Which color change indicates the presence of water when using anhydrous copper(II) sulfate?

• A. From white to blue.
• B. From blue to pink.
• C. No color change.
• D. From white to green.

Answer: (A)

The test relies on hydration of copper(II) sulfate. Anhydrous copper(II) sulfate is white, but when it absorbs water, it forms copper sulfate pentahydrate, CuSO4·5H2O, which is blue. So seeing a white solid turn blue shows water is present and has hydrated the compound. If no water is present, the solid remains white. The other color changes aren’t produced by this hydration process, so they don’t indicate water presence.