What would be observed when aqueous ammonia is added in drops and then in excess to a solution of copper(II) ions?

1. Analyze the question You are asked what happens when NH₄OH (aqueous ammonia) is added first in drops, then in excess, to Cu²⁺ solution. This is a qualitative analysis (cation test) question.

2. The concept Aqueous ammonia supplies OH⁻ ions, which precipitate many metal hydroxides. Some hydroxides then dissolve in excess ammonia because the metal ion forms a soluble complex ion with NH₃.

3. Step-by-step solution

Step 1: A few drops of ammonia Cu²⁺ reacts with OH⁻ to form copper(II) hydroxide:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

Observation: a pale blue precipitate.

Step 2: Excess ammonia The precipitate dissolves because the tetraamminecopper(II) complex forms:

Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2OH⁻(aq)

Observation: a deep blue (inky blue) solution.

Checking the other options:

• B: Brick red is not the colour of any copper(II) hydroxide precipitate. (Brick-red is the colour of calcium flame, or of Cu₂O in Fehling's test.)
• C: White precipitates are formed by Zn²⁺, Pb²⁺, or Al³⁺. With Cu²⁺ the precipitate is blue.
• D: A green precipitate is typical of Fe²⁺, and it does not dissolve in excess ammonia.

4. Answer The blue precipitate dissolves in excess ammonia to give a deep blue solution, so the answer is A.

5. Tip Remember: Cu²⁺ = blue ppt → deep blue solution in excess NH₃. The cations whose precipitates dissolve in excess ammonia are Cu²⁺, Zn²⁺, and Ag⁺ (in dilute solution). A quick way to recall them is "Come Zee Ammonia". Fe²⁺, Fe³⁺, Pb²⁺, and Al³⁺ do not dissolve in excess ammonia.