Decay
Q1.
Fresh milk contains bacteria.
Some students investigated decay caused by the bacteria in fresh milk.
This is the method used:
1.   Put 200 cm3 of fresh milk in a sterilised flask.
2.   Leave the flask for 3 days at 20 °C.
3.   Measure the pH of the milk each day using universal indicator paper.
Figure 1 and Figure 2 show the apparatus the students used.
 
(a)     Give one reason why the students sterilised the flask before adding the milk.
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(1)
(b)     Describe how the students could sterilise the flask in a school laboratory.
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(2)
(c)     Why did the students put a cap on top of the flask?
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(1)
(d)     The table shows the students’ results.
 
Table 1	
Time in days	Colour of universal indicator paper	pH	
0	Olive-green	 	
1	Olive-green	 	
2	Olive-green	 	
3	Orange-green	 	
Complete Table 1.
Use information from Figure 2.
(1)
(e)     The students repeated their investigation with two changes to the method:
•   they used a pH meter to measure the pH
•   they left the apparatus set up for 6 days instead of for 3 days.
Suggest a reason why each of these changes improves the investigation.
Using a pH meter ____________________________________________________
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Leaving the apparatus set up for 6 days ___________________________________
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(2)
Table 2 shows the results of the students’ second investigation.
 
Table 2	
Time in days	pH	
0	7.0	
1	7.0	
2	6.7	
3	6.0	
4	5.0	
5	4.5	
6	4.5	
(f)      Complete the graph below.
You should:
•   label the x-axis
•   plot the data from Table 2
•   draw a line of best fit.
 
(4)
(g)     Give one reason for each of the following.
Use information from Table 2 and the graph above.
The pH did not change during the first day: ________________________________
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The pH decreased after day 1: __________________________________________
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There was no change in pH between days 5 and 6: __________________________
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(3)
(h)     The students did both of their investigations at 20 °C
The students then repeated the investigation with the pH meter, but at 25 °C
Predict how the new results would be:
•   similar to the results at 20 °C
•   different from the results at 20 °C
Similarity ___________________________________________________________
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Difference __________________________________________________________
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(2)
(Total 16 marks)