Diffusion & Osmosis (Cambridge O Level Biology)

Exam Questions

2 hours30 questions

9 marks

A student measured the distance moved by different concentrations of citric acid solution through agar jelly. The agar contained Universal Indicator which changed colour in the presence of acid. The agar mixed with Universal Indicator was green at the beginning of the investigation.

Step 1

Three test-tubes were labelled A, B and C. Three different concentrations of citric acid solution were made.

Table 1 shows the volumes of 5% citric acid solution and distilled water that were used to make each solution.

Table 1

Step 2

The base of a Petri dish containing agar and Universal Indicator was labelled A, B and C.

Three holes were cut into the agar. This is shown in Fig.1

cie-2019-p6-q1a-petri

Fig. 1

Step 3	The student was provided with one dropping pipette. Three drops of solution A were placed into the hole in section A of the Petri dish.
Step 4	Three drops of solution B were placed into the hole in section B of the Petri dish.
Step 5	Three drops of solution C were placed into the hole in section C of the Petri dish.
Step 6	A stop-clock was started.
Step 7	After 30 minutes the student observed the colour change in the agar around the hole in each section of the Petri dish. The colour change was caused by the diffusion of the citric acid solution through the agar.
Step 8	A ruler was used to measure the distance travelled by each concentration of citric acid solution through the agar.

Fig. 2 shows the appearance of the Petri dish after 30 minutes.

_RIHQH5c_cie-2019-p6-q1a-petri-results

Fig. 2

Measure the distance travelled by each concentration of citric acid solution after 30 minutes in Fig. 2 Rulers have been added to the image to assist you in taking these measurements.

Prepare a table and record these results.

You should include:

The concentration of the citric acid solutions
The distance travelled by the citric acid solutions.

[3]

ii)

Describe how you decided where to measure the distance travelled by the citric acid solutions.

[1]

iii)

State a conclusion for these results.

[1]

iv)

The citric acid moves through the agar by diffusion. The diffusion coefficient is used to show the effect of concentration on diffusion.

The formula to calculate the diffusion coefficient is:

cie-2019-p6-q1a-diffusion-coefficient

Calculate the diffusion coefficient, in mm² per minute, for a 10 % solution of citric acid that travelled 14 mm in 30 minutes.

Give your answer to two significant figures.

[2]

Universal Indicator is used to estimate the pH value of substances.

Estimate the pH value for the green agar and the red agar.

[2]

How did you do?

4 marks

State two variables that have been kept constant in this investigation.

[2]

ii)

Identify one potential source of error in this investigation and suggest how the error could affect the results.

[2]

How did you do?

6 marks

Describe how you could adapt this method to find the effect of temperature on the rate of diffusion.

Agar melts at 70 °C.

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6 marks

A student investigated the effect of different concentrations of salt solution on a hollow plant stem.

They were provided with a 2% salt solution and distilled water. The student used these to make up different concentrations of salt solution.

Step 1	Four test-tubes were labelled 1, 2, 3 and 4.
Step 2	The information in Table 1 was used to make up the four different salt solutions in the test-tubes.

Table 1

Test-tube	Volume of 2% salt solution / cm³	Volume of distilled water / cm³	Final percentage concentration of salt solution
1	0	20	0.0
2	5	15
3	10	10	1.0
4	20	0	2.0

Complete Table 1 by calculating the final percentage concentration of the salt solution in test-tube 2.

[1]

Step 3	The contents of each test-tube were poured into four Petri dishes labelled 1, 2, 3 and 4.
Step 4	A hollow stem was cut into 12 rings using a sharp scalpel. Each stem ring was approximately 2mm long, as shown in Fig.1

p63fig1-1

Fig. 1

Step 5

Each stem ring was then cut open as shown in Fig. 2.

p63fig1-2

Fig. 2

Step 6

Three cut stem rings were put into each of the different salt solutions in the labelled Petri dishes and left for 10 minutes.

Fig. 3 shows the appearance of the cut stem rings after 10 minutes.

p63fig1-3

Fig. 3

Step 7

The distance between the two cut ends of each stem ring can be measured, as shown in Fig. 4

In the example shown in Fig. 4 the distance is 12 mm.

p63fig1-4

Fig. 4

The measurements of each stem ring were as follows:

Petri dish 1: 18 mm, 20 mm, 17 mm
Petri dish 2: 10 mm, 12 mm, 12 mm
Petri dish 3: 7 mm, 10 mm, 12 mm
Petri dish 4: 1 mm, 1 mm, 1 mm

ii)

Prepare a table and record your measurements in your table.

Your table should show:

all of your results
a calculated average for each solution.

[4]

iii)

Use Table 1 and your measurements to describe the results shown in Fig. 3

[1]

How did you do?

2 marks

Identify one hazard in step 4 and describe a suitable safety precaution.

How did you do?

2 marks

Explain why more than one ring of the hollow stem was placed into each Petri dish.

How did you do?

3 marks

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

[1]

ii)

Identify two variables that were kept constant in this investigation.

[2]

How did you do?

4 marks

There are potential errors in steps 4 and 7.

Identify two of these errors and suggest an improvement for each.

How did you do?

2 marks

Fig. 5 shows a section through a hollow plant stem observed through a light microscope.

p63fig1-5

Fig. 5

The measured length of AB on Fig. 5 is 28 mm.

Calculate the actual length of AB using the following equation:

$magnification = \frac{measured length of AB}{actual length of AB}$

Show your working.

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