Copper sulphate crystals are blue, but when strongly heated they turn white. What explains this colour change, and what happens to the colour when a few drops of water are added to the white powder?
Generated by claude-sonnet-4-6 · 2026-06-26 01:02 · grounding rag
Model Answer
Copper sulphate crystals contain water of crystallisation (CuSO₄·5H₂O), which gives them their blue colour. On strong heating, this water is lost (thermal decomposition), forming anhydrous copper sulphate (CuSO₄), which is white.
When a few drops of water are added to the white powder, it reabsorbs water and turns blue again, as the hydrated form is restored.
Source: Chapter 1, Section 1.2.2 Decomposition Reaction
Explanation
- The key concept is water of crystallisation: its presence = blue; its absence = white.
- Examiners expect you to use the term anhydrous for the white powder and mention that heating causes loss of water of crystallisation (a thermal decomposition).
- The reversibility (white → blue on adding water) shows this is not a permanent chemical change — a point worth stating clearly.
- This is a classic 2-mark question: one mark for explaining the blue-to-white change, one mark for the white-to-blue reversal on adding water.