The standard enthalpy change of hydration, ΔHhydo, is the change in enthalpy when one mole of an ion (or molecule) in the gaseous state dissolves in water to form an infinitely dilute solution under standard conditions. This means that we need to dissolve the solute in excess water until there is no change in the energy absorbed or released by the system.
Some examples are:
The standard enthalpy of hydration is usually exothermic (negative) and becomes more negative for ions with with higher charge-to-radius ratios and for molecules with greater polarity.