Amphoteric Nature of aluminium , zinc, and lead - metals, oxides, and hydroxides

Amphoteric Nature

Amphoteric substances react with both acids and bases, acting as acids with bases (accepting OH⁻) and as bases with acids (donating OH⁻ or accepting H⁺). Aluminium, zinc, and lead (to a lesser extent) exhibit this property in their metals, oxides, and hydroxides.

Summary

• Aluminium: Metal, oxide, and hydroxide react readily with NaOH (forming aluminates) and HCl (forming chlorides), showing strong amphoterism.
• Zinc: Similar to aluminium, forms zincates with NaOH and chlorides with HCl, though metal requires concentrated alkali.
• Lead: Oxide and hydroxide are amphoteric, forming plumbites with NaOH and chlorides with HCl; metal is less reactive with NaOH.

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DETAILS

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1. Aluminium (Al)

Aluminium Metal

• With NaOH: Reacts to form sodium aluminate and hydrogen gas: $$2Al(s) + 2NaOH(aq) + 6H₂O(l) \rightarrow 2NaAl(OH)₄(aq) + 3H₂(g)$$
• 
  
• With Dilute HCl: Forms aluminium chloride: $$2Al(s) + 6HCl(aq) \rightarrow 2AlCl₃(aq) + 3H₂(g)$$

Aluminium Oxide (Al₂O₃)

• With NaOH: Dissolves to form sodium aluminate: $$Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) \rightarrow 2NaAl(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms aluminium chloride: $$Al₂O₃(s) + 6HCl(aq) \rightarrow 2AlCl₃(aq) + 3H₂O(l)$$

Aluminium Hydroxide (Al(OH)₃)

• With NaOH: Dissolves to form sodium aluminate: $$Al(OH)₃(s) + NaOH(aq) \rightarrow NaAl(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms aluminium chloride: $$Al(OH)₃(s) + 3HCl(aq) \rightarrow AlCl₃(aq) + 3H₂O(l)$$

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2. Zinc (Zn)

Zinc Metal

• With NaOH: Reacts in hot, concentrated NaOH to form sodium zincate: $$Zn(s) + 2NaOH(aq) + 2H₂O(l) \rightarrow Na₂Zn(OH)₄(aq) + H₂(g)$$
• 
  
• With Dilute HCl: Forms zinc chloride: $$Zn(s) + 2HCl(aq) \rightarrow ZnCl₂(aq) + H₂(g)$$

Zinc Oxide (ZnO)

• With NaOH: Forms sodium zincate: $$ZnO(s) + 2NaOH(aq) + H₂O(l) \rightarrow Na₂Zn(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms zinc chloride: $$ZnO(s) + 2HCl(aq) \rightarrow ZnCl₂(aq) + H₂O(l)$$

Zinc Hydroxide (Zn(OH)₂)

• With NaOH: Dissolves to form sodium zincate: $$Zn(OH)₂(s) + 2NaOH(aq) \rightarrow Na₂Zn(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms zinc chloride: $$Zn(OH)₂(s) + 2HCl(aq) \rightarrow ZnCl₂(aq) + 2H₂O(l)$$

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3. Lead (Pb)

Lead Metal

• With NaOH: Does not react significantly under standard conditions.
• With Dilute HCl: Reacts slowly due to insoluble PbCl₂ formation: $$Pb(s) + 2HCl(aq) \rightarrow PbCl₂(s) + H₂(g)$$

Lead(II) Oxide (PbO)

• With NaOH: Forms sodium plumbite: $$PbO(s) + 2NaOH(aq) + H₂O(l) \rightarrow Na₂Pb(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms lead(II) chloride (sparingly soluble): $$PbO(s) + 2HCl(aq) \rightarrow PbCl₂(s) + H₂O(l)$$

Lead(II) Hydroxide (Pb(OH)₂)

• With NaOH: Dissolves to form sodium plumbite: $$Pb(OH)₂(s) + 2NaOH(aq) \rightarrow Na₂Pb(OH)₄(aq)$$
• 
  
• With Dilute HCl: Forms lead(II) chloride: $$Pb(OH)₂(s) + 2HCl(aq) \rightarrow PbCl₂(s) + 2H₂O(l)$$

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Special action of Ammonium Hydroxide on solutions of copper salts and sodium hydroxide on ammonium salts

 Special action of Ammonium Hydroxide on solutions of copper salts and sodium hydroxide on ammonium salts

>>>  When ammonium hydroxide is added to solutions of copper salts, it produces a unique deep blue solution.

>>>  Similarly, when sodium hydroxide is mixed with ammonium salts and heated, it generates ammonia gas.

Part 1: "Special action of Ammonium Hydroxide on solutions of copper salts"

• Ammonium Hydroxide (NH₄OH): This is a solution of ammonia (NH₃) in water, often used in chemistry labs. It acts as a base and can form complexes with certain metals.
• Solutions of copper salts: These are liquids containing dissolved copper compounds, like copper sulfate (CuSO₄) or copper chloride (CuCl₂). Copper salts usually contain copper(II) ions (Cu²⁺).
• Special action: When you add ammonium hydroxide to a copper salt solution, something unique happens:
  1. First step: A pale blue solid (precipitate) forms, called copper(II) hydroxide (Cu(OH)₂). This shows copper is present.
     • Example: CuSO₄ + 2NH₄OH → Cu(OH)₂ (blue solid) + (NH₄)₂SO₄
  2. Second step: If you add more ammonium hydroxide, the blue solid dissolves, and the solution turns a bright, deep blue color. This is because a new compound forms, called tetraamminecopper(II) complex ([Cu(NH₃)₄]²⁺).
     • Example: Cu(OH)₂ + 4NH₃ → [Cu(NH₃)₄]²⁺ (deep blue solution) + 2OH⁻
• Why it matters: This deep blue color is a clear sign of copper(II) ions, so chemists use this reaction to test for copper in a sample.

Part 2: "Sodium hydroxide on ammonium salts"

• Sodium Hydroxide (NaOH): This is a strong base, often called caustic soda, used in many chemical reactions.
• Ammonium salts: These are compounds containing the ammonium ion (NH₄⁺), like ammonium chloride (NH₄Cl) or ammonium sulfate ((NH₄)₂SO₄).
• Action: When you mix sodium hydroxide with an ammonium salt and gently heat the mixture, it produces ammonia gas (NH₃), which you can smell (it’s sharp and pungent).
  • Example reaction: NH₄Cl + NaOH → NaCl + H₂O + NH₃↑ (ammonia gas)
• Why it matters: The smell of ammonia gas confirms the presence of ammonium ions in the salt. Chemists use this as a test to detect ammonium in a sample.

Putting It Together

• The statement describes two separate chemical tests:
  1. Ammonium hydroxide + copper salts → Forms a deep blue solution, proving copper is present.
  2. Sodium hydroxide + ammonium salts → Releases ammonia gas when heated, proving ammonium is present.
• These are "special" because they give clear, recognizable results (color change or gas) that help identify specific chemicals in a lab.

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