RP10: Decay

Question 1

What is the aim of this experiment?

Answer 1

• Investigate the effect of temperature on the rate of decay of fresh milk by m measuring pH change

Answer 2

1. Write down a hypothesis of the effect of temperature on the rate of decay of milk.
2. Carefully, fill half of a beaker with hot water (60°C or below) from the kettle for a water bath.
3. Use a syringe to transfer 5 cm3​ o​ f lipase solution into a boiling tube and label as ‘lipase’.
4. Add 5 drops of Cresol red into another boiling tube and label as ‘milk’.
5. Use a calibrated pipette to transfer 5 cm3​ m​ ilk into the ‘milk’ tube.
6. Use another calibrated pipette to transfer 7 cm3​ s​ odium carbonate solution to the ‘milk’ tube, which should make a purple solution.
7. Place a thermometer into the ‘milk’ tube.
8. Place both boiling tubes into the water bath.
9. Allow time for the solutions in the boiling tubes to reach the same temperature as the water
   bath.
10. Use another pipette to transfer 1 cm3​ o​ f lipase from the ‘lipase’ tube into the ‘milk’ tube and start timing immediately.
11. Record the time required for the colour change to yellow in a table such as below.
12. Repeat steps 2-11 at the same temperature twice and take a mean value.
13. Repeat steps 2-11 using a range of different temperatures of water baths (a range from
    20°C - 60°C).
14. Plot a graph of time taken against temperature.1

Question 3

Name some sources of error

Answer 3

• the colour change at the end point may be difficult to judge

Question 4

Why does the addition of lipase accelerate the decay of milk?

Answer 4

• Lipase is an enzyme which catalyses the breakdown of lipid to fatty acids and glycerol

Question 5

Why does the pH of the solution change during this reaction?

Answer 5

• Fat in milk is broken down into fatty acids by the enzyme lipase, lowering the pH

Question 6

What colour is cresol red in alkaline and acidic solutions?

Answer 6

• Alkaline: purple
• Acidic: yellow

Question 7

Why does an increase in temp increase the rate of this reaction?

Answer 7

• Enzyme and reacting particles have high kinetic energy and collide more frequently - faster rate of reaction

Question 8

Why might the rate of reaction decrease if the temp is very high?

Answer 8

• Enzymes become denatured as active site changes shape - cannot catalyse reaction