Biochemical Tests

Question 1

Reducing sugars

Answer 1

Benedict’s

Question 2

Reducing sugars test method

Answer 2

Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube

Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes

If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water

It is important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present

A positive test result is a colour change somewhere along a colour scale from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar) to brown/brick-red (a high concentration of reducing sugar)

This test is semi-quantitative as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present

Question 3

Non-reducing sugars

Answer 3

Hydrochloric acid & Benedicts

Question 4

Non-reducing sugars test method

Answer 4

Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil

Neutralise the solution with sodium hydrogencarbonate

Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict’s test to work

Then carry out the Benedict’s test as normal; add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs, a reducing sugar is present

Question 5

Starch

Answer 5

Iodine

Question 6

Starch test method

Answer 6

To test for the presence of starch in a sample, add a few drops of orange/brown iodine in potassium iodide solution to the sample

The iodine is in potassium iodide solution as iodine is insoluble in water

If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour

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

Question 7

Protein

Answer 7

Biuret’s

Question 8

Protein test method

Answer 8

Add Biuret’s reagent to sample

If protein present: colour change from blue to purple

Question 9

Lipids

Answer 9

Emulsion test

Question 10

Lipids test method

Answer 10

Dissolve sample in ethanol

Pour solution slowly into water

If lipids present: white emulsion will form