AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, e.g., CuSO₄·5H₂O has 5 water molecules.
The definition is directly from the "What you have learnt" summary of Chapter 2. Examiners expect the exact phrase "fixed number of water molecules" and "one formula unit of a salt". Adding an example (like blue vitriol) strengthens a 1-mark answer. Do not write more than one line.