Testing For Reducing And Non-reducing Sugars

Question 1

What is a reducing sugar?

Answer 1

A sugar that can donate an electron to another molecule.

Question 2

What sugars are all monosaccharides?

Answer 2

Reducing sugars.

Question 3

What disaccharides are reducing sugars?

Answer 3

Maltose and lactose.

Question 4

What disaccharide is a reducing sugar?

Answer 4

Sucrose.

Question 5

What do we do at the start of each test?

Answer 5

Grind up the food with distilled water and filter the solid food particles.

Question 6

What solution do we add to test for reducing sugars?

Answer 6

Benedicts solution.

Question 7

How much Benedicts solution do we add?

Answer 7

3cm^3

Question 8

What does Benedicts solution contain?

Answer 8

The copper ion Cu^2+

Question 9

What does the copper ion Cu^2+ do in benedicts solution?

Answer 9

Makes the solution blue.

Question 10

What do we do after we add the benedicts solution?

Answer 10

Place the boiling tube into a beaker of boiling water and leave for 5 mins.

Question 11

What do we add the 3cm^3 of food solution to at the start?

Answer 11

A boiling tube.

Question 12

What colour would the solution be if no reducing sugar is present?

Answer 12

Remains blue.

Question 13

What happens if a reducing sugar is present?

Answer 13

An electron will be added to the copper 2+ ion to form the copper 1+ ion.

Question 14

What is formed if a reducing sugar is present?

Answer 14

A red precipitate.

Question 15

What does it mean if the solution turns green?

Answer 15

There is only a small amount of reducing sugar present.

Question 16

What does it mean if the solution turns yellow?

Answer 16

There is more reducing sugar present then the green solution.

Question 17

What does it mean if the solution turns orange?

Answer 17

There is a higher level of reducing sugar present then the green and yellow solutions.

Question 18

What does it mean if the solution turns a brick red colour?

Answer 18

There is a lot of/most reducing sugar present.

Question 19

Why do the sugars turn different colours?

Answer 19

There are different amount of the red precipitate mixing with the blue Benedicts solution.

Question 20

Why is the Benedicta test semi-quantitative?

Answer 20

It only shows a narrow range of colour changes that are approximate idea of the amount of reducing sugars.

Question 21

What do we need to do to test for non-reducing sugars?

Answer 21

Break the glycosidic bond to release the monosaccharides.

Question 22

What is the first step to test for non-reducing sugars?

Answer 22

Carry out the Benedicts test on the unknown solution and note down any colour change.

Question 23

What do we do after we carry out the benedicts test on our unknown solution?

Answer 23

Take a new boiling tube and add 3cm^3 of our unknown solution again.

Question 24

What do we add the the unknown solution in the boiling tube?

Answer 24

3cm^3 of dilute hydrochloric acid.

Question 25

What do we do after we add the dilute hydrochloric acid?

Answer 25

Gently boil the solution in a water bath for 5 mins.

Question 26

What happens if a non-reducing sugar is present?

Answer 26

Then the acid hydrolyses the glycosidic bonds releasing the monosaccharides.

Question 27

What do we add after the water bath?

Answer 27

3cm^3 of a dilute alkali such as sodium hydroxide.

Question 28

What do we do after we add the dilute alkali?

Answer 28

Use pH paper to check the solution is alkaline.

Question 29

Why do we add a dilute alkali?

Answer 29

To turn the solution alkaline as the benedicts test can’t work under acidic conditions.

Question 30

What do we do last when testing for non-reducing sugars?

Answer 30

Carry out the Benedicts test and note down any colour change (add solution then water bath).

Question 31

Why do we do a first Benedicts test in the test for non-reducing sugars?

Answer 31

To see if the unknown solution contains any reducing sugar.