Decay RPA

Question 1

Method

Answer 1

1) use a pipette to add 5cm cubed lipase solution to a boiling tube and label ‘lipase’
2) use a pipette to add 5 drops of cresol red to another boiling tube and label ‘milk’
3) use a pipette to transfer 5cm cubed to the ‘milk’ and another pipette to transfer 7cm cubed sodium carbonate solution to the ‘milk’
4) the ‘milk’ should now be purple- place both boiling tubes in a water bath at 20 degrees and leave a thermometer in the milk tube
5) wait for the ‘milk’ and ‘lipase’ to both reach 20 degrees
6) use pipette to add 1cm cubed of lipase to the milk and start stopwatch
7) record time taken for solution to turn from purple to yellow
8) repeat steps 1-7 2 more times and calculate mean time taken for 20 degrees
9) repeat whole investigation at different temperatures
10) plot results on graph with time taken against temperature

Question 2

What colour is cresol red in alkaline conditions (provided by sodium carbonate)

Answer 2

Purple

Question 3

What colour is cresol red in acidic conditions

Answer 3

Yellow
(Solution turns acidic due to fatty acid produced when lipase breaks down fat into fatty acids and glycerol)

Question 4

Source of error

Answer 4

Colour change is gradual so may be hard to time the exact point of colour change accurately

Question 5

Why must the test tubes be cleaned between each different experiment

Answer 5

So there are no traces of lipase left which will increase rate of reaction otherwise the time taken for colour change will be quicker than it should be