The correct option is Blue colour reappears.
Explanation
Certain salts contain water molecules within their crystal structure, known as water of crystallization. The presence or absence of these water molecules often determines the physical appearance and color of the salt.
Detailed Analysis:
- Anhydrous Copper Sulphate: In its dry form, copper sulphate ($CuSO_4$) lacks water of crystallization. It appears as a white powder.
- Hydration Process: When water is added to anhydrous copper sulphate, the water molecules chemically bond with the salt to form copper(II) sulphate pentahydrate ($CuSO_4 \cdot 5H_2O$).
- Color Change: The formation of the pentahydrate structure restores the characteristic blue color of the salt.
- Reaction: The chemical equation for this change is:
$CuSO_4 \text{ (White)} + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O \text{ (Blue)}$
Key Takeaway:
Anhydrous copper sulphate is white, while hydrated copper sulphate is blue. This distinct color change allows anhydrous copper sulphate to be used as a test for the presence of moisture.