Cation Tests with Sodium Hydroxide
This topic covers the identification of specific metal cations in solution by adding aqueous sodium hydroxide. You need to know the characteristic colours of the solid precipitates that form, which is 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.