What's the difference between BaCl2 and BaCl2·2H2O?
Only two water molecules—but that matters.
Anhydrous barium chloride has a molecular weight of 208.23, while the dihydrate is 244.26. Confusing them during quantitative solution preparation produces a real concentration error.
Dissolve either in enough water, however, and both ultimately provide the same Ba2+ and Cl- ions.
The distinction becomes important again in dry or high-temperature chemistry, where a hydrate brings water into a process that may not tolerate it. Anhydrous BaCl2 can also participate in molten chloride mixtures, where ions become mobile without any water at all.
In chemistry, "water" is not always just water. Sometimes it is part of the material.
Global Chemical Database