Chemical Tests

Question 1

Biuret test for proteins

Answer 1

(Detect the presence of peptide bonds)

1) sample in test tube
2) add sodium hydroxide
3) add a few drops of copper (II) sulfate

Positive result:
Blue👉Purple

Question 2

Lipids emulsion test

Answer 2

1) sample in test tube
2) add ethanol, mix until dissolved
3) add water, mix gently

Positive result:
Cloudy white substance formed on the top

(Lipids can’t dissolve in water (hydrophobic) so they form layer at the top)

Question 3

Benedict’s test - Reducing sugars

Answer 3

1) sample in test tube
2) add equal volume of benedict’s reagent (copper II sulfate)
3) solution is heated in boiling water bath for 3-5 mins

Positive result: (reducing sugars present)
Light blue to green to yellow to orange to red
(Indicates concentration of reducing sugars)

Question 4

Reducing sugars - Reagent test strip

Answer 4

1) sample put in test tube
2) reagent strip dipped in sample, blue end first
3) colour of test strip compared to calibration card to estimate concentration of reducing sugars

Question 5

Benedict’s test - non reducing sugars

Answer 5

1) sample in test tube
2) add hydrochloride acid
3) water bath (boiling) for 3-5 mins
4) add sodium hydrogen carbonate (neutralises acid)
Boiling water bath, 3-5 mins

Positive result:
Blue to green to yellow to orange to red

Question 6

Iodine test for starch

Answer 6

1) sample in test tube
2) add potassium iodine solution
3) gently shake

Positive result
Yellow/brown👉blue/black