Both sucrose and lactose possess the same molecular formula, but sucrose is a non-reducing sugar and lactose is a reducing sugar. Why?

Explanation

In sucrose, hemiacetal hydroxyl groups of glucose and fructose are involved in the glycosidic linkage. Hence sucrose is a non-reducing sugar.
In lactose, hemiacetal hydroxy groups of galactose involved in the glycosidic linkage but of glucose not involved in glycosidic linkage.
Hence lactose is a reducing sugar.