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.
Text Solution
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The hydrolysis of sucrose by boiling with a mineral acid (HCl), produces a mixture of equal molecules of 
-glucose and D-fructose. Sucrose solution is dextrorotatory, specific rotation 
. But on hydrolysis, it becomes laevorotatory. The specific rotation of 
-glucose is 
and of 
-fructose is 
. Therefore, the net specific rotation of an equimolar mixture of both is
∴ It becomes laevorotatory.
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