Chemistry · Ch 8 — Organic Chemistry – Some Basic Principles and Techniques
Detection of Carbon and Hydrogen
Detection of Carbon and Hydrogen
The first task in analysing an organic compound is to identify which elements it contains. Carbon and hydrogen are almost always present, and their detection relies on a simple combustion principle: the organic compound is heated strongly with copper(II) oxide (CuO), which acts as an oxidising agent.
When the mixture is heated, the carbon in the compound is oxidised to carbon dioxide (), and the hydrogen is oxidised to water (). The copper(II) oxide itself gets reduced to metallic copper. The reactions that take place are:
The carbon dioxide and water produced are then detected using specific chemical tests.
Detection of Carbon
The carbon dioxide gas evolved during heating is passed through lime water, which is a dilute solution of calcium hydroxide, . Carbon dioxide reacts with lime water to form insoluble calcium carbonate, which appears as a milky white precipitate or turbidity.
The appearance of a white precipitate (turbidity) in the lime water confirms the presence of carbon in the original organic compound.
If carbon dioxide is passed through lime water for too long, the initially formed white precipitate of calcium carbonate dissolves to form a clear solution of calcium hydrogen carbonate, . This does not mean carbon is absent — it simply means the test has been overdone. The initial appearance of turbidity is the positive test.
Detection of Hydrogen
The water vapour produced during heating is tested using anhydrous copper sulphate, , which is a white powder. Water vapour reacts with anhydrous copper sulphate to form the blue pentahydrate crystal.
The colour change from white to blue confirms the presence of hydrogen in the original organic compound. …