Topic 1: disaccharides and biochemical tests

Question 1

Describe how disaccharides are formed:

Answer 1

By a condensation reaction between two monosaccharides.

Question 2

Write the word equation for the formation of each disaccharide:

Answer 2

1 Glucose + glucose –> maltose + water

2 Glucose + fructose –> sucrose + water

3 Glucose + galactose –> lactose + water

Question 3

Describe the test for a reducing sugar (3 marks):

Answer 3

1.Add Benedict’s reagent

2. Heat
3. A positive result is a colour change from blue to green/yellow/orange/red

Question 4

Suggest why the colour change occurs at the top of the test tube first:

Answer 4

Heat rises (convection currents).
The reaction occurs at the top of the test tube first.

Question 5

Describe the test for a non-reducing sugar (4 marks):

Answer 5

1. Following a negative reducing sugars test
2. Boil the sample in acid (acid hydrolysis). Then add alkaline solution to neutralise.
3. Add Benedict’s reagent and heat
4. A positive result will go from blue to orange/red

Question 6

Describe the test for starch (2 marks)

Answer 6

1.Add iodine to the sample

2.A positive result will go from
orange to blue/black

Question 7

Describe the test for lipids (3 marks)

Answer 7

1. Add ethanol and shake to dissolve
2. Then add distilled water
3. A positive result is a white emulsion

Question 8

Describe the test for proteins (2 marks)

Answer 8

1.Add biuret reagent to the sample

2.A positive result will go from blue to purple