This diagram covers Sodium Hydroxide (NaOH) Precipitate Tests within Tests for Ions for GCSE Chemistry. Revise Tests for Ions in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 3 of 15 in this topic. Focus on the labels, the relationships between parts, and the explanation that turns the diagram into an exam-ready answer.
Figure 1: Adding NaOH solution identifies metal ions by precipitate colour — white (Al³⁺/Ca²⁺), pale blue (Cu²⁺), green (Fe²⁺), rusty brown (Fe³⁺).
Sodium Hydroxide (NaOH) Precipitate Tests
Adding sodium hydroxide solution to a solution of a metal salt produces a metal hydroxide precipitate. The colour of the precipitate identifies the metal ion.
| Metal Ion | Result with NaOH | Precipitate Formula |
|---|---|---|
| Al³⁺ (Aluminium) | White precipitate; redissolves in excess NaOH | Al(OH)₃(s) |
| Ca²⁺ (Calcium) | White precipitate (does NOT redissolve) | Ca(OH)₂(s) |
| Cu²⁺ (Copper) | Blue precipitate | Cu(OH)₂(s) |
| Fe²⁺ (Iron II) | Green precipitate | Fe(OH)₂(s) |
| Fe³⁺ (Iron III) | Brown/orange precipitate | Fe(OH)₃(s) |
Key distinguishing point: Only aluminium hydroxide Al(OH)₃ redissolves in excess NaOH, forming the soluble aluminate ion [Al(OH)₄]⁻. Calcium hydroxide does NOT redissolve — this distinguishes Al³⁺ from Ca²⁺ when both give white precipitates.
Practice questions for Tests for Ions
Which reagents are used to test for carbonate ions in a solution?
Describe how sodium hydroxide solution can be used to distinguish between iron(II) ions and iron(III) ions in solution, including the expected observations.