3.4 Chemical Tests

Question 1

What is the test for starch?

Answer 1

Iodine Test:
- few drops of iodine dissolved in Potassium Iodide solution are mixed with a sample
- yellow/brown to purple/black = positive result
- stays yellow/brown = negative result

Question 2

What is the test for lipids?

Answer 2

Emulsion Test:
- sample is mixed with ethanol, then water and shaken
- if the emulsion forms a white layer on top of the solution = positive result
- if stays clear = negative result

Question 3

What is the test for proteins?

Answer 3

Biuret Test:
- the basis of this test is from peptide bonds forming violet coloured complexes with copper ions in alkaline solution
- mixture of an alkali and copper sulphate solution is called a biuret reagent
- purple = positive result
- blue = negative result

Question 4

What is the test for reducing sugars, eg. maltose, fructose, galactose and glucose?

Answer 4

Benedicts Test:
1. Add 2cm3 of the sample to be tested into a test tube (if not liquid, grind in water).
2. Add 2cm3 of Benedicts Reagent.
3. Heat mixture in gently boiling water bath for 5 minutes.
- Starting colour is blue = negative result
- Final colour can be green (v. low), Yellow (low), Brown (medium), Brick-red (high) = positive result (quantitative/ semi-quantitative).

Question 5

What is the test for non-reducing sugars, eg. sucrose?

Answer 5

Benedicts Test:
1. Add 2cm3 of the sample to be tested in a test tube (if not liquid, grind in water).
2. Then add 2cm3 of dilute Hydrochloric acid, heat for a few minutes to hydrolyse the Glycosidic Bond.
3. Add Sodium Hydrogen Carbonate powder to neutralise the solution - fizzing.
4. Add 2cm3 of Benedicts Reagent.
5. Heat mixture in gently boiling water bath for five minutes.
- Starting colour is blue = negative result
- Final colour can be green (v. low), Yellow (low), Brown (medium), Brick-red (high) = positive result (quantitative/ semi-quantitative).