Question

what is water of crystallisation

Answer 1

water that occurs as a constituent of crystalline substances in a definite  ratio: it can be removed from them by the application of heat at 100°C (212°F) and its loss usually results in a change in the crystalline structure

Answer 2

In chemistry, water of crystallization or water of hydration or crystallization water is waterthat occurs inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions.[1] In some contexts, water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.
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