Metal Hydroxides | Chemistry

📖 In-Depth Theory

Testing for Metal Ions with NaOH

Adding SODIUM HYDROXIDE SOLUTION (NaOH) to solutions of metal salts produces PRECIPITATES — insoluble hydroxide compounds.

The COLOUR of the precipitate identifies the metal ion.

METHOD:

1. Add a few drops of NaOH solution to the sample solution.

2. Observe whether a precipitate forms and note its colour.

3. Add excess NaOH and observe if the precipitate dissolves.

PRECIPITATE COLOURS:

Copper(II) Cu²⁺: blue precipitate — Cu(OH)₂

Iron(II) Fe²⁺: green precipitate — Fe(OH)₂

Iron(III) Fe³⁺: brown/rust-red precipitate — Fe(OH)₃

Calcium Ca²⁺: white precipitate — Ca(OH)₂ (sparingly soluble)

Magnesium Mg²⁺: white precipitate — Mg(OH)₂

Aluminium Al³⁺: white precipitate — Al(OH)₃

Ammonium NH₄⁺: no precipitate — but on warming with NaOH, gives ammonia gas (test with damp red litmus → turns blue)

Reactions and Equations

IONIC EQUATIONS for precipitate formation:

M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)

M³⁺(aq) + 3OH⁻(aq) → M(OH)₃(s)

SPECIFIC EXAMPLES:

Cu²⁺ + 2OH⁻ → Cu(OH)₂ (blue precipitate)

Fe²⁺ + 2OH⁻ → Fe(OH)₂ (green precipitate)

Fe³⁺ + 3OH⁻ → Fe(OH)₃ (brown precipitate)

ALUMINIUM — DISSOLVES IN EXCESS NaOH:

Al(OH)₃ is AMPHOTERIC — dissolves in both acids and excess alkali.

Al³⁺ + 3OH⁻ → Al(OH)₃ (white precipitate, initially)

Al(OH)₃ + OH⁻ → Al(OH)₄⁻ (dissolves in excess — aluminate ion)

This distinguishes aluminium from calcium and magnesium (white precipitates that don't dissolve in excess).

DISTINGUISHING WHITE PRECIPITATES:

Al³⁺: white precipitate dissolves in excess NaOH.

Ca²⁺: white precipitate does NOT dissolve in excess NaOH.

Mg²⁺: white precipitate does NOT dissolve in excess NaOH.

Combining Tests for Identification

SYSTEMATIC APPROACH:

Flame test first — identifies Na, K, Li, Ca, Cu positively.

NaOH test — identifies transition metals (Cu, Fe²⁺, Fe³⁺) and distinguishes Al from Ca/Mg.

For white precipitates: add excess NaOH to check if it dissolves (Al) or remains (Ca, Mg).

AMMONIUM ION TEST:

NH₄⁺ does not give a precipitate with NaOH.

Warm the mixture → ammonia gas released.

Damp red litmus paper held over the tube → turns blue (ammonia is alkaline).

COMBINATION EXAMPLE:

Solution gives blue precipitate with NaOH AND green flame → likely contains copper(II) ions (but the flame test wouldn't show green for dissolved Cu²⁺ in solution — would use spectroscopy for that).

⚠️ Common Mistake

Iron(II) gives a GREEN precipitate; iron(III) gives a BROWN precipitate. These are often confused. Aluminium gives a WHITE precipitate that DISSOLVES in excess NaOH — calcium and magnesium also give white precipitates but they DON'T dissolve in excess.

📐 Key Equations

M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s)

📌 Key Note

NaOH test for metal ions: Cu²⁺ = blue, Fe²⁺ = green, Fe³⁺ = brown/rust, Al³⁺ = white (dissolves in excess), Ca²⁺ = white (stays), Mg²⁺ = white (stays). NH₄⁺: warm with NaOH → ammonia gas (blue litmus). Aluminium unique: amphoteric — dissolves in excess NaOH.

🎯 Matching Activity — NaOH Precipitate Colours

Match each metal ion to the colour of its precipitate with sodium hydroxide. — drag the symbols on the right to match the component names on the left.

Copper(II) Cu²⁺

Drop here

Iron(II) Fe²⁺

Drop here

Iron(III) Fe³⁺

Drop here

Aluminium Al³⁺

Drop here

Calcium Ca²⁺

Drop here

Blue precipitate — Cu(OH)₂

Green precipitate — Fe(OH)₂

White precipitate that remains in excess NaOH

Brown/rust-red precipitate — Fe(OH)₃

White precipitate that dissolves in excess NaOH

🔬 Triple Science Only

Metal hydroxides (4.8.3.2) is chemistry-only — not in Combined Science.

🧪 Required Practical

🔬 RP Chemistry 4 (chemistry-only) — Test for metal ions using NaOH solution. Observe and interpret precipitate colours.

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself

Question 1 of 2

1. An unknown solution gives a brown precipitate with NaOH solution. Which metal ion is present?

2. A white precipitate forms when NaOH is added to a solution, then dissolves when excess NaOH is added. Which ion is present?

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🧪 Metal Hydroxides

Testing for Metal Ions with NaOH

Reactions and Equations

Combining Tests for Identification

🔬 RP Chemistry 4 (chemistry-only) — Test for metal ions using NaOH solution. Observe and interpret precipitate colours.

Mr. Badmus AI