Amphoteric oxides, such as aluminium oxide, are soluble both in strongly acidic and in strongly basic solutions : In acid : Aℓ2O3(s) + 6H3O+ (aq) ⇌ 2Aℓ3+ (aq) + 9 H2O (ℓ) In base : Aℓ(OH)3(s) + OH¯ (aq) ⇌ Aℓ(OH)4¯ (aq) .........................Keq = 40 Dissolution of Al(OH)3 in excess base is just a special case of the effect of complex-ion formation on solubility. Al(OH)3 dissolves because excess OH¯ ions convert it to the soluble complex ion Al(OH)4¯ (aluminate ion). The effect of pH on the solubility of Al(OH)3 is shown in figure. Other examples of amphoteric hydroxides include Zn(OH)2, Cr(OH)3, Sn(OH)2 and Pb(OH)2, which react with excess OH¯ ions to form the soluble complex ions Zn(OH)42– (zincate ion), Cr(OH)4¯ (chromite ion), Sn(OH)3¯ (stannite ion), and Pb(OH)3¯ (plumbite ion), respectively. By contrast, basic hydroxides, such as Mn(OH)2, Fe(OH)2,and Fe(OH)3, dissolve in strong acid but not in strong base.

Amphoteric oxides, such as aluminium oxide, are soluble both in strongly acidic and in strongly basic solutions :

In acid : Aℓ₂O₃(s) + 6H₃O⁺(aq) ⇌ 2Aℓ³⁺ (aq) + 9 H₂O (ℓ)

In base : Aℓ(OH)₃(s) + OH¯ (aq) ⇌ Aℓ(OH)₄¯ (aq) .........................K_eq = 40

Dissolution of Al(OH)₃ in excess base is just a special case of the effect of complex-ion formation on solubility. Al(OH)₃ dissolves because excess OH¯ ions convert it to the soluble complex ion Al(OH)₄¯ (aluminate ion). The effect of pH on the solubility of Al(OH)₃is shown in figure.

Other examples of amphoteric hydroxides include Zn(OH)₂, Cr(OH)₃, Sn(OH)₂ and Pb(OH)₂, which react with excess OH¯ ions to form the soluble complex ions Zn(OH)₄^2– (zincate ion), Cr(OH)₄¯ (chromite ion), Sn(OH)₃¯ (stannite ion), and Pb(OH)₃¯ (plumbite ion), respectively. By contrast, basic hydroxides, such as Mn(OH)₂, Fe(OH)₂,and Fe(OH)₃, dissolve in strong acid but not in strong base.

Linked Question 1

Aluminium is mined as bauxite (Al₂O₃.xH₂O) a hydrated oxide that is always contaminated with Fe₂O₃ and SiO₂. In Bayer process, Al₂O₃ is purified by some reagents, which of the following reagents will be the best for this purification (Note : Strong heating of precipitate of Al(OH)₃ produces Al₂O₃)

first treat bauxite with a weak acid (like CO₂) and then treat the remaining solution with NaOH solution and heat the precipitate obtained

First treat the bauxite with NaOH solution and filter it, then treat the remaining filtrate with a weak acid (like CO₂) and heat the precipitate obtained.

First treat bauxite with a strong acid solution (like HCl) and filter it, then treat the remaining filtrate with NaOH solution and heat the precipitate obtained.

First treat bauxite solution with a strong alkali like NaOH and filter it, then to filtrate add a strong acid like HCl and heat the precipitate obtained.

First heat bauxite with NaOH so, Al₂O₃ get dissolved in it (SiO₂ will also get dissolved as it is an acidic oxide but not Fe₂O₃ (basic acidic),then filter lt. To filtrate add a weak acid like CO₂ so Al(OH)₃ forms Al³⁺ ions). On heating the ppt of Al(OH)₃ we will get pure Al₂O₃

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Linked Question 2

How many mg of solid Al(OH)₃ would dissolve in 100 mL of 5M NH₃(aq.) (K_b = 1.62 × 10^–5).

[Mol. wt. of Al(OH)₃ = 78]

2108

8202

208

2808

[OH^–] = $\sqrt{1.62 \times 10^{- 5} \times 5}$ = 9 × 10^–3 M

Al(OH)₃ + OH^– ⇌ [Al(OH)₄]^–_aq. K = 40

(s) aq.

40 = $\frac{{[Al {(OH)}_{4}]}^{-}}{9 \times 10^{- 3}}$

∴ [Al(OH)₄]^– = 0.36 M

mgms of Al(OH)₃ dissolved = $\frac{0.36 \times 78 \times 1000}{10}$

= 2808

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Linked Question 3

Which of the following curves best represents the variation of solubility of ferrous hydroxide Fe(OH)₂ with the concentration of [H⁺] ions in the solution :

On increasing concentration of [H⁺] ions the solubility of basic hydroxide Fe(OH)₂, will increase.

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