Water of crystallisation is...

• A. The water trapped inside a crystal
• B. Water released when heated
• C. Water that dissolves in water
• D. Water produced during crystallisation

Answer: (A)

Water of crystallisation refers to water molecules that are chemically bound within the crystal lattice of a hydrated salt. These water molecules are part of the solid structure, not free in solution, and can be driven off as steam when the compound is heated, leaving the anhydrous salt. That makes the description “water trapped inside a crystal” the best fit. The other statements describe processes or ideas that don’t define what water of crystallisation is: simply water released on heating is dehydration, water dissolving in water is about solubility, and water produced during crystallisation isn’t what hydrates the crystal in the first place.