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AQA GCSE Biology (9-1): Getting the most out of Required Practicals - Decay

Posted by James Simms on November 20, 2019

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AQA GCSE Biology (9-1) Required Practical: Decay

Specification location: AQA GCSE Biology Paper 2, Section 4.7 - Ecology

Note: There is more than one way to do this experiment. I have seen this done with actual milk decaying over a week, but by far the more popular method seems to be the one outlined below.

Summary of method: Milk, Cresol Red and sodium carbonate are combined in a test tube and heated to a chosen temperature, along with a tube of lipase. When the tubes have reached the required temperature, lipase is transferred into the milk tube and a timer is started. When the solution turns from purple to yellow, the lipase has finished digesting the fat in the milk (the milk has been ‘decayed’) and the timer is stopped. This experiment is repeated at a variety of temperatures (for example: 20^oC, 30^oC, 40^oC, 50^oC,60^oC).

For more detail on the set-up of this experiment, watch our high-quality required practical tutorial on YouTube.

Decay required practical variables:

Independent variable: Temperature of milk solution

Dependent variable: Time taken to turn milk solution yellow

Control variables: Volume of milk used, volume of lipase used, volume of Cresol Red used, volume of sodium carbonate used, fat content of milk, age of milk, degree of mixing

Common issues	Potential solutions
Confusion	There are multiple liquids that need to be accurately measured up and it is critical that the equipment used to measure the lipase is not used to measure the liquids for the other tube because the lipase will start digesting fats immediately. This is best solved by explaining the method carefully and thoroughly and also by modelling best practice.
Time	In my experience, it has proven perfectly possible to clearly explain and conduct this experiment, but often there is insufficient time to properly analyse and contextualise the results. This is especially true if the content on enzymes is reintroduced. By far, the best solution I have found is for pupils to preload knowledge of the enzymes topic through a flipped learning technique (e.g. viewing and note-taking tutorials and undertaking quizzes on https://www.theeverlearner.com/). This minimises the time needed to be spent on recapping content and allows more time for context to be established and analyses conducted.
End points	It is sometimes difficult for students to recognise the endpoint of the experiment. It may be worth showing students both the purple ‘starting’ colour and yellow ‘finished’ colours so that they have a reference.

Sample decay required practical results:

Temperature of milk (^oC)	Time taken for solution to turn yellow (s)
Temperature of milk (^oC)	Your results	Class repeat 1	Class repeat 2	Mean
20	365	360	345	357
35	21	22	26	23
45	16	19	22	19
55	>600	>600	>600	>600
65	>600	>600	>600	>600

Source: AQA GCSE Biology Required Practical Handbook

AQA GCSE Science subject-specific terminology:

As this experiment is unlikely to determine the optimal temperature for lipase, this is a good opportunity to discuss intervals.
As, necessarily, results are being shared between groups, a discussion around the idea of reproducibility is a good fit here

Other blog posts in our AQA GCSE Biology (9-1) Required Practical series:

Getting the most out of GCSE Biology Required Practicals: Microscopy

Getting the most out of GCSE Biology Required Practicals: Microbiology

Getting the most out of GCSE Biology Required Practicals: Osmosis

Getting the most out of GCSE Biology Required Practicals: Food tests

Getting the most out of GCSE Biology Required Practicals: Enzyme activity

Getting the most out of GCSE Biology Required Practicals: Photosynthesis

Getting the most out of GCSE Biology Required Practicals: Reaction time

Getting the most out of GCSE Biology Required Practicals: Plant responses

Getting the most out of GCSE Biology Required Practicals: Field investigations