Biology 5090/62 — October/November 2018

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

questions

marks

minutes

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

Q115MUse of Techniques, Apparatus and MaterialsExperimental ContextsObservations and MeasurementsPlanning Experiments and InvestigationsFree 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.

(a)

A yoghurt-milk mixture was produced by adding $5\,\text{cm}^3$ of live yoghurt to $100\,\text{cm}^3$ of milk. The mixture was stirred well. Given suitable conditions this mixture would then form yoghurt.

The consistency of the yoghurt-milk mixture was compared to the consistency of the milk and the yoghurt. The pH of each was measured.

The observations are recorded in the table.

sample	consistency	pH
milk	watery	7
yoghurt-milk mixture	watery	7
yoghurt	thick and creamy	5

(i)

If you were making the yoghurt-milk mixture in your laboratory, state which pieces of equipment you could use for obtaining the most accurate measurement of:

$100\,\text{cm}^3$ of milk,
______

$5\,\text{cm}^3$ of yoghurt.
______

(ii)

Describe, in detail, how you would use pH indicator paper to measure the pH of the samples in the table.

(iii)

Referring to the pH of the samples in the table, describe any change in pH that occurs as the 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 reached 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.

______
______

(iv)

You are given milk and natural live yoghurt. Describe an experiment you could carry out to show that the bacteria in the natural yoghurt must be alive for milk to be turned into more yoghurt.

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2 more questions

Q2Experimental Contexts · Planning Experiments and Investigations · Analysis, Conclusions and Evaluation · Microscopy and Biological Drawing · Observations and Measurements12M
Q3Microscopy and Biological Drawing · Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation13M

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