Enthalpy change of hydration: the definition that scores the mark
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
The enthalpy change of hydration is the enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions, in an infinitely dilute solution. For example, Mg2+(g) becomes Mg2+(aq).
Words that must appear
- One mole of gaseous ions
- Dissolves in water
- To form aqueous ions
- Infinitely dilute
The sign
Always exothermic, so the value is negative. Ion-dipole attractions form between the ions and the polar water molecules, and forming attractions releases energy.
Do not confuse with
Enthalpy of solution, which starts from the solid and so includes breaking the lattice.
Common questions
What is the enthalpy change of hydration?
The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions, in an infinitely dilute solution.
Is the enthalpy change of hydration endothermic or exothermic?
Always exothermic.
What is the difference between enthalpy of hydration and enthalpy of solution?
Hydration starts with gaseous ions. Solution starts with the solid, so it also includes the energy to break the lattice.
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