2.1 Testing for Biological Molecules

Question 1

What is Benedict’s solution testing for? (2)

Answer 1

reducing sugars
–> glucose, fructose, maltose

non reducing sugars
–> sucrose

Question 2

Outline Benedict’s test for reducing sugars. (2)
Starting color & positive result.

Answer 2

1. add 2cm^2 Benedict’s solution to an equal amount of the test sample
2. heat over a water bath

Starting color: blue
Positive result: green → yellow → orange → brick red

Question 3

Name a non-reducing sugar. (1)
Outline Benedict’s test for non-reducing sugars. (3)
Starting color & positive result.

Answer 3

Sucrose

1. Boil the test sample with HCl (to break the glycosidic bonds)
2. Neutralise with sodium hydroxide (NaHCO₃) / alkali –> otherwise the HCl would react with Benedict’s solution
3. Add Benedict’s solution to the test sample, heat in a water bath

Starting color: blue
Positive result: orange/red

Question 4

How can the Benedict’s test be made for quantitative? (2)

Answer 4

Measure the time from solution in the water bath til the first color change is produced

or

Create standard solutions of known concentrations, and compare color change change to estimate concentration

Question 5

What is biuret testing for?

Answer 5

proteins

Question 6

Outline the Biuret test for proteins. Starting color & positive result.

Answer 6

add a few drops of Biuret reagent to the test sample

Starting color: blue
Positive result: lilac/purple

Question 7

What is iodine solution testing for?

Answer 7

starch

Question 8

Outline the iodine test for starch. Starting color & positive result.

Answer 8

Add a few drops of iodine solution to the test sample

Starting color: yellow/brown
Positive result: blue-black

Question 9

What is the emulsion test with ethanol testing for?

Answer 9

lipids – fats and oils

Question 10

Outline the emulsion test test for lipids (fats and oils). (2) Starting color & positive result.

Answer 10

1. shake some of the test samples with ethanol
2. add the same amount of water

Starting color: clear
Positive result: formation of a cloudy white emulsion