Chapter 3 - Bio Molecules - Chemical Tests

Question 1

What do reducing sugars include?

Answer 1

Include all monosaccharides and some disaccharides such as maltose and lactose.

Question 2

What do non-reducing sugars include?

Answer 2

Include some disaccharides such as sucrose and all polysaccharides.

Question 3

What is the test for reducing and non-reducing sugars?

Answer 3

Benedict’s test

Question 4

What are the steps to find out whether a sample contains a reducing sugar?

Answer 4

1. Place 2 cm3 of your food sample into a test tube.
2. Add an equal volume of Benedict’s reagent.
3. Heat the mixture in a gently boiling water bath for 5 minutes.
4. If a reducing sugar is present, the mixture will change from a blue solution to a brick red precipitate.

Question 5

What is the conc. of a reducing sugar of it is blue?

Answer 5

This indicates no reducing sugar is present.

Question 6

What is the conc. of a reducing sugar of it is green?

Answer 6

This indicates a low concentration.

Question 7

What is the conc. of a reducing sugar of it is orange?

Answer 7

This indicates a medium concentration.

Question 8

What is the conc. of a reducing sugar of it is brick red?

Answer 8

This indicates a high concentration.

Question 9

What are more accurate methods to determine the conc of reducing sugars

Answer 9

Use a colorimeter to measure the absorbance of each solution.

Filter the solution and weigh the precipitate.

Question 10

Describe the test for starch

Answer 10

Iodine test.

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.

Question 11

How do you test for non-reducing sugars?

Answer 11

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 12

What are the steps to test for non-reducing sugars?

Answer 12

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.
6. If non-reducing sugars were present at the start, the mixture will now change from a blue solution to a brick red precipitate.

Question 13

What is the test for lipids?

Answer 13

the emulsion test

Question 14

How do you test for lipids?

Answer 14

1. Place your food sample in a test tube.
2. Add 2 cm3 of ethanol.
3. Shake.
4. Add 2 cm3 of distilled water.
5. If lipids are present, a milky white emulsion will appear.

Question 15

What is the test for proteins?

Answer 15

Biuret test

Question 16

How do you test for proteins?

Answer 16

1. Place your food sample in a test tube.
2. Add an equal volume of Biuret solution (sodium hydroxide and copper sulfate).
3. If proteins are present, the solution will turn from blue to purple. If no protein is present, the solution remains blue.