Detection of Carbon and hydrogen: Put Copper Oxide Test

Carbon and hydrogen are detected by heating the compound with copper (II) oxide. Carbon present in the compound is oxidized to carbon dioxide (tested with lime-water, which develops turbidity) and hydrogen to water (tested with anhydrous copper sulphate, which turns blue).

Detection of Other elements

Lassaigne’s test:

Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by ‘Lassaigne’s test’. Covalent compounds are converted into ionic form by fusing the compound with sodium metal. Following reaction occurs:

Test for Nitrogen

• The substance is heated strongly with sodium metal.

• The water extract of the fused mass is boiled with ferrous sulphate solution

• To the cooled solution is then added a little ferric chloride solution and excess of conc. Hydrochloric acid.

The formation of Prussian blue or green colour confirms the presence of nitrogen.

Test for sulphur:

The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicated the presence of sulphur.

Test for Halogens:

The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish ppt. sparingly soluble in ammonium hydroxide shows the presence of bromine, a yellowish ppt. insoluble in ammonium hydroxide shows the presence of iodine.

Test of Phosphorous:

The compound is heated with an oxidising agent (sodium peroxide). The phosphorous present in the compound is oxidised to phosphate.
The solution is boiled with HN0₃ and treated with ammonium molybdate. A yellow colored ppt. indicates the presence of phosphorous.