When sugars react with strong mineral acids, they undergo dehydration and form furfurals. Furfurals condense with phenolic compounds such as α-napthol, thymol, and resorcinol to produce coloured complexes. This property of the sugars helps in the identification of sugars.

1. Molisch’s test: 
2. Seliwanoff’s test: 

Note: This test is used for distinguishing between glucose and fructose. If boiled for a long time, glucose will also give positive results.

(c) Bial’s test:

Note: This test is used for the detection of pentoses.