Practical Assessment

Question 1

Test for reducing sugars

Answer 1

Benedict’s Test

Question 2

Procedure for Benedict’s Test

Answer 2

1) Place 2cm3 of test solution into a test tube.
2) Add an equal volume of Benedict’s reagent.
3) Stir to mix the mixture.
4) Heat the mixture by placing it in a boiling water bath for 2 minutes.
5) Observe the contents of the test tube.

Question 3

Basis of Benedict’s Test

Answer 3

• based on ability of reducing sugars to reduce Cu2+ to Cu+
• due to presence of carbonyl groups that are rich in electrons
• blue alkaline solution of CuSO4 receives an electron and is reduced to red, insoluble Cu2O

Question 4

Observations for Benedict’s Test

Answer 4

Red precipitate  - large 
Orange precipitate - moderately large
Yellow precipitate  - moderate
Green precipitate - small
Remains blue - no

Question 5

Test (procedure) for non-reducing sugars

Answer 5

If a -ve result is obtained in the Benedict’s test:

1) Boil fresh test sample with an equal amount of dilute hydrochloric acid for about 1 minute to hydrolyse disaccharides into monosaccharides.
2) Cool contents of the test tube.
3) Neutralise contents with sodium hydrogen carbonate as Benedict’s solution only works in alkaline conditions.
4) Carry out Benedict’s test on solution.
5) Record observations.

Question 6

Basis of non-reducing sugars test

Answer 6

• no precipitate observed when heated directly with Benedict’s reagents as there are no electron-rich carbonyl groups available to reduce other compounds
• non-reducing sugar has to hydrolysed into its reducing sugar components

Question 7

Observations for non-reducing sugars test

Answer 7

Red precipitate  - large 
Orange precipitate - moderately large
Yellow precipitate  - moderate
Green precipitate - small
Remains blue - no

Question 8

Test for starch

Answer 8

Iodine test

Question 9

Procedure for Iodine Test

Answer 9

1) Add a few drops of potassium iodide solution to 1cm3 of test sample or piece of test specimen
2) Observe any colour change

Question 10

Basis of Iodine Test

Answer 10

• as ioidine is not very soluble in water, iodine reagent is made by dissolving iodine in water in the presence of potassium iodide; forms a soluble, linear triiodide ion complex
• blue-black colouration observed when starch is present as triiodide ion complex fits into centre of each turn of the amylose helix, forming a starch-ion complex
• boiling causes temporary unwinding of the amylose helix, releasing the iodine molecules; colouration reforms on cooling

Question 11

Observations for Iodine Test

Answer 11

Mixture turns from yellow-brown to blue-black - starch is present
Mixture remains yellow-brown - starch is absent

Question 12

Test for proteins

Answer 12

Biuret test

Question 13

Procedure for Biuret Test

Answer 13

1) Place 2cm3 of test solution in a test tube.

2) Add an equal amount of 5% NaOH solution.
3) Stir to mix mixture well.
4) Add 1% copper sulphate solution, drop by drop, shaking well after each drop.
OR
- add an equal volume of Biuret reagent and shake to mix well
5) Observe any colour change.

Question 14

Basis of Biuret Test

Answer 14

• In presence of dilute copper sulphate in alkaline solution, nitrogen atoms in peptide chain form a purple complex with copper (II) ions, Cu2+
• Biuret reagent is derived from urea which also contains the -CONH- group, also giving a postive result

Question 15

Observations for Biuret Test

Answer 15

Mixture turns from blue to purple - proteins are present

Mixture remains blue - proteins are absent

Question 16

Test for lipids

Answer 16

Ethanol emulsion test

Question 17

Procedure for Ethanol Emulsion Test

Answer 17

1) Place 2cm3 of absolute ethanol in test tube.
2) Add 2cm3 of test solution.
3) Dissolve contents by shaking vigorously.
4) Add 4cm3 of cold water.
5) Observe any colour change.

Question 18

Basis of Ethanol Emulsion Test

Answer 18

• lipids dissolve in organic solvents like ethanol but not water
• thus, emulsion is seen when lipids are shaken vigorously in water

Question 19

Observations for Ethanol Emulsion Test

Answer 19

Mixture is a homogenous/clear solution with ethanol, and remains clear when water is added - lipids are absent
Mixture is a homogenous/ clear solution with ethanol, and emulsion is formed when water is added - lipids are present