Which statement best describes the copper test result that yields four marks?

• A. A clear blue solution forms immediately.
• B. A light blue precipitate forms, which dissolves in excess to give a deep blue solution.
• C. A green precipitate forms that does not dissolve.
• D. No reaction occurs.

Answer: (B)

Copper(II) ions with ammonia show a distinctive two-step change: first a pale blue precipitate of copper(II) hydroxide forms, and then, in excess ammonia, that precipitate dissolves to give a deep blue solution due to the formation of the [Cu(NH3)4]2+ complex. The four marks come from recognizing both parts of this test and understanding why the color changes as more ammonia is added. The description that best fits is a light blue precipitate forming, which dissolves in excess to give a deep blue solution. This matches the observed sequence and the reason behind the color change. An immediate clear blue solution would skip the precipitate stage, a green precipitate isn’t characteristic of copper with ammonia, and no reaction would ignore the well-known copper–ammonia behavior.