Assertion (A) : Cu2+ in water is more stable than Cu+.Reason (R) : Enthalpy of hydration for Cu2+ is much less than that of Cu+.

The assertion and the reason are both correct and they are related. Copper (II) ions (Cu2+) are indeed more stable in water than Copper (I) ions (Cu+). This is because the enthalpy of hydration of Cu2+ is less than that of Cu+.

Enthalpy of hydration is the amount of energy released when one mole of ions undergoes hydration, which is the process of ion dissolving in water. A lower enthalpy of hydration means less energy is required for the process, making it more favorable.

In the case of copper ions, Cu2+ ions have a higher charge density than Cu+ ions. This means they can attract water molecules more strongly and thus get hydrated more easily. As a result, the hydration of Cu2+ ions releases more energy (i.e., has a lower enthalpy of hydration) than the hydration of Cu+ ions, making Cu2+ more stable in water.