test for biological molecules

Question 1

Test for lipids

Answer 1

emulsion test

Question 2

emulsion test

Answer 2

Lipids dissolve in ethanol because ethanol has a non-polar ethyl group

The dissolved lipid is added
to pure water. The lipid is insoluble in water and a white cloudy emulsion
forms on the surface.

Question 3

Benedict’s test for non-reducing sugars

Answer 3

hydrolysed into simple sugars by adding dilute HCl
solution is first boiled and then the acid is neutralised by adding sodium hydrogencarbonate
Then Benedict’s solution is added
the sample is heated, a change in colour

Question 3

Benedict’s test for reducing sugars

Answer 3

2cm3 of the Benedict’s solution is added
to 2cm3 of the solution to test

heated in the presence of a
reducing sugar, the Cu2+ ions are reduced
to Cu+ ions in the form of copper(I) oxide,
which forms a brick-red precipitate

The colour of the precipitate indicates the relative concentration of reducing sugar.

Question 4

Iodine test for starch

Answer 4

few drops of iodine added to sample
present turn blue-black

Question 5

Biuret test for proteins

Answer 5

few drops of biuret reagent added to solution
test tube shaken after each drop
present turn violet
not remain blue