Define seliwanoff's Test or Resorcinol Hydrochloric Acid Reaction?

seliwanoff's test is specific test for ketonic groups and therefore is positive for ketose sugars like fructose and sucrose.

Principle

Ketoses on treatment with hydrochloric acid form 5 hydroxy methyl furfural, which on condensation with resorcinol give a wine or cherry red colour.  Sucrose gets converted to glucose and fructose in the presence of HCl and also gives this test.