Required Practical 6 – Tests for Functional Groups

Question 1

How do you test for an alkene? Give the result.

Answer 1

Add bromine water (0.5 cm³) to a few drops of the unknown and shake. Alkenes decolourise the orange bromine water.

Question 2

What is Tollens’ reagent and how is it made?

Answer 2

Silver nitrate solution + a drop of NaOH (gives Ag2O precipitate), then add aqueous ammonia dropwise until the precipitate just dissolves. Active species is [Ag(NH3)2]⁺.

Question 3

What is the result of Tollens’ test with an aldehyde vs a ketone?

Answer 3

Aldehyde: silver mirror forms on the inside of the test tube. Ketone: no change.

Question 4

What is Fehling’s solution and what is its result with an aldehyde?

Answer 4

Contains blue Cu²⁺ ions (Fehling’s A + Fehling’s B). Aldehyde: blue solution forms a red precipitate of Cu2O on gentle heating. Ketone: no change.

Question 5

How do you test for a carboxylic acid? Give the result.

Answer 5

Add a small amount of sodium carbonate solid. Carboxylic acids effervesce (produce CO2). Bubble gas through limewater — turns cloudy if CO2 is present.

Question 6

How do you test for a primary or secondary alcohol using dichromate?

Answer 6

Add acidified sodium dichromate(VI) and warm. Primary and secondary alcohols (and aldehydes) give orange to green colour change. Ketones and tertiary alcohols: no colour change.

Question 7

What reagent distinguishes an acyl chloride from a chloroalkane?

Answer 7

Add silver nitrate solution at room temperature. Acyl chloride → rapid white precipitate of AgCl with vigorous reaction and steamy HCl fumes. Chloroalkane → slow formation of white AgCl precipitate only on warming.