Testing for Metal Ions with Sodium Hydroxide Revision Notes | GCSE AQA Higher Chemistry

1. Why Test for Metal Ions?

Picture a forensic scientist with an unknown white powder, or a water company checking for contamination. How do they know which metals are present? You can't tell aluminium from calcium by looking — they both form colourless solutions.

This is where sodium hydroxide becomes your detective tool. Add a few drops to any metal ion solution, and you'll see distinctive patterns that reveal the metal's identity.

2. The Sodium Hydroxide Test

When you add sodium hydroxide solution to a metal ion solution, the hydroxide ions (OH⁻) react with the metal ions to form metal hydroxides. Most metal hydroxides are insoluble, so they appear as coloured precipitates.

The general pattern is: Metal ion + Hydroxide ions → Metal hydroxide precipitate. For example: $Al^{3+}(aq) + 3OH^-(aq) \rightarrow Al(OH)_3(s)$

3. The Three White Precipitates

Three common metal ions all produce white precipitates with sodium hydroxide: aluminium (Al³⁺), calcium (Ca²⁺), and magnesium (Mg²⁺). At first glance, they look identical — all white, all insoluble.

**Trap**: Many students think you can't tell these apart because they're all white. Actually, there's a crucial second step that separates aluminium from the other two.

Worked example: Testing for Aluminium, Calcium and Magnesium Ions

You have three test tubes containing Al³⁺, Ca²⁺, and Mg²⁺ solutions. How do you identify which is which?

1. Add 2-3 drops of sodium hydroxide solution to each test tube — All three will form white precipitates at this stage
2. $Al^{3+} + 3OH^- \rightarrow Al(OH)_3(s)$ (white) $Ca^{2+} + 2OH^- \rightarrow Ca(OH)_2(s)$ (white) $Mg^{2+} + 2OH^- \rightarrow Mg(OH)_2(s)$ (white) — Cannot distinguish between them yet — all appear white
3. Continue adding sodium hydroxide solution until in large excess — This is the key step that reveals the difference
4. Al(OH)₃: dissolves (clear solution) Ca(OH)₂: remains as white precipitate Mg(OH)₂: remains as white precipitate — Only aluminium hydroxide dissolves in excess NaOH

Answer: Aluminium ion identified by precipitate that dissolves in excess NaOH; calcium and magnesium remain as white precipitates

The key insight is that aluminium hydroxide is amphoteric — it can act as both an acid and a base. In excess sodium hydroxide, it dissolves to form a soluble product. Calcium and magnesium hydroxides cannot do this.

4. Why Does Aluminium Behave Differently?

Aluminium sits in an unusual position in the periodic table. It's a metal, but it's close to the metalloids. This gives aluminium hydroxide special properties that calcium and magnesium hydroxides don't have.

When excess sodium hydroxide is present, aluminium hydroxide dissolves because it can act as an acid, donating protons to the excess hydroxide ions. The exact chemistry involves complex ion formation, but you don't need to write equations for this process in GCSE exams.

5. Exam Technique and Common Mistakes

Exam questions often give you observations and ask you to identify the metal ion. The key is recognising the two-stage pattern: initial precipitate colour, then behaviour with excess sodium hydroxide.

**Trap**: Students often forget to mention 'excess' sodium hydroxide. Writing 'dissolves in NaOH' gets no marks — you must specify 'excess NaOH' for the mark.

Exam precision note: In Testing for Metal Ions with Sodium Hydroxide - Metal hydroxides - Identification of ions by chemical and spectroscopic means (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Sodium hydroxide tests help identify some metal ions because different hydroxide precipitates behave differently in excess sodium hydroxide. Aluminium hydroxide dissolves in excess, while calcium and magnesium hydroxides remain as white precipitates. At GCSE, students only need the observation and the conclusion, not complex-ion equations.