Which statement about water of crystallisation is true?

• A. It is water trapped inside a crystal and part of the chemical formula
• B. It is water only within solutions
• C. It is water that forms the salt's charge
• D. It is water that is produced when the salt forms

Answer: (A)

Water of crystallisation means water molecules that are physically incorporated into the solid crystal of a hydrated salt. These water molecules are part of the compound’s formula, not just present in solution. For example, copper sulfate forms the hydrated salt CuSO4·5H2O, where five water molecules are included in the crystal lattice and shown in the formula. These waters can be driven off by heating, leaving the anhydrous salt, which shows that the water is integral to the solid’s structure, not produced during salt formation or simply present as solvent. The water molecules do not affect the salt’s charge; they are neutral guests embedded in the crystal.