Sodium Hydroxide (NaOH) Precipitate Tests
Part of Tests for Ions · GCSE GCSE Chemistry revision
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.
Topic position
Section 3 of 15
Practice
20 questions
Recall
14 flashcards
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.