7.10 Required practical: temperature and the rate of decay
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Explain why the pink colour disappears when lipase is added to the milk mixture.
Mark scheme
- lipase breaks down fat / lipids (1)
- into fatty acids (and glycerol) (1)
- pH falls / becomes less alkaline, so phenolphthalein turns colourless (1)
Model answer
Lipase breaks down the fat in milk into fatty acids, which lower the pH, so the phenolphthalein indicator turns from pink to colourless.
Why is sodium carbonate added to the milk at the start?
Mark scheme
to make the mixture alkaline (so the indicator is pink to start with)
Model answer
It makes the mixture alkaline, so the phenolphthalein is pink at the start.
At 30 °C the colour disappeared after 125 seconds.
Calculate the rate using rate = 1 / time. Give your answer in standard form.
Mark scheme
- 1 ÷ 125 (1)
- = 8 × 10-3 (s-1) (1)
Model answer
rate = 1 ÷ 125 = 0.008 = 8 × 10-3 s-1.
The rate was lower at 60 °C than at 40 °C. Explain why.
Mark scheme
- lipase / enzyme is denatured (1)
- active site changes shape so fat cannot fit / be broken down (1)
Model answer
At 60 °C the lipase is denatured: its active site changes shape, so it cannot break down the fat.
Suggest how the measurement of the end point could be made more accurate.
Mark scheme
use a pH meter / data logger / compare against a colour standard
Model answer
Use a pH meter or data logger to measure the pH, instead of judging the colour by eye.
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