Lactose is a dissacharide composed of two monosacharides(glucose and galactose). Glucose and galactose are isomers, which means they have the same molecular formula(C6H12O6) but different structural formulas.
During hydrolysis the bond between the two monosacharides is broken and a water molecule is added.
lactose(C12H22O11)+H2O-->glucose(C6H12O6)+galactose(C6H12O6)
the enzyme that catalyzes this reaction is called lactase and is secreted by the small intestine