Sodium Hydroxide (NaOH) Precipitate Tests
Part of Tests for Ions — GCSE Chemistry
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 topic appears regularly enough that it should still be part of a steady revision cycle. It is section 3 of 14 in this topic. Focus on the labels, the relationships between parts, and the explanation that turns the diagram into an exam-ready answer.
Topic position
Section 3 of 14
Practice
20 questions
Recall
14 flashcards
Figure 1: Sodium hydroxide tests produce characteristic precipitate colours for different metal ions.
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.