Biological Molecules Tests

Question 1

Describe the iodine test for starch

Answer 1

1- add a few drops of iodine in potassium solution
2- if present, iodine ions interact with centre of starch molecules, producing a complex with blue/black colours

Question 2

Why is the iodine in potassium solution for starch test?

Answer 2

Because iodine is insoluble in water

Question 3

What is the colour change in a positive test for starch?

Answer 3

Orange/brown –> blue/black

Question 4

Describe the emulsion test for lipids

Answer 4

When shaken with ethanol, lipids will create a milky emulsion

Question 5

How do you know how much lipid is present from an emulsion test?

Answer 5

More obvious milky colour = more lipid

Question 6

Why is ethanol used in an emulsion test for lipids?

Answer 6

Lipids are non-polar and dissolve in organic solvents (e.g. ethanol)

Question 7

Describe the biuret test for proteins

Answer 7

1- treat liquid solution of sample with either NA or KOH to make it alkaline
2- add a few drops of copper (II) sulphate

Question 8

What will the colour change be in a positive biruet test for proteins?

Answer 8

Blue –> lilac/purple

Question 9

When won’t a biruet test work?

Answer 9

If the protein has 2 or less peptide bonds present, so the test will show negative for amino acids and dipeptides

Question 10

Describe the Benedict’s test for reducing sugars

Answer 10

1- add Benedict’s reagent to sample
2- heat sample in boiling water bath

Question 11

Describe a positive result for reducing sugars

Answer 11

A coloured precipitate will form
blue (no reducing sugar) –> brown/brick red (high conc. reducing sugars)