Define Molisch's Test or alpha naphthol reaction?

The Molisch test is a common test for the existence of carbohydrates.

Principle

The reaction is due to the formation of furfural or furfural derivatives. Furfural and furfural derivatives are derived from the dehydration of sugars. The dehydration is caused by the action of concentrated sulphuric acid on sugars. Disaccharides, oligosaccharides and polysaccharides are hydrolyzed to yield their repeating monomers by the acid. The alpha-naphthol reacts along with the cyclic aldehydes to make purple coloured condensation products.