Identifying Zinc, Aluminium and Lead Ions

Introduction

Zinc(II), aluminium(III) and lead(II) can all give white hydroxide precipitates that dissolve under suitable excess strong hydroxide. That shared response narrows the candidate set but does not identify one ion. Aqueous ammonia and separate anion-forming tests create a decision path: zinc hydroxide can dissolve in excess ammonia, whereas aluminium and lead hydroxides commonly persist under standard classroom conditions.

Core explanation

The formal precipitation equations are Zn²⁺ + 2OH⁻ → Zn(OH)₂(s), Al³⁺ + 3OH⁻ → Al(OH)₃(s), and Pb²⁺ + 2OH⁻ → Pb(OH)₂(s). All are white or nearly white, so colour alone is unhelpful. Their charges and hydroxide formulas differ; a quantitative hydroxide-to-metal mole ratio could support an assignment if other reactions were controlled, but the visual test usually cannot measure that ratio.

In excess strong NaOH, zinc and aluminium can form soluble [Zn(OH)₄]²⁻ and [Al(OH)₄]⁻ in simplified aqueous models. Lead(II) can also form soluble hydroxo species. This is amphoteric redissolution and reflects coupled complexation, not a temperature-induced change in the original Ksp. Consequently, “white then soluble in NaOH” is a group clue shared by all three.

Excess aqueous ammonia gives a stronger contrast. Zn(OH)₂ can dissolve as a zinc ammine complex, commonly represented [Zn(NH₃)₄]²⁺. Al(OH)₃ usually remains insoluble under the standard ammonia test, and Pb(OH)₂ also tends to remain as a white solid. Therefore dissolution in both NaOH and NH₃ favours zinc among the three. Insolubility in NH₃ leaves aluminium and lead as candidates.

Lead can be distinguished through its chloride or sulfate chemistry under suitable conditions. PbCl₂ is sparingly soluble and belongs to the classical first chloride fraction; PbSO₄ is also sparingly soluble. Aluminium's chloride is generally soluble. A reported white chloride precipitate plus amphoteric hydroxide behaviour supports Pb²⁺ over Al³⁺, while absence of a chloride precipitate under a sensitive valid test weakens lead. The final conclusion should still consider concentration, temperature and other ions.

Analytical sequence matters. If lead was already removed in the first chloride group, an aluminium-versus-zinc question in a later fraction may be simpler. If the starting sample was not separated, the three-way table remains necessary. A group flowchart should record whether each candidate could have survived earlier stages.

The Royal Society of Chemistry comparison chart at https://edu.rsc.org/download?ac=523339 reports the common NaOH and NH₃ patterns. Real concentrations and ligands can shift observations; a clear solution after excess ammonia is more informative when the starting white solid was verified and a control showed the reagent itself was clear.

Do not infer original anion from a hydroxide test. An aluminium nitrate solution and an aluminium chloride solution can both produce Al(OH)₃; their original counterions require separate evidence. Lead compounds require controlled handling in practice; this note remains a conceptual interpretation of reported observations.

Step-by-step reasoning

1. Identify a white hydroxide and balance its possible formulas. 2. Check excess strong-NaOH dissolution for amphoteric behaviour. 3. Check excess ammonia: zinc may dissolve, aluminium and lead usually persist. 4. If aluminium versus lead remains, use chloride or another independent response. 5. Account for prior group separations and original anion separately.

Visual explanation

Draw a three-row table for Zn²⁺, Al³⁺ and Pb²⁺. All rows show white precipitate and NaOH dissolution; only the zinc row shows routine ammonia dissolution, while lead has a separate chloride-precipitate clue.

Real-world analogy

Three keys share the same first notch and pass the first checkpoint. A second notch singles out zinc, and a third check separates the two keys still together.

Real-world example

An unknown white hydroxide dissolves in excess NaOH and excess NH₃. Within this three-ion candidate set, zinc is favoured. If it dissolves only in NaOH but also forms a sparingly soluble chloride, lead(II) becomes more plausible.

Why?

Why does aluminium's NaOH solubility not imply ammonia solubility? Strong hydroxide can form soluble hydroxoaluminate, while weak-base ammonia does not provide the same OH⁻ level or an equally favourable aluminium ammine pathway.

Common misconception

“All amphoteric hydroxides behave alike with every base.” NaOH and NH₃ differ in basicity and metal-ligand chemistry, so the same white solid can respond differently.

Worked example

A cation gives white M(OH)n that dissolves in excess NaOH but remains solid in excess NH₃. Zinc is less likely under the standard table because its hydroxide can dissolve in ammonia. Aluminium and lead remain. A separate chloride test gives no precipitate at conditions that would detect PbCl₂ at the sample concentration, so aluminium becomes the better-supported candidate. The negative chloride result must be qualified by detection limits.

Quick check

1. Which one of Zn²⁺, Al³⁺ and Pb²⁺ commonly forms a soluble ammine complex after an initial white hydroxide in excess NH₃? Answer: Zn²⁺, commonly represented as [Zn(NH₃)₄]²⁺ in the simplified test model.

Exam focus

Do not identify a white hydroxide from NaOH alone. Compare excess NH₃ and a separate lead/aluminium discriminator.

Advanced insight

Hydroxo and ammine formation constants, pH and analytical metal concentration determine whether the textbook visual pattern appears. A quantitative model tracks free and complexed metal separately.

Summary

Zn²⁺, Al³⁺ and Pb²⁺ can share white amphoteric hydroxide behaviour. Zinc's dissolution in excess ammonia distinguishes it in a common classroom comparison; lead's chloride or sulfate chemistry can help separate it from aluminium. Multiple observations are necessary.

Practice questions

1. What is the formal hydroxide formula for Al³⁺? Answer: Al(OH)₃. 2. Which reagent commonly dissolves all three white hydroxides under suitable excess conditions? Answer: Strong NaOH, through amphoteric hydroxo-species formation. 3. Which two usually remain insoluble in excess aqueous ammonia in this comparison? Answer: Aluminium and lead(II) hydroxides. 4. Why can PbCl₂ evidence distinguish lead from aluminium? Answer: Lead(II) chloride is sparingly soluble under the classical group conditions, whereas aluminium chloride is generally soluble.