Cation Tests with Sodium Hydroxide
Adding aqueous sodium hydroxide identifies specific metal cations in solution, so you need to know the characteristic colours of the solid precipitates that form, a key skill in qualitative chemical analysis.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Adding aqueous sodium hydroxide (NaOH) to solutions containing metal cations can produce insoluble metal hydroxides, which appear as precipitates.
- Copper(II) ions, Cu²⁺, form a blue precipitate of copper(II) hydroxide, Cu(OH)₂.
- Iron(II) ions, Fe²⁺, form a green precipitate of iron(II) hydroxide, Fe(OH)₂.
- Iron(III) ions, Fe³⁺, form a brown precipitate of iron(III) hydroxide, Fe(OH)₃.
- Aluminium (Al³⁺), Calcium (Ca²⁺), and Magnesium (Mg²⁺) ions all form a white precipitate. Their hydroxides are Al(OH)₃, Ca(OH)₂, and Mg(OH)₂ respectively.
Formulae
Mn+(aq) + n OH^-(aq) → M(OH)n(s) This general ionic equation describes the formation of a metal hydroxide precipitate when an alkali like sodium hydroxide is added to a solution containing metal ions.
Definitions
- Precipitate
- An insoluble solid that emerges from a liquid solution during a chemical reaction.
- Cation
- An ion with a positive electrical charge, such as a metal ion like Fe²⁺.
- Aqueous
- A solution where the solvent is water, denoted by (aq) in chemical equations.
Worked example
Aqueous sodium hydroxide is added to a test tube containing a clear, blue solution. A solid is observed to form. Identify the cation responsible for this observation and state the chemical formula of the solid formed.
- 1
The starting solution is blue, which is characteristic of aqueous copper(II) compounds.
- 2
The addition of sodium hydroxide is a standard test for metal cations.
- 3
A blue precipitate is the specific result for the reaction between copper(II) ions and hydroxide ions.
- 4
Therefore, the cation present is the copper(II) ion, Cu²⁺.
- 5
The solid formed is copper(II) hydroxide.
The Cu²⁺ ion requires two OH⁻ ions (each with a 1- charge) to form a neutral compound.
- 6
The chemical formula for the precipitate is Cu(OH)₂.
Answer: Cation: Cu²⁺. Formula: Cu(OH)₂.
Common mistakes
- ×Writing incorrect chemical formulae for the precipitates by not balancing the charges correctly. For example, writing FeOH₃ for iron(III) hydroxide instead of the correct Fe(OH)₃.
- ×Confusing the colours for the two iron ions. Remember Fe²⁺ forms a green precipitate, while Fe³⁺ forms a brown one.
- ×Forgetting that the test with sodium hydroxide alone cannot distinguish between Al³⁺, Ca²⁺, and Mg²⁺ as they all produce a white precipitate.
No-calculator tips
- ✓This topic relies on memory, not calculation. Use mnemonics or visual associations: Cu²⁺ compounds are often blue, and Fe³⁺ is the colour of rust.
- ✓Focus on the Roman numeral in the ion's name, e.g., iron(II) vs iron(III). This number directly tells you the positive charge on the cation (Fe²⁺ vs Fe³⁺), which you need to write the correct hydroxide formula.
- ✓Practice writing the ionic equations for each precipitate formation to reinforce your knowledge of the correct formulae.