Science — AI-generated practice question

Copper sulphate crystals (CuSO₄·5H₂O) are blue because they contain water of crystallisation. On heating, this water is lost and the crystals become anhydrous copper sulphate (CuSO₄), which is white. This is a decomposition reaction.

$$\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\text{Heat}} \text{CuSO}_4 + 5\text{H}_2\text{O}$$

When water is added to the white anhydrous copper sulphate powder, it reabsorbs the water and turns blue again, as the hydrated form is restored. This reaction is also exothermic.

Source: Chapter 1, Chemical Reactions and Equations

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• The key concept here is water of crystallisation — water molecules that are part of the crystal structure and give CuSO₄ its blue colour.
• Examiners expect you to name both forms: hydrated (blue) and anhydrous (white).
• Mentioning that the blue colour is restored on adding water scores the third mark.
• The equation is a bonus but strengthens the answer; write it if you remember it.