3.4 - Testing For Carbohydrates

Question 1

Test for reducing sugars

Answer 1

Benedicts reagent

Question 2

What is a reducing sugar

Answer 2

All monosaccharides and some disaccharides that can donate electrons or reduce another molecule or chemical

Question 3

What is benedicts reagent made up of

Answer 3

Alkaline solution of copper(II) Sulfate

Question 4

How do you carry out benedicts test

Answer 4

1) place sample in boiling tube (if not a liquid then grind or blend into water)
2) add equal volume of benedicts reagent
3) heat mixture gently in boiling water bath for 5 minutes

Question 5

How does benedicts work

Answer 5

Reducing sugars react with Cu2+ ions in the reagent. Results in addition of electrons to Cu2+ ions, reducing them to brick red Cu+ ions.

Question 6

Colour change in benedicts

Answer 6

Blue to brick red precipitate. Actual colour seen will be mixture of precipitate and unchanged blue ions and will depend on the concentration of reducing sugar present.

Question 7

What type of test is benedicts

Answer 7

Qualitative

Question 8

Most common non reducing sugar

Answer 8

Sucrose

Question 9

How can you test for non reducing sugar using benedicts reagent

Answer 9

Boil with dilute HCl. Will show positive result when warmed with Benedicts solution as sugar has been hydrolysed by acid to glucose and fructose which are reducing sugars.

Question 10

Test for starch

Answer 10

Iodine test

Question 11

How to carry out iodine test

Answer 11

Few drops of iodine dissolved in potassium iodide solution are mixed with a sample. If solution changes from yellow/brown to purple/black then starch is present.

Question 12

Another test for presence of reducing sugars

Answer 12

Manufactured reagent test strips can be used to test for the presence of reducing sugars most commonly glucose.

Question 13

Advantage of using reagent strips

Answer 13

With the use of a colour coded chart, concentration of sugar can be determined