2

1 A student investigated the effect of surface area on the rate of diffusion.

The student prepared different sized blocks of agar. The blocks of agar had different surface
areas.

The agar contained universal indicator which made the agar green in colour.

The blocks of agar were put in hydrochloric acid. Universal indicator turns red in the presence of
acid.

As the acid diffused into the agar block, the indicator changed from green to red in colour. The
time taken for the acid to diffuse to the centre of the block was measured.

Step 1 10 cm3 of 1 mol per dm3 hydrochloric acid was put into each of four test-tubes.

Step 2 The student used a ruler and a knife to measure and cut four blocks of agar, A, B, C and
D, from a larger piece of agar. Each block was a different size, as shown in Fig. 1.1.

A B
1.0 cm 1.0 cm
agar block

1.0 cm 1.0 cm

white tile 1.0 cm 0.5 cm

C
D
1.0 cm 0.5 cm

0.5 cm 0.5 cm

0.5 cm 0.5 cm

Fig. 1.1

Step 3 Agar block A was placed into one of the test-tubes containing hydrochloric acid. A stop-
clock was started. The colour of agar block A was observed.

The time taken for agar block A to change in colour from green to completely red was
recorded.

Step 4 Step 3 was repeated with the remaining blocks B, C and D.

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Fig. 1.2 shows the time taken for each agar block to become completely red.

minutes:seconds minutes:seconds minutes:seconds minutes:seconds

06:56 03:02 01:54 01:24

A B C D

Fig. 1.2

(a) (i) Prepare a table to record the results.

Convert the times shown in Fig. 1.2 to seconds and record them in your table.

[4]

(ii) State a conclusion for the results of this investigation.

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(iii) State the variable that the student measured (dependent variable) in this investigation.

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(iv) State the variable that the student changed (independent variable) in this investigation.

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..................................................................................................................................... [1]

(v) State two variables that were kept constant in this investigation.

1 ........................................................................................................................................

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2 ........................................................................................................................................

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[2]

(vi) Suggest one way of improving the method used in this investigation.

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(vii) Describe how step 2 could be carried out safely.

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(b) Table 1.1 shows the surface areas and volumes of the blocks of agar that were used in the
investigation.

Table 1.1

surface area to
length of sides / cm surface area / cm2 volume / cm3
volume ratio
1.0 × 1.0 × 1.0 6.00 1.000 6:1
1.0 × 1.0 × 0.5 4.00 0.500 8:1
1.0 × 0.5 × 0.5 2.50 0.250
0.5 × 0.5 × 0.5 1.50 0.125 12:1

Calculate the surface area to volume ratio for the 1.0 cm × 0.5 cm × 0.5 cm block of agar.

.......................................................... [1]

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(c) (i) Many organisms have adaptations that increase the area of their gas exchange surfaces.

Fig. 1.3 is a photomicrograph of part of a fish gill.

magnification ×550

Fig. 1.3

Draw a large diagram of the part of the fish gill shown in the box in Fig. 1.3.

[4]
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(ii) Measure the length of line PQ in Fig. 1.3.

length of line PQ .............................. mm

Calculate the actual length of the part of the fish gill using the formula and your
measurement.
length of line PQ
magnification = actual length of the part of the fish gill

Give your answer to three decimal places.

Space for working.

......................................................... mm
[3]

(d) Fish is a source of protein in the diet.

State how you could test a sample of food to show that it contains protein.

Include the result of a positive test.

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(e) Photographic film consists of a plastic sheet coated in crystals. The crystals are fixed to
the plastic sheet by gelatin, which is made of protein. If the gelatin is digested by protease
enzymes the crystals fall off and the film will become transparent, as shown in Fig. 1.4.

before protease treatment after protease treatment

Fig. 1.4

Plan an investigation to determine the effect of temperature on the activity of a protease

enzyme, using photographic film.

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[Total: 27]

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2 Cabbage leaves contain vitamin C. Boiling the cabbage in water reduces the vitamin C content of
the leaves. Some students investigated the effect of boiling time on the concentration of vitamin C
remaining in the cabbage leaves.

(a) The students prepared the cabbage samples using this method:

Step 1 Heat a large volume of water until it begins to boil.

Step 2 Measure the mass of chopped cabbage leaves.

Step 3 Place the chopped cabbage leaves into the boiling water.

Step 4 Continue to boil the water.

Step 5 Remove a sample of cabbage leaves from the water at each time interval.

State two pieces of apparatus, other than safety equipment, that would be required to carry
out the method described in steps 1 and 2.

1 ................................................................................................................................................

2 ................................................................................................................................................
[2]

(b) The vitamin C concentration of each cabbage leaf sample was then determined.

The results of the investigation are shown in Table 2.1.

Table 2.1

vitamin C
boiling time concentration
/ minutes / mg per 100 g of
cabbage leaves
0 34
5 31
10 26
15 22
25 13

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(i) Plot a line graph on the grid of the data in Table 2.1.

[4]

(ii) Use your graph to estimate the concentration of vitamin C in a sample of cabbage leaves
that has been boiled for 20 minutes.

Show on your graph how you obtained your estimate.

.......................................... mg per 100 g

[2]

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(iii) Table 2.1 shows that at the start of the investigation 100 g of unboiled cabbage leaves
contained 34 mg of vitamin C.

Using the information in Table 2.1, calculate the percentage decrease in the vitamin C
content of 100 g of cabbage leaves after boiling for 25 minutes.

Give your answer to two significant figures.

Space for working.

............................................................ %
[3]

(c) The students wanted to know if the vitamin C had moved from the cabbage leaves into the
water that the leaves were boiled in.

State how the students could test the water for the presence of vitamin C.

Include the result of a positive test.

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[Total: 13]

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