Biochemical tests!

Here are quick notes on the four biochemical tests that need to be known for AS biology

Hope the notes help :)


Emulsion test

Test for Lipids;


1. If the sample if solid crush it up (such as seeds).

2. Then add 5cm3 of ethanol (measurements don't really need to be learnt).

3. Shake the mixture.

4. Now add water.

5. Shake again.


Positive test: Solution turns cloudy because the lipid is dispersed throughout the liquid.

Negative test: Stays the same colour as before.

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Reducing sugar test

Test for Reducing sugars;


1. Add 1cm3 of benedicts reagent to sample in a test tube.

2. Mix carefully and heat in a water bath for 3 minutes.

Positive test: Solution turns from blue to brisk red.

Negative: Stays blue.

(Glucose, Fructose, and Maltose produce a positive test but Sucrose doesn't)

Test for Non-reducing sugars (Confirmation of Sucrose);


1. Same steps 1 & 2 as above but then you boil the solution with an acid.

Positive result: Sucrose previously stayed blue but goes brick red.

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Iodine test

Test for Starch;


1. Add 1cm3 of iodine solution to a test tube containing the sample. (Note: Iodine SOLUTION not iodine alone).

Positive result: Solution turns blue/black

Negative result: Solution stays yellow.

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Biuret test

Test for Proteins;


1. Add 1cm3 of sodium hydroxide to a test tube containing the sample.

2. Mix gently.

3. Add around 8 drops of copper sulphate to the test tube.

4. Mix again.

Positive test: Solution turns purple.

Negative test: Solution stays blue 

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