Biochemical tests

Question 1

What reagent is used to test for sugars?

Answer 1

Benedict’s reagent

Question 2

What colour is Benedict’s reagent?

Answer 2

Pale blue

Question 3

Describe the test for reducing sugars and the results.

Answer 3

• Add excess Benedict’s reagent to the sample
• Heat in a water bath above 80°C (boiling)
• If a reducing sugar is present the sample will turn from blue to green to yellow to orange to brick red, depending on how many reducing sugars there are.#

Question 4

What are reducing sugars?

Answer 4

All monosaccharides and some disaccharides including maltose and lactose

Question 5

Describe the test for non-reducing sugars and the results.

Answer 5

• Following a negative result of reducing sugars, the sample will remain blue
• Add dilute hydrochloric acid and heat in a water bath (to break the bonds and produce monosaccharides)
• Cool the solution then add an alkali to neutralise
• Add excess benedict’s reagent to the sample and heat in a water bath above 80°C
• If a non-reducing sugar is present the sample will turn from blue to green to yellow to orange to brick red, depending on how many non-reducing sugars there are

Question 6

Describe how we can obtain quantitative data from a positive result of reducing sugars

Answer 6

• filter the solution and weigh the precipitate
• remove the precipitate
• use a colorimeter to measure the absorbance of the remaining Benedict’s solution

Question 7

What are non-reducing sugars?

Answer 7

Disaccharides and some polysaccharides like sucrose

Question 8

What are the hazards and precautions when testing for sugars?

Answer 8

Benedict’s reagent is an irritant
- wear goggles
- wash hands on contact with skin

Hot water can cause burns
- use caution when poring / carrying hot water

Question 9

What reagent is used to test for starch?

Answer 9

Iodine

Question 10

What colour is iodine?

Answer 10

Yellow / brown

Question 11

Describe the test for starch and the results.

Answer 11

• Add iodine to the sample
• If starch is present the sample will turn from brown/yellow to blue/black

Question 12

What reagent is used to test for lipids?

Answer 12

Ethanol and water

Question 13

What colour is ethanol and water?

Answer 13

Clear

Question 14

Explain the emulsion test for lipids.

Answer 14

• Add ethanol to the sample
• Then add water and shake for a minute
• If a white milky emulsion appears, there is a lipid present

Question 15

What are the hazards and precautions when testing for lipids?

Answer 15

Ethanol is flammable
- do not conduct this test near a naked flame

Question 16

What reagent is used to test for amino acids?

Answer 16

Biuret’s reagent

Question 17

What colour is Biuret’s reagent?

Answer 17

Pale blue

Question 18

Describe the test for amino acids and the results.

Answer 18

• Add excess Biuret’s reagent to the sample
• Shake and leave at room temperature
• If amino acids are present the sample will turn from a pale blue to purple

Question 19

Describe how we can obtain quantitative data from a positive result of amino acids

Answer 19

Chromatography can be used to identify amino acids in a mixture, separating them by mass

Question 20

What are the hazards and precautions when testing for amino acids?

Answer 20

Biuret’s reagent is an irritant
- wear goggles
- wash hands on contact with skin

Question 21

Why do we add Benedict’s or Biuret’s reagent in excess?

Answer 21

To ensure all reducing sugars or amino acids have reacted fully to produce accurate results