Lesson 7.5 - Testing for Carbohydrates

Question 1

Reducing sugars examples?

Answer 1

Maltose
Lactose
Glucose
Fructose
Ribose

Question 2

Meaning of reducing sugars?

Answer 2

They can donate electrons, or reduce another chemical / molecule.

Question 3

Reducing meaning in chemistry?

Answer 3

Reaction involving the gain of electrons

Question 4

What is Benedicts reagent?

Answer 4

alkaline solution of Copper (II) sulfate.

Question 5

What is used in test for reducing sugars?

Answer 5

Benedicts reagent

Question 6

Test for reducing sugars:

Answer 6

1. Place sample to be tested in a boiling tube. If it is not a liquid, grind it up with water in a mortar and pestle or blend it.
2. Add equal volume of benedicts reagent.
3. Heat the mixture gently in a boiling (100 degrees celcius) water bath for 5 minutes.

Question 7

Colour change of Benedicts reagent?

Answer 7

Blue –> Brick-red

Question 8

Why is there a colour change in the test for reducing sugars?

Answer 8

• The reducing sugars react with the copper (II) ions.
• Results in the addition of electrons to the blue Cu+2 ions. This reduces them to brick-red Cu+ ions.

Question 9

What happens when reducing sugar is mixed with benedicts solution and warms?

Answer 9

Brick-red precipitate forms

Question 10

How is the test for reducing sugars qualitative?

Answer 10

More reducing sugars present = more Cu+
Colour of solution is a mix of brick-red Cu+ ions and blue Cu+2 ions.

Question 11

Colour changes of reducing sugars:

Answer 11

High: Brick-red
Medium: Orange / Yellow / Amber
Low: Green
None: Blue

Question 12

Iodine test:

Answer 12

Test for starch

Question 13

How does the iodine test work for starch?

Answer 13

Few drops of iodine dissolved in potassium iodine solution and are mixed with the solution.

If solution colour changes from yellow/brown to black/blue then starch is present.

Question 14

Reagent strips?

Answer 14

Manufactured tests to test for reducing sugars. Most commonly glucose.

Question 15

Advantage of reagent strips?

Answer 15

Colour-coded chart to determine the concentration of reducing sugars.

Question 16

Non-reducing sugars examples?

Answer 16

Sucrose

Question 17

Describe how the investigation could be changed to give quantitative results

Answer 17

• Use a colorimeter
• filter and dry precipitate, then weigh it.

Standardises the method
Colour change is subjective.

Question 18

How to test for non-reducing sugars?

Answer 18

To test for these types of sugars, you must first hydrolyse them into their monosaccharide components before we can do the rest of the test.

Question 19

Test for non-reduing sugars

Answer 19

1. Carry out the test for reducing sugars, and if the result is negative (turns blue), continue with the next steps.
2. Add 2 cm3 of the food sample to 2 cm3 of dilute hydrochloric acid.
3. Heat the mixture in a gently boiling water bath for 5 minutes (the acid will hydrolyse disaccharides into monosaccharides).
4. Neutralise the mixture by adding sodium hydrogencarbonate solution.
5. Retest this mixture using the test for reducing sugars.
   If non-reducing sugars were present at the start, the mixture will now change from a blue solution to a brick red precipitate.

Question 20

Steps to test for iodine

Answer 20

1. Place 2 cm3 of your food sample into a test tube.
2. Add a couple of drops of iodine solution and shake.
3. If starch is present, the solution will turn from orange to blue-black.