When water was added to a white anhydrous substance X, the colour changed to blue. X is

Step 1: Analyze the question A white anhydrous (water-free) solid turns blue when water is added. This is a classic chemical test for water.

Step 2: Explain the concept Many salts form hydrates, meaning they take water into their crystal structure (water of crystallisation). Their colour can change when they hydrate:

• Anhydrous copper(II) sulphate, CuSO₄, is white.
• Hydrated copper(II) sulphate, CuSO₄·5H₂O, is blue.

The reaction is:

CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue)

Step 3: Check the options

• A. CuSO₄: white when anhydrous, turns blue with water. ✔
• B. FeSO₄: the hydrated form, FeSO₄·7H₂O, is green, and the anhydrous form is not a typical white-to-blue test. ✘
• C. Na₂SO₄: white both when anhydrous and when hydrated, so there is no colour change. ✘
• D. PbSO₄: white and insoluble in water, so there is no colour change. ✘

Step 4: Answer X is CuSO₄, which is option A.

Exam tip: Remember the two water tests:

• White anhydrous CuSO₄ turns blue.
• Blue anhydrous cobalt(II) chloride paper turns pink.

"Copper sulphate: white to blue means water is true!"