The presence of Carbon and Hydrogen is known when an organic compound is heated with cupric oxide. They gets oxidised to carbon dioxide and water.
When the carbon dioxide formed in the reaction when passed through limewater, a milky white precipitate is formed due to the formation of calcium carbonate, indicates the presence of Carbon dioxide.
Anhydrous copper sulphate absorbs moisture and turns blue due to the formation of hydrated copper sulphate, indicates the presence of hydrogen. The presence of Nitrogen, Sulphur and halogens can be detected by Lassaigne’s test. In Lassaigne’s test, the organic compound is heated with a small amount of sodium metal in an ignition tube.
For the detection of Nitrogen, a small amount of sodium fusion extract is boiled with ferrous sulphate and then acidified with concentrated sulphuric acid.
Sulphur can be detected in an organic compound by the lead acetate test (or) by the sodium nitroprusside test.
To detect the presence of phosphorus, sodium extract is heated with sodium peroxide and then extracted with water. The resultant solution is boiled with nitric acid and then treated with ammonium molybdate. Fomation of yellow precipitate (ammonium phosphomolybdate) indicates the presence of phosphorus.