7. Ecology (required practical 10 - decay)

Question 1

What is the practical investigating?

Answer 1

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

Question 2

Why does milk turn sour it left at room temperature, after a couple of days?

Answer 2

• bacteria in milk start the decay process
• the bacteria use enzymes to produce acidic molecules

Question 3

Why is decay very hard to observe during lesson?

Answer 3

as it is a very slow process

Question 4

What are the steps for investigating the effect of temperature on the rate of decay of fresh milk by measuring the pH change?

Answer 4

1. Start by labelling a test tube “lipase”
2. Use a pipette to place 5cm3 of lipase solution into the test tube
3. Label another test tube “milk” and add 5 drops of the indicator Cresol red
4. Now add 5cm3 of milk and 7cm3 of sodium carbonate solution to the milk test tube. At this point the solution should be purple. (cresol red is purple in alkaline conditions)
5. Place a thermometer into the test tube containing the milk
6. Place both of the test tubes into a beaker of water at the first chosen temperature (20cm3)
7. Let the temperature of the solutions become the same as the water in the beaker
8. Use a pipette to transfer 1cm3 of lipase solution to the test tube containing milk and stir the solution, and at the same time, start a timer
9. The enzymes lipase will start to break down fat molecules in the milk which released fatty acids and causes the milk solution to become acidic
10. Once the solution turns yellow, stop the timer and record the results
    Repeat the experiment at a range of different temperatures
11. Plot a graph of the results

Question 5

What is the IV in this experiment?

Answer 5

the temperature

Question 6

What is the DV in this experiment?

Answer 6

the time taken for the milk solution to turn yellow

Question 7

What is the CV in this experiment?

Answer 7

volumes of the different solutions (lipase solution, milk, sodium carbonate solution and cresol red)

Question 8

Why is it important that we use a clean test tube for the milk solution for each experiment?

Answer 8

any traces of lipase from the previous experiment will trigger the reaction before we are ready

Question 9

How can we reduce the effect of it being difficult to decide when to stop the timer?

Answer 9

sharing data with other groups and calculating a mean