Biology 5090/32 — October/November 2018

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation · Use of Techniques, Apparatus and Materials · Planning Experiments and Investigations · Microscopy and Biological Drawing

Q111MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationUse of Techniques, Apparatus and MaterialsFree sample

Certain bacteria are important in the production of yoghurt. Live yoghurt contains these living bacteria.

A small volume of live yoghurt can be added to milk and, given suitable conditions, the bacteria will change the milk into yoghurt.

You are provided with labelled samples of milk and live yoghurt.

Label a small, empty beaker yoghurt-milk mixture.
Use the measuring cylinder to measure $50\text{ cm}^3$ of milk and add it to the beaker labelled yoghurt-milk mixture.
Use the syringe to measure $2.5\text{ cm}^3$ of live yoghurt and add it to the milk in the beaker labelled yoghurt-milk mixture.
Stir this mixture well.

(a)

Examine the contents of the containers of milk, yoghurt-milk mixture and live yoghurt to observe their colour and how liquid they are.

(i)

Record your observations in the table.

sample	observations	pH
milk
yoghurt-milk mixture
live yoghurt

(ii)

Use pH indicator paper to find the pH of the milk, yoghurt-milk mixture and live yoghurt.
Record your results in the table above.

(iii)

Referring to the pH values you have recorded, describe any change in pH that occurs when yoghurt-milk mixture forms yoghurt.

______

(b)

When making yoghurt, the milk is first heated to $85\,^\circ\text{C}$ and then allowed to cool to $45\,^\circ\text{C}$ .

(i)

Suggest why the milk is heated to $85\,^\circ\text{C}$ .

______

(ii)

State how you could determine when the milk reaches a temperature of $85\,^\circ\text{C}$ .

______

(iii)

After the milk has been cooled to a temperature of $45\,^\circ\text{C}$ , a small volume of live yoghurt is added to it. This mixture is stirred and left at $45\,^\circ\text{C}$ for 8 hours. After this time, the milk has turned to yoghurt which can be cooled and eaten.

Using information in the table on page 2, suggest two ways you could check that the milk has been turned into yoghurt.

______
______

The rest of this paper

2 more questions

Q2Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation · Planning Experiments and Investigations14M
Q3Microscopy and Biological Drawing · Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation · Use of Techniques, Apparatus and Materials15M

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