Conc. Sulphuric acid dehydrates sugars to furfuryl derivates. Furfural is derived from the dehydration of pentoses and pentosans, while hydroxymethyl furfural is produced from hexoses and hexosans. α-Naphthol reacts with the cyclic aldehydes(furfural) to form purple-coloured condensation products (furfuryl-diphenyl-methane-dyes). Although this test will be positive for compounds other than carbohydrates (i.e. glycoproteins), a negative result indicates the absence of carbohydrates. A red-violet layer at the interface between the acid (bottom) and aqueous (upper) layers is a positive test for carbohydrates. This test is called Molisch’s test and it can be considered as a test for the presence of pentoses and hexoses.