AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The human digestive system: Required practical activity 3: use qualitative reagents to test for a range of carbohydrates, lipids and proteins.
This required practical uses qualitative reagents to identify food molecules.
Study notes
From the checked teaching notes for this specification point.
The idea
This required practical uses qualitative reagents to identify food molecules. Benedict's solution tests for reducing sugars such as glucose: add it to the food sample and heat in a water bath, and a blue to green, yellow, orange or brick-red colour indicates a reducing sugar. Iodine solution tests for starch: add a few drops to the sample and a blue-black colour is a positive result. Biuret reagent tests for protein: add it to the sample and a purple or violet colour indicates protein. Ethanol emulsion tests for lipids: add ethanol to the sample, shake, then pour into water, and a white emulsion indicates lipid. A colour change shows the food molecule is present, but these tests are qualitative and do not give an exact concentration.
What earns the marks
- Benedict's solution is used to test for reducing sugars and requires heating in a water bath; a colour change from blue through green and yellow to brick-red indicates a positive result.
- Iodine solution is used to test for starch; a blue-black colour is a positive result.
- Biuret reagent is used to test for protein; a purple or violet colour is a positive result.
- Ethanol emulsion test is used to test for lipids; shaking the sample with ethanol and then adding water gives a white emulsion if lipid is present.
- Qualitative tests show whether a food molecule is present, not its exact concentration.
- Control tests using water instead of food sample help show the colour of a negative result.
Common mistakes and how to put them right
Using iodine to test for sugar or Benedict's solution to test for starch.
Put it right: Iodine tests for starch and Benedict's solution tests for reducing sugars.
Forgetting to heat the Benedict's test.
Put it right: Benedict's solution must be heated in a water bath for the colour change to occur.
Adding water before ethanol in the lipid test.
Put it right: Add ethanol to the sample first, shake, then pour the mixture into water.
Exam tips
- Name the reagent, the food molecule it tests for, and the positive colour in each answer.
- State that Benedict's test needs heating in a water bath, not a direct flame.
- Use the word qualitative to explain that the tests show presence, not amount.
Try the lesson
The start of the The human digestive system: Required practical activity 3: use qualitative reagents to test for a range of carbohydrates, lipids and proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
Which reagent is used to test a food sample for starch?
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