3.4 Testing for Carbohydrates

Question 1

What is a reducing sugar?

Answer 1

sugar molecule

which can donate electrons

to reduce another species

Question 2

Name a non-reducing sugar:

Answer 2

Sucrose

Question 3

Name the reducing sugars:

Answer 3

Glucose, Fructose, Galactose

Maltose, Lactose

Question 4

What test is used to detect reducing sugars?

What is the positive result?

Answer 4

Benedict’s reagent

blue solution ⟶ brick red precipitate

Question 5

What is Benedict’s reagent used to test the presence of?

Answer 5

reducing sugars

Question 6

Outline Benedict’s test on a solution of reducing sugars:

Answer 6

1. Form solution by grinding sample & adding water in a test tube.
2. Add Benedict’s reagent to the solution.
3. Place test tube in water bath.
4. Since reducing sugars are present, Benedict’s solution will form brick red precipitate.

Question 7

What is missing from this method of testing for reducing sugars?

1. Grind sample & form solution within test tube by adding water.
2. Add a few drops of Benedict’s reagent.
3. If brick red precipitate forms, reducing sugars are present.

Answer 7

Heating in water bath

Question 8

How does Benedict’s reagent work to determine the presence/concentration of reducing sugars?

Answer 8

Benedict’s solution contains blue Cu²⁺ ions

reducing sugars donate e^- to Cu²⁺

Cu²⁺ reduced to Cu⁺, forming brick red precipitate

the higher the concentration of reducing sugar, more Cu²⁺ are reduced

less blue Cu²⁺ present in end mixture, so mixture will be more red

Question 9

How is Benedict’s test carried out on non-reducing sugars?

Answer 9

1. Add dilute HCl to sample to hydrolyse non-reducing disaccharide ⟶ reducing monosaccharides.
   
   1. e.g sucrose ⟶ glucose + fructose
2. Neutralise by adding alkali.
3. Perform Benedict’s test as you would on reducing sugars.

Question 10

How do you use Benedict’s test to quantitatively determine the concentration of reducing sugars present in a solution?

Answer 10

After performing Benedict’s test on solutions of different concentrations:

1. Place colour filter in colorimeter
2. Calibrate colorimeter using distilled water (should have transmission of 100%)
3. Filter out brick red precipitate to leave solutions of blue Cu²⁺.
4. Place solutions in colorimeter & measure %transmission.
5. Higher transmission = less Cu2+ left unreduced = higher concentration of reducing sugar in original solution

Question 11

What is the test for starch?

What is the positive result?

Answer 11

Iodine

yellow ⟶ blue/black

Question 12

How does the iodine test work to determine the presence of starch within a solution?

Answer 12

Iodine particles trapped in helical structure of amylose/amylopectin

form complex which is blue/black

Question 13

When the solution formed from the positive test for starch is heated, it reverts back to the original yellow colour. Suggest why.

Answer 13

Hydrogen bonds in helical structure of amylose/amylopectin are broken

releases iodine molecules from starch-iodine complex

Question 14

What is the test for amino acids?

Answer 14

Chromatography

Question 15

Outline how can you determine the number/type of amino acids present in a mixture:

Answer 15

1. Spot amino acid solutions onto pencil line of chromatography paper.
2. Place chromatography paper in solvent/water.
3. Calculate retention factors of each amino acid. (distance travelled by compound/distance travelled by solvent front)
4. Compare results to amino acid retention factor data.

Question 16

How do you calculate the retention factor?

Answer 16

Distance travelled by compound / distance travelled by solvent front