Required practical - Biochemical tests
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the structure of biological molecules including carbohydrates, lipids, and proteins.
What is a polymer and give one biological example?
A polymer is a large molecule made from smaller units (monomers); e.g. starch is a polymer of sugars.
What is a protein made of?
Amino acids.
What is the monomer of starch?
Sugar / glucose.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains biochemical tests, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Aim and Overview
To use qualitative reagents to test for the presence of biological molecules - carbohydrates (sugars and starch), proteins and lipids - in food samples.
A qualitative test does not tell you how much of the molecule is present, only whether or not it's present.
Overview of Tests
|
Biological Molecule |
Test Used |
Method Summary |
Positive Result |
|
|
Sugars e.g., glucose |
Benedict’s test |
Add Benedict’s solution; heat in a water bath for 5 minutes. |
Blue → green / yellow / brick red, depending on the concentration of sugar. |
|
|
Starch |
Iodine solution |
Add a few drops of iodine solution to the food sample. |
Orange-brown → blue-black |
|
|
Protein |
Biuret reagent |
Add Biuret reagent to the sample. Mix gently. |
Blue → purple / lilac |
|
|
Lipids |
Ethanol test (known as the emulsion test) |
Add ethanol to the sample and shake. Then add water. |
Clear → cloudy, white emulsion |
|
Key Terms
- Qualitative test – A test that shows presence or absence, not amount.
- Reagent – A chemical used to cause a visible reaction with a substance.
No answer provided.
Exam Tips
- Always describe the colour change and name the reagent in your answers. Saying “it goes red” is not enough – you must state Benedict’s solution turns from blue to brick red.
- Don't forget to mention that the benedicts test requires heating.
- Make sure to say iodine solution, not just iodine, as it has to be a liquid form that is used.
- Lots of students find this topic difficult, but you really just need to learn the reagents and their colour changes!
No answer provided.
Practice Question
Describe how a student could test cow’s milk to show whether it contains protein and different types of carbohydrate. (6 marks)
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add Biuret reagent to milk, biuret reagent tests for protein
-
solution will turn (from blue) to purple if protein is present
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add iodine solution to milk, iodine solution tests for starch
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solution will turn (from orange / brown) to blue / black if starch is present
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add Benedict’s reagent to milk and heat, Benedict’s reagent tests for sugars
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solution will turn (from blue) to (brick) red / orange / yellow / green if sugars are present.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!