4. Record the number of days taken to reach pH 5.

5. Repeat steps 1 to 4 with goats’ milk and with almond milk.

Give one way the pH can be measured.

Give two control variables the student should have used in this investigation.

The student improved the investigation to produce valid results.

Figure 1 shows the results.

Which type of milk stays fresh the longest at 10 C?

Describe the effect of temperature on the time taken for goats’ milk to reach pH 5. Use data from Figure 1 in your answer.

The time taken for cows’ milk to reach pH 5 at 10 °C is less than the time taken for cows’ milk to reach pH 5 at 5 °C. Suggest one reason why.

Suggest two reasons why the different types of milk took different lengths of time to reach pH 5.

The student said: ‘The temperature milk is stored at affects how likely the milk is to cause food poisoning.’ How can the investigation be developed to find out if the student is correct? Tick (✓) one box.

Gardeners sometimes make compost heaps from dead plant material.

The dead plants decay in the compost heap.

Figure 8 shows a compost heap.

The thin layers of soil contain organisms that cause decay.

Which two types of organism cause decay?

Tick (✓) two boxes.

The rate of decay in the compost heap depends on several environmental factors.

Explain how the rate of decay would be affected by:

• an increase in oxygen concentration

• a temperature increase from 5 C to 25 C

Give one environmental factor needed for decay.

Do not refer to oxygen or temperature in your answer.

Dead plant material can also be decayed in a biogas generator.

Figure 9 shows the percentages of the gases found in a sample of biogas.

Gas X is the main fuel gas found in the biogas.

What is gas X ?

Tick (✓) one box.

What is the percentage of gas X in the biogas?

The dead plant material in the compost heap and biogas generator does not decay completely.

Explain why a farmer might spread the remaining dead plant material onto his fields.