Sophie wanted to investigate the changes in pH during the production of
yoghurt from milk.
Yoghurt tastes sour because the bacteria convert lactose (a sugar) into
lactic acid.
The teacher predicted that the pH of the fermenting mixture would
decrease with time. The results were obtained using data-logging
equipment, and the experiment was carried out at 40 °C.
The table below shows how the pH changes over a 4-hour period.
Time (hours)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
pH of milk
6.5
5.6
5.4
4.7
4.5
4.3
4.1
4.1
4.1
How much did the pH change by in the first 90 minutes?
(1 mark)
Plot the data onto the grid below.
(2 marks)
When does the reaction stop?
Explain how you know this. Use evidence from the table and your graph
to help you.
(2 marks)
What conclusion can Sophie from her collected data. Use data from the
table and the graph in your answer.
(2 marks)
Suggest and explain one way in which Sophie could have ensured the
data she collected was reliable.
(2 marks)
Sophie wanted to speed up the rate at which the bacteria produce lactic
acid. She changed the temperature of the bacteria and milk solution.
Look at the table. It shows the data that Sophie collected.
Temperature
( C)
0
5
10
15
20
25
30
35
40
45
50
Time taken for the reaction to
stop (hours)
*
*
9.5
8.0
6.5
5.0
3.5
2.5
3.0
3.5
*
* = time taken was over 10 hours
Which is the best temperature to use? Explain your answer.
(1 mark)