In the copper test, the best four-mark observation is:

• A. A white precipitate that is insoluble.
• B. A blue precipitate that remains unchanged on excess.
• C. Light blue precipitate which is soluble in excess to form dark blue solution.
• D. Green precipitate that dissolves in water.

Answer: (C)

The test for copper(II) ions relies on how copper behaves with hydroxide and ammonia: a pale blue Cu(OH)2 precipitate forms, and in excess ammonia it dissolves to give a deep blue solution containing the [Cu(NH3)4(H2O)2]2+ complex. The described observation—light blue precipitate that dissolves in excess to form a dark blue solution—fits perfectly: it shows both the initial precipitate color and its dissolution with excess reagent, which is a hallmark of copper in this test.

This combination is what earns the marks, because it demonstrates the characteristic change from a pale blue solid to a deep blue solution when excess ammonia is added, confirming copper’s presence. The other descriptions don’t match copper’s behavior: a white precipitate points to a different ion, a blue precipitate that stays unchanged suggests no complex formation with the excess reagent, and a green precipitate dissolving in water doesn’t align with the copper hydroxide–ammonia pattern.