Aqueous Ammonia Tests

Why it matters

Add a few drops of ammonium hydroxide to clear metal salt solutions. Distinct coloured precipitates instantly cloud the liquids — and in excess NH₄OH the copper and zinc hydroxides dissolve again (copper giving a deep-blue solution), while lead hydroxide remains.

The idea in plain words

Confirm the equation: CuSO₄ + 2NH₄OH → Cu(OH)₂ + (NH₄)₂SO₄. The pale-blue solid carries the copper out of solution.

Predict first

Why do different metal ions form precipitates of different colours when NH₄OH is added?

The metal cation determines the identity and color of the insoluble hydroxide formed in this double decomposition reaction.

• The hydroxide ion (OH⁻) changes color depending on the metal it binds to.
• Each metal cation forms its own insoluble hydroxide with a characteristic colour, though some overlap — Zn²⁺ and Pb²⁺ both give white precipitates. — correct
• The ammonium ions react differently with each metal.
• The water in the solution evaporates, leaving colored salts behind.

What you do

Add NH₄OH to each tube and match the precipitate colour to the metal ion — pale-blue for copper, white for zinc and for lead.

Check yourself

Which observation correctly identifies the precipitate formed from Copper(II) Sulphate and Ammonium hydroxide (NH₄OH)?

Copper(II) ions form a characteristic pale-blue precipitate of copper(II) hydroxide.

Both ZnSO₄ and Pb(NO₃)₂ give white precipitates with NH₄OH. Which one dissolves when excess NH₄OH is added?

Zn(OH)₂ dissolves in excess ammonia while Pb(OH)₂ never does — the classic way to tell the two white precipitates apart.

Both Zinc Sulphate and Lead(II) Nitrate form white precipitates with NH₄OH. What is the formula of the precipitate from Lead(II) Nitrate?

Lead(II) nitrate reacts with NH₄OH to give a white precipitate of lead(II) hydroxide, Pb(OH)₂ — insoluble even in excess ammonia.

• Dirty-green precipitate
• Pale-blue precipitate of Cu(OH)₂ — correct
• White precipitate
• No reaction occurs
• Zn(OH)₂ dissolves; Pb(OH)₂ stays — correct
• Pb(OH)₂ dissolves; Zn(OH)₂ stays
• Both dissolve in excess reagent
• Zn(OH)₂
• Pb(OH)₂ — correct
• PbO
• NaPbO₂