Assertion : A solution of sucrose in water is dextrorotatory. But on hydrolysis in the presence of a little hydrochloric acid, it becomes laevorotatory. Reason : Sucrose on hydrolysis gives unequal amounts of glucose and fructose. As a result of this, change in sign of rotation is observed.

Step-by-step solution

The hydrolysis of sucrose by boiling with a mineral acid ( HCl ) , produces a mixture of equal molecules of D -glucose and D -fructose. Sucrose solution is dextrorotatory, specific rotation =+66.5^ . But on hydrolysis, it becomes laevorotatory. The specific rotation of D -glucose is +52^ and of D -fructose is -92^ . Therefore, the net specific rotation of an equimolar mixture of both is : +52^ -92^ 2 =-20^ laevorotatory.