Biological molecules

Benedicts test

Benedicts test

for reducing sugar

How to perform benedicts

1. Add Benedict's reagent (which is blue)

2. Heat the test tube in a beaker of water that has been brought to a boil

3. If a reducing sugar is present, a coloured precipitate will form

4. A positive test is, a colour change scaling from blue to brown/brick-red

Iodine test for

starch

How to carry out iodine test

1. add a few drops of orange/brown iodine to the sample

2. If starch is present, a distinctive blue-black colour can be seen

   (This test is useful in experiments for showing that starch in a sample has been digested by enzymes)

Emulsion test for

lipids

How to carry out the emulsion test

1. Add ethanol to the sample to be tested, shake to mix

2. Add the mixture to a test tube of water

3. If lipids are present, a milky emulsion will form

4. If no lipids are present, solution remains clear

Biuret test for

protein

How to carry out biuret test

1. Add biuret reagent

2. Positive result turns the solution lilac

Example of non-reducing sugar

Sucrose

How to carry out test for non-reducing sugar

1. Carry out benedicts test first to ensure that it is a non-reducing sugar

2. Add acid to the sucrase and boil (acid hydrolysis)

3. Cool the acid and add an alkali to neutralize

4. Then add the benedicts solution and heat

5. A positive test turns from blue to

   green —> yellow —> orange —> red