In the copper test, what is the color of the initial precipitate?

• A. White solid.
• B. Blue solid.
• C. Light blue precipitate.
• D. Green precipitate.

Answer: (C)

The test is looking at what happens when copper ions are treated with a base. Copper(II) ions react with hydroxide ions to form copper(II) hydroxide, Cu(OH)2, which appears as a gelatinous blue precipitate. It’s described as light blue because it’s a pale blue solid. If you add excess ammonia later, this precipitate dissolves to give a deep blue solution of a copper-ammonia complex. So the initial, visible solid is the light blue precipitate, which is a hallmark of Cu(OH)2 formation in this test.