A Level Chemistry OCR Practice Exam

Which type of bonding is primarily responsible for the inclusion of water of crystallisation in crystals?

1. Ionic bonding

2. Covalent bonding

3. Hydrogen bonding

4. Metallic bonding

The correct answer is: Hydrogen bonding

The presence of water of crystallisation in crystalline compounds is primarily attributed to hydrogen bonding. In many hydrated salts, water molecules are integrated into the crystal lattice, where they interact with the ions or molecules through hydrogen bonds. These bonds occur due to the polar nature of water, where the hydrogen atoms have a partial positive charge and the oxygen atom has a partial negative charge. This polarity allows water to form attractive interactions with other polar or charged species present in the crystal structure, stabilising the entire lattice. For instance, in hydrated salts like copper(II) sulfate pentahydrate (CuSO₄·5H₂O), the water molecules are not merely trapped but play a crucial role in defining the structure and stability of the crystal. The hydrogen bonds help to hold the water molecules in place within the crystal lattice, contributing to the overall integrity of the structure and influencing properties like solubility. Other types of bonding, such as ionic, covalent, and metallic, play essential roles in the formation of the core structure of a crystal, but they do not account for the inclusion of water of crystallisation. While ionic bonds are present in ionic compounds, and covalent or metallic bonds can be found in other types of materials, the specific interactions