What is referred to as 'water of crystallisation'?

Water of crystallisation refers specifically to the water molecules that are integrated into the crystal structure of certain salts. When these salts form crystals, they often incorporate water molecules into their lattice structure in a specific ratio. This water is essential for maintaining the integrity and stability of the crystalline form.

For example, in hydrated copper(II) sulfate (CuSO₄·5H₂O), the "5H₂O" indicates that five water molecules are present in the structure of each formula unit of copper(II) sulfate. This embedded water is part of the solid crystalline material and can influence the properties of the salt, such as its color and solubility.

In contrast, the other choices do not accurately describe water of crystallisation. Water absorbed from the atmosphere refers to moisture that can affect materials but is not part of a crystalline structure. Water used to dissolve salts describes the role of water in forming solutions, which is different from being an integral part of the structure. Lastly, water lost during heating typically refers to the process of dehydration, where water is expelled from solid materials, particularly heated hydrated salts, which becomes relevant when considering how hydrated salts change upon heating but is not what defines water of crystallisation.