Biology 9700/33 — May/June 2023

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Use of the Light Microscope

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

When milk is dropped into a test-tube containing copper sulfate solution, the drop of milk sinks to the bottom of the test-tube. The speed at which the drop of milk sinks will depend on the amount of protein, fat and other solids that are present in the milk.

You will investigate the effect of different concentrations of milk on the time taken for the drop to sink to the bottom of the test-tube.

You are provided with the materials shown in Table 1.1.

Table 1.1

labelled	contents	hazard	volume / $\text{cm}^3$
C	copper sulfate solution	irritant	200
M	100% milk	none	50
W	distilled water	none	30

If any solution comes into contact with your skin, wash off immediately with cold water.

It is recommended that you wear suitable eye protection.

Carry out step 1 to step 3 to practise releasing one drop of M from a pipette.

step 1 Put some of M into a pipette.

step 2 Hold the pipette containing M over an empty test-tube, as shown in Fig. 1.1. Squeeze the pipette gently to release one drop. It is important that the drop does not contain air bubbles.

step 3 Repeat step 2 until you can release one drop at a time.

Fig. 1.1

You will investigate the effect of different concentrations of milk on the time taken for one drop of milk to sink through copper sulfate solution.

You will need to:

prepare different concentrations of milk
record the time taken for one drop of each concentration of milk to sink through copper sulfate solution.

(a)

You will need to use proportional dilution to make different concentrations of milk, M, between 95% and 55%.

You will need to prepare $10\ \text{cm}^3$ of each concentration, using M and W.

Table 1.2 shows two of the concentrations of milk you will use and how to prepare one of the concentrations.

Decide which other concentrations of milk you will use.

(i)

Complete Table 1.2 to show how you will prepare the concentrations of milk you will use.

Table 1.2

percentage concentration of milk	volume of M / $\text{cm}^3$	volume of W / $\text{cm}^3$
95.0



55.0	5.5	4.5

(ii)

Carry out step 4 to step 14.

step 4 Prepare the concentrations of milk, shown in Table 1.2, in the beakers provided.

step 5 Draw a line $2\ \text{cm}$ from the top of a clean, dry test-tube, as shown in Fig. 1.2. You will start timing from this line in your investigation.

Fig. 1.2

step 6 Fill this test-tube with C, up to the top, as shown in Fig. 1.2.

step 7 Stir the 95% concentration of milk.

step 8 Put some of the 95% milk into a pipette.

step 9 Put one drop of 95% milk into the test-tube containing C, as shown in Fig. 1.3. Start timing when the drop passes the line you have drawn.

Fig. 1.3

step 10 Stop timing when the drop reaches the bottom of the test-tube.

If the drop has not reached the bottom of the test-tube after 120 seconds, stop timing and record 'more than 120'.

step 11 Record your result in (a)(ii).

step 12 Carry out step 7 to step 10 as many times as you need, so that you are confident in your results for 95% milk. Remove any milk remaining at the surface of C with a pipette between each trial and add fresh C up to the top of the test-tube.

Record all your results in (a)(ii).

step 13 When you are confident in your results for 95% milk, empty the contents of the test-tube into the container labelled For waste and rinse out the test-tube.

step 14 Repeat step 6 to step 13 for the other concentrations of milk you prepared in step 4.

Record your results in an appropriate table.

(iii)

Describe the trend in your results in (a)(ii).

(iv)

Using Table 1.1 and step 4 to step 13, assess the risk of this procedure as low, medium or high. Give one reason for your answer.

risk ______

reason ______

(v)

Complete Table 1.3 to identify two possible sources of error in this investigation and describe how you could improve the procedure to reduce the effect of each source of error.

Table 1.3

error	improvement

(b)

A student investigated the effect of temperature on the density of a milk sample.

The results from the investigation are shown in Table 1.4. The density of milk is shown in arbitrary units (au).

Table 1.4

temperature / °C	density of milk / au
5	34
10	33
20	31
25	26
30	24
35	21

(i)

Plot a graph of the data shown in Table 1.4 on the grid in Fig. 1.4.

Use a sharp pencil.

Fig. 1.4

(ii)

The student measured the temperature of the laboratory as 23°C. Use your graph to determine the density of milk at 23°C.

density of milk = ______ $\text{au}$

(iii)

A student suggested the hypothesis:

As the temperature increases by 10°C the density of the milk decreases by 2 au.

Explain whether or not the data provides evidence to support the hypothesis.

The rest of this paper

1 more questions

Q2Use of the Light Microscope20M

Loading the full paper…