2.2 Biological Molecules (TESTS)

Question 1

Benedict tests = Reducing + Non reducing sugars

Answer 1

• Reducing sugars include monosaccharides and certain disaccharides that can donate electrons.
• The test involves adding an alkaline solution of blue copper (II) sulphate to the sugar.
• When heated in the presence of a reducing sugar, it results in the formation of a red precipitate of copper (I) oxide.
• In the absence of reducing sugar, there is no change in the colour of the Benedict’s reagent.

Question 2

Biurets tests = Test for proteins

Answer 2

• The biuret test is employed to detect proteins.
• A sample of the solution is placed in a test tube.
• An equal amount of NaOH (sodium hydroxide) is added to the test tube.
• A few drops of dilute copper (II) sulphate solution are gently mixed into the solution.
• In the presence of a protein, the solution changes colour to lilac, indicating the presence of peptide bonds.
• In the absence of protein, the solution retains its blue colour

Question 3

Emulsion test = Test for lipids

Answer 3

The emulsion test is employed to detect lipids.
Start by adding 2ml of the sample to 5ml of ethanol.
Ensure thorough mixing by shaking to dissolve the lipids in the ethanol.
Add 5ml of water to the mixture and mix again.
The solution turns cloudy, indicating the presence of lipids due to the formation of an emulsion where light is refracted as it passes from droplets of oil to droplets of water.

Question 4

Iodine test = Test for starch

Answer 4

In the presence of starch, potassium iodide solution changes colour from yellow to black or blue.