Carbohydrates biochemical tests

Question 1

What chemical do we use to test for starch? What is the positive result?

Answer 1

• Potassium iodide.
• Iodine turns from orange to blue-black.

Question 2

True or False

All monosaccharides and disaccharides are reducing sugars.

Answer 2

• False.
• All monosaccharides and SOME disaccharides are reducing sugars.

Question 3

What is the test for reducing sugars?

Answer 3

• Add a liquified form (make liquified by griding/ adding water) of sample to test tube.
• Add equal volume of Benedict’s
• Heat for 5 minutes

Question 4

What is the positive test for reducing sugars?

Answer 4

• Benedict’s turns from blue –> brick-red

Question 5

What colours can benedict’s reagant go - starting from colour that shows lowest concentration of sugar?

Answer 5

• Blue
• Green
• Yellow
• Orange
• Brick-Red.

Question 6

Give an example of a non-reducing and reducing sugar.

Answer 6

Non-reducing: sucrose
Reducing: glucose.

Question 7

How do you test for a non-reducing sugar?

5 main steps

Answer 7

1.) Test the sample with Benedict’s to ensure it is a non-reducing sugar.
2.) Remove sample/ Benedict’s from test-tube and add more sample to an empty test-tube.
3.) Add acid to the test-tube/ BOIL (allow water in bath to reach 100 degrees.)
4.) Cool solution + add alkali to neutralise.
5.) Add Benedict’s reagant and heat.

Question 8

What is it called when you add acid to a non-reducing sugar and BOIL them?

Answer 8

• Acid hydrolysis.

Question 9

What two monosacharides would sucrose break into after undergoing acid hydrolysis?

Answer 9

• Glucose
• Fructose.

Question 10

What is a reducing sugar?

Answer 10

• One that can give electrons to another chemical (ie. can reduce the other chemicals.)

Question 11

What makes reducing sugars go from blue to brick-red in Benedict’s?

Answer 11

• The sugar reduces the copper oxide (blue) in Benedict’s to copper sulphate (brick-red.)

Question 12

Why do non-reducing sugars not change colour in Benedict’s? Why DOES the Benedict’s reagant work after acid hydrolysis?

Answer 12

• Reducing group (allows sugar to reduce other chemicals) is involved in glycosidic bond.
• When sucrose is hydrolysed –> the glycosidic bond is broken and the reducing group is exposed
• Allowing the sugar to reduce the copper oxide

Question 13

EXAM Q

A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter,that this student could use to measure the quantity of reducing sugar in a solution.

Answer 13

• Filter/ dry precipitate.
• Find mass (of precipitate.)

Question 14

What is a colorimeter?

Answer 14

• A device that is able to measure light absorbance (therefore, determing how much of a sugar solute there is in a solution of Benedict’s)

() - extra info for clarification.