In a copper test that carries four marks, which description best describes the copper product?

• A. A white precipitate that remains insoluble.
• B. A light blue precipitate which is soluble in excess to form a dark blue solution.
• C. A green precipitate that turns yellow on heating.
• D. A colorless solution.

Answer: (B)

When identifying copper ions with a qualitative test, you look for a distinct color change that shows how copper reacts with a reagent. Here, copper(II) ions react with ammonia to first form a light blue precipitate of copper(II) hydroxide. If you add more ammonia, that precipitate dissolves and a deep blue solution appears. This happens because the copper is binding with ammonia to form a soluble copper–ammine complex, so the solid turns into a blue solution. That progression—a pale blue solid forming, then turning into a dark blue solution with excess reagent—is characteristic of copper in this test, which is why this description matches best. The other descriptions don’t fit copper’s behavior in this test: they describe colors or states you wouldn’t see with copper under these conditions.