Biochemical tests for molecules

Question 1

What is a qualitative test?

Answer 1

it only determines whether the substance is present or not.

Question 2

What is a quantitative test?

Answer 2

tells you the amount of a substance that is present in a molecule

Question 3

What test would you do if you needed to find out whether a solution contained proteins?

Answer 3

a biuret test
step 1: add a few drops of sodium hydroxide solution to make it alkaline

step 2: add some copper sulfate solution

if protein is present solution turns purple
if protein is absent then solution stays blue

Question 4

What test would you do to test for starch?

Answer 4

the iodine test
step 1: add iodine dissolved in potassium iodide

if starch is present, solution turn blue/black
if starch is absent, solution stays brown/orange

Question 5

What test would you do to test for the presence of lipids?

Answer 5

the emulsion test
step 1: shake test substance with ethanol for about a minute

step 2: add solution to water

if lipids are present, solution turns milky
if lipids are absent, solution stays clear

Question 6

How would you test for reducing sugars?

Answer 6

the benedict’s test
step 1: add benedict’s reagent to the sample

step 2: then heat the sample up in a water bath

if reducing sugars are present, it will form a coloured precipitate that goes from green-yellow-orange-brick red

if reducing sugars are not present then sample stays blue

Question 7

how would you test for non-reducing sugars?

Answer 7

step 1: add dilute HCL to the test solution to break them down into monosaccharides

step 2: heat in a water bath

step 3: neutralise it by adding sodium hydrogencarbonate

step 4: carry out the benedict’s test

if non-reducing sugars are present, then a coloured precipitate will form that goes from green-yellow-orange-brick red.
if non-reducing sugar is absent then solution stays blue.