Q1. [2] medium thorough-understanding
Blue copper sulphate crystals turn white when heated strongly, but regain their blue colour when a few drops of water are added to the white powder. What does this experiment reveal about water of crystallisation? Write the chemical equations for both the forward and reverse changes observed.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
Water of crystallisation is the fixed number of water molecules chemically bonded within a crystal. This experiment shows that water of crystallisation is essential for the blue colour of copper sulphate crystals, and its removal/addition causes a reversible change.
Forward change (heating):
$$\text{CuSO}_4 \cdot 5\text{H}_2\text{O}(s) \xrightarrow{\Delta} \text{CuSO}_4(s) + 5\text{H}_2\text{O}(g)$$
$$\text{(Blue)} \hspace{4.5cm} \text{(White)}$$
Reverse change (adding water):
$$\text{CuSO}_4(s) + 5\text{H}_2\text{O}(l) \rightarrow \text{CuSO}_4 \cdot 5\text{H}_2\text{O}(s)$$
$$\text{(White)} \hspace{3.5cm} \text{(Blue)}$$
---
Explanation
- The key concept tested is water of crystallisation — examiners expect you to define it and link it to the colour change.
- Write both balanced equations; each is worth a mark in a 2-mark question.
- Note the state symbols and the $\cdot 5\text{H}_2\text{O}$ notation — these show the examiner you understand the chemical formula of hydrated copper sulphate (blue vitriol).
- The reaction is reversible, which is also a good point to mention briefly.