3.4 Testing for Carbohydrates

Question 1

Give two examples of disaccharide reducing sugars. (2 marks)

Answer 1

Maltose
Lactose

Question 2

Are monosaccharides reducing or non-reducing sugars? (1 mark)

Answer 2

Reducing

Question 3

How do you test for reducing sugars? (3 marks)

Answer 3

Place sample in boiling test tube
Add equal volume of Benedict’s reagent
Heat the mixture gently in a hot water bath for 5 minutes.

Question 4

What is a positive result for reducing sugars, explain why this happens? (3 marks)

Answer 4

Changes from a blue colour to a brick-red colour, this is due to the addition of electrons to the Cu2+ ions to make Cu+ ions.

Question 5

Give an example of a non-reducing sugar. (1 mark)

Answer 5

Sucrose

Question 6

How do you test for non-reducing sugars? (2 marks)

Answer 6

Boil sample, and add dilute HCl.
Complete same test as for reducing sugars.

Question 7

How do you test for starch, and what is a positive result? (2 marks)

Answer 7

Add a few drops of iodine to the sample.
If it changes from yellow/brown to purple/black starch is present in the sample.

Question 8

What are reagent test strips most commonly used to test for and name one advantage of them? (2 marks)

Answer 8

Reducing sugars
(accept glucose)
Advantage: the concentration of sugar can be determined, with the use of a colour-coded chart.