A Level Chemistry OCR Practice Exam

How does water of crystallisation typically affect the properties of a compound?

1. It increases the solubility of the compound.

2. It alters the color of the crystalline substance.

3. It influences the melting and boiling points of the solid.

4. It decreases the reactivity of the compound.

The correct answer is: It influences the melting and boiling points of the solid.

Water of crystallisation is an integral part of certain crystalline compounds and directly influences their physical properties. One of the most significant effects it has is on the melting and boiling points of the solid. When water molecules are embedded within the crystal lattice of a compound, they contribute to the stabilization of the structure, which generally results in a higher melting point compared to the anhydrous form of the compound. This is because additional energy in the form of heat is required to break the interactions between the water molecules and the rest of the crystal during the phase change. Additionally, the presence of water molecules can also result in variations in the lattice energy of the crystalline solid. This effect can lead to better-defined melting points due to the organized nature of the hydrated lattice. The amount and arrangement of water in the structure can also influence its thermal properties significantly. While one might consider the influence of water of crystallisation on solubility, color, or reactivity, these are secondary effects or could vary greatly depending on the specific substance in question and cannot be generalized as primarily influenced by water of crystallisation as melting and boiling points can be.