Explain the Observation or Inference for seliwanoff's test?

1. Wine or cherry red colour seen. Wine red colour confirms the presence of a ketose sugar.

Note: On overheating aldoses also give the same reaction so continuous boiling should be avoided and test tube should be heated for 30 seconds to detect fructose.

2. seliwanoff's test is specific test for ketonic groups and therefore it is positive in case of sucrose. As studied in monosacccharides, seliwanoff's reagent contains HCl. In the presence of dil. HCl, sucrose  is hydrolyzed to glucose and fructose which then responds to the test.

3. Maltose and lactose on hydrolysis do not give fructose and so do not respond to this test. Starches and dextrins are made up mainly of glucose molecules. They do not contain a ketose and do not answer seliwanoff's test.