3.4 Tests For Reducing & Non-Reducing Sugars

Question 1

Describe the method for testing for reducing sugars.

Answer 1

1) Set up a thermostatic water bath. Ensure temperature is at least 80*C using a thermometer
2) Add 2cm^3 of each test solution to labelled test tubes (If test substance is food, grind it up using a pestle and mortar, and add 1cm^3 of the powder and 2cm^3 of distilled water
3) Add 0.5cm^3 of Benedict’s reagent to each test solution (making sure the pipette doesn’t touch the test tubes)
4) Once all test tubes are prepared, place them all in the water bath and set a timer for 5 minutes
5) At 5 minutes, retrieve the test tubes, place them in the test tube rack and record the colour changes.

Question 2

What does a positive result for reducing sugars look like?

Answer 2

A colour change from blue, to green, orange or red.

Question 3

What does a negative result for reducing sugars look like?

Answer 3

No colour change, the solution stays blue.

Question 4

What is the safety risk of using Benedict’s reagent?

Answer 4

Benedict’s reagent is alkaline and an irritant, so can irritate skin. So, wear safety glasses to prevent it from entering eyes.

Question 5

What is a qualitative result?

Answer 5

A result based on what you see

Question 6

What is a quantitative result?

Answer 6

A result based on numerical measurements.

Question 7

Is the Benedict’s test for reducing sugars qualitative or quantitative?

Answer 7

Qualitative, because it tells us whether a substance is present or not, and roughly how much based on observation only.

Question 8

Why is Benedict’s Reagent blue?

Answer 8

Because it contains Copper ions (Cu2+).

Question 9

Why does Benedict’s Reagent change colour when subjected to reducing sugars?

Answer 9

-Reducing sugars are able to donate electrons to the copper ions
-Copper ions in the Benedict’s solution are blue
-The copper ions are reduced, and a coloured precipitate is formed
-The more reducing sugar is present, the more of the blue copper ions will be reduced.
The colour of the precipitate indicates the concentration of reducing sugar present

Question 10

What does the colour of the precipitate indicate?

Answer 10

The concentration of reducing sugar present

Question 11

What is a reducing sugar?

Answer 11

Any sugar that is capable of giving electrons away (acting as a reducing agent)

Question 12

What does a reagent strip test for?

Answer 12

Reducing sugars

Question 13

What is a reagent strip used for?

Answer 13

Determining the approximate concentration of glucose in urine, used to indicate diabetes.

Question 14

What substance is used to test for reducing sugars?

Answer 14

Benedict’s reagent

Question 15

Describe the method for testing for non-reducing sugars.

Answer 15

1) Test for reducing sugars using Benedict’s solution + method
2) If any solutions remain blue, that indicates that there are no reducing sugars
3) Boil the sugar solution with dilute hydrochloric acid
4) Neutralise the mixture with sodium hydrogencarbonate (an alkali)
5) Add Benedict’s reagent and heat in a water bath
6) Look for a positive result (green/yellow/orange/brick red precipitate)

Question 16

What is Acid Hydrolysis?

Answer 16

Using acid to break down chemical bonds

Question 17

How come sucrose gives a positive result for reducing sugars after acid hydrolysis?

Answer 17

The acid hydrolyses the glycosidic bonds in sucrose and it is broken down into glucose and fructose. Glucose and fructose are both reducing sugars, so they give a positive result

Question 18

Why do we have to neutralise an acidic solution before performing a Benedict’s test?

Answer 18

To prevent the acidic environment from interfering with the reaction

Question 19

What substance do you use to test for the presence of starch?

Answer 19

Iodine solution (iodine dissolved in a potassium iodide solution)

Question 20

How to test for the presence of starch?

Answer 20

Mix a sample with a few drops of iodine dissolved in a potassium iodide. If starch is present, solution changes from yellow/brown to purple/black

Question 21

What is the colour change of iodine solution if starch is present?

Answer 21

Yellow/brown to purple/black.

Question 22

What is colorimetry?

Answer 22

A piece of equipment used to quantitatively measure the absorbance or transmission of light by a coloured solution.

Question 23

What does a biosensor do?

Answer 23

Uses biological components to determine the presence and concentration of molecules.