2.1.2 - Testing

Question 1

Iodine test

Answer 1

Used to test for the presence of starch

Question 2

How to carry out an iodine test

Answer 2

1. The sample is placed in a test tube
2. Iodine solution is added to the test tube and gently shaken
3. If starch is present, the solution changes colour from yellow-brown to blue-black

Question 3

How the iodine test works

Answer 3

• Starch dissolves in KI solution
• A triiodide ion forms
• It intercalated into the middle of the amylose helix and causes a colour change

Question 4

Biuret Test

Answer 4

It can be used to test for proteins by detecting the presence of peptide bonds

Question 5

How to carry out a Biuret Test

Answer 5

1. The sample is placed in a test tube
2. Sodium hydroxide is added to the test tube
3. Few drops of Copper(II) sulphate is added and gently mixed
4. If protein is present the colour changes from light blue to purple

Question 6

Emulsion Test

Answer 6

It can be used to detect the presence of lipids

Question 7

How to carry out an Emulsion Test

Answer 7

1. Sample placed in test tube and mixed with ethanol
2. Water is added and mixed gently
3. Cloudy white emulsion indicates that lipids are present.

Question 8

How the emulsion test works

Answer 8

Lipids are not soluble in water, so they come out of solution and form an emulsion

Question 9

Reagent strips

Answer 9

The presence of reducing sugars can be tested

Question 10

How a reagent strip test is carried out

Answer 10

1. The strip is dipped in the sample

2. The colour that the test strip turns is compared to a calibration card to estimate the concentration

Question 11

Benedict’s Test

Answer 11

It can be used to detect the presence of reducing sugars

Question 12

Reducing sugars

Answer 12

Reducing sugars are sugars that can donate electrons to other molecules.
Reducing sugars include all monosaccharides and most disaccharides

Question 13

How to carry out the Benedict’s Test

Answer 13

1. Sample placed in test tube
2. Equal volume of Benedict’s reagant( Copper sulphate) is added
3. Solution heated in water bath
4. Colour change from blue to orange-red if reducing sugars are present

Question 14

Benedict’s Test colour change

Answer 14

Blue -> Green -> Yellow -> Orange -> Red

None present -> Low conc -> High conc

Question 15

Colorimeter

Answer 15

• Colorimeter gives a quantitative measure of reducing sugar concentration
• Low conc : High amount of unreacted CuSO4 : Low percentage transmission : High absorbance

Question 16

How the Benedict’s Test works

Answer 16

If reducing sugars are present they will reduce the Cu2+ ions in Cu+ forming an orange precipitate

Question 17

Testing for non-reducing sugars

Answer 17

1. Add Hydrochloric acid to sample
2. Heat in water bath (This results in hydrolysis of non-reducing disaccharide to reducing monosaccharide)
3. Sample cooled and neutralised with sodium hydrogencarbonate
4. Tested using usual method

Question 18

Biosensors

Answer 18

Use biological components to determine the presence and concentration of molecules

Question 19

Paper chromatography

Answer 19

Stationary phase - Paper
Mobile phase - Water
1. Draw a pencil line
2. Put sample in the line
3. Put the paper in water
4. Take the paper out before it reaches the end
5. Draw solvent front

Question 20

Thin layer chromatography

Answer 20

Stationary phase - Silica gel on sheet of glass or metal

Mobile phase - Organic solvent

Question 21

How they move

Answer 21

The rate at which different substances move depends on the interaction (hydrogen bonds ) they have with the stationary phase and their solubility in the mobile phase

Question 22

Rf

Answer 22

Rf = D traveled by component / D travelled by solvent