A Level Chemistry OCR Practice Exam

In crystalline structures, what role does water of crystallisation play?

1. It serves as a color indicator.

2. It stabilizes the structure through hydrogen bonds.

3. It increases the reactivity of the compound.

4. It acts as a solvent for reactions.

The correct answer is: It stabilizes the structure through hydrogen bonds.

Water of crystallisation refers to water molecules that are incorporated into the crystalline structure of certain compounds. This water plays a crucial role in stabilizing the overall crystalline lattice. In many hydrated salts, the presence of water molecules contributes to the formation of hydrogen bonds between the water and the other ions or molecules within the crystal. These hydrogen bonds help to maintain the integrity of the structure and can impact the physical properties of the compound, such as its melting point, solubility, and stability. This stabilization occurs because the interaction between the water molecules and the surrounding ions or molecules creates a more cohesive and organized structure, which would be less stable without the water molecules. Consequently, the loss of water of crystallisation can lead to a structural collapse or alteration in the compound’s properties. In contrast, while the presence of water can affect the color of some substances, its primary role in hydrated crystal structures is not to act as an indicator. Water does not inherently increase the reactivity of the compound in a general sense; instead, it can alter reaction pathways or conditions when it is involved in a reaction. Additionally, while water can act as a solvent in many chemical reactions, this is not its role when it is a part of the crystalline structure of solid compounds.