A Level Chemistry OCR Practice Exam

What term describes water molecules that are integral to the crystal structure of a compound?

1. Hydration shell

2. Water of crystallisation

3. Solvent molecules

4. Interstitial water

The correct answer is: Water of crystallisation

The term that describes water molecules that are integral to the crystal structure of a compound is indeed "water of crystallisation." This refers specifically to the water molecules that are included within the crystalline lattice of a solid compound, forming part of its structure and often impacting its physical properties such as color, stability, and reactivity. Typically, these water molecules are not just present as a result of solvation or merely associated with the surface of the crystals but are structurally and chemically bound within the crystal matrix. This is in contrast to a hydration shell, which involves water molecules surrounding a solvated ion or molecule in solution, or solvent molecules that aid in facilitating a reaction or dissolution process without being part of the solid structure. Interstitial water is a less commonly used term that can refer to water present in the spaces between particles, but it does not specifically define water that is bound within a crystal lattice. Therefore, "water of crystallisation" most accurately describes the water that is structurally significant within the crystalline formation itself.