A Level Chemistry OCR Practice Exam

In which type of compound is water of crystallisation most commonly found?

1. Gases

2. Liquids

3. Solids

4. Aqueous solutions

The correct answer is: Solids

Water of crystallisation is most commonly associated with solid compounds, particularly in the context of hydrated salts. Many ionic compounds incorporate water molecules into their solid structures, which can significantly influence their properties, such as solubility and thermal stability. These water molecules are localized in the crystal lattice of the solid and are integral to maintaining the structure of the compound, often giving hydrated salts their characteristic appearance and properties. In contrast, gases and liquids do not typically form fixed crystal structures that would accommodate water of crystallisation. Aqueous solutions, while they contain water, do not have the solid crystal framework necessary to classify water as crystallisation in this manner. Thus, the presence of water of crystallisation is intrinsically linked to the solid state of compounds.