Amphoteric Oxides

Introduction

We have seen that metal oxides are generally basic and non-metal oxides generally acidic. A handful of oxides refuse to take sides. Aluminium oxide and zinc oxide react with hydrochloric acid, like bases, but they also react with sodium hydroxide solution, like acids. Such oxides are called amphoteric , from a Greek word meaning "both". They occupy the middle ground where metallic and non-metallic behaviour overlap.

Core explanation

Definition. An amphoteric oxide reacts with acids to form a salt and water, and also reacts with alkalis to form a salt (and sometimes water). It can act as a base or as an acid depending on what it meets.

Common amphoteric oxides. The most important at this level are:

- aluminium oxide, Al₂O₃ - zinc oxide, ZnO - lead(II) oxide, PbO

Other examples include tin(II) oxide and beryllium oxide. Their hydroxides, such as Al(OH)₃ and Zn(OH)₂, are also amphoteric.

Acting as a base. With an acid, the oxide behaves like any basic metal oxide:

ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)

The metal ends up as a positive ion in the salt: Zn²⁺ or Al³⁺.

Acting as an acid. With a concentrated alkali, the oxide reacts to form a salt in which the metal is part of a negative ion. With sodium hydroxide solution, zinc oxide forms sodium zincate and aluminium oxide forms sodium aluminate. In simplified form:

ZnO(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂O(l)

Al₂O₃(s) + 2NaOH(aq) → 2NaAlO₂(aq) + H₂O(l)

In aqueous solution these ions are more accurately written with hydroxide groups, for example [Zn(OH)₄]²⁻ and [Al(OH)₄]⁻:

ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂Zn(OH)₄

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄

Why these elements? Aluminium, zinc and lead sit near the boundary between metals and non-metals in character. Their ions are small and highly charged (especially Al³⁺), so their oxides are not fully ionic. They have enough metallic character to accept H⁺ from acids, and enough non-metallic character to combine with OH⁻ from alkalis.

Amphoteric hydroxides in testing. When sodium hydroxide solution is added drop by drop to a solution of aluminium or zinc ions, a white precipitate of the hydroxide forms first. Adding excess sodium hydroxide makes the precipitate dissolve again, because the amphoteric hydroxide reacts with the extra alkali to form a soluble aluminate or zincate. Magnesium hydroxide, which is only basic, stays as a precipitate. This difference helps identify metal ions.

Across a period. In Period 3 the pattern runs: Na₂O and MgO basic, Al₂O₃ amphoteric, SiO₂, P₄O₁₀ and SO₃ acidic. Aluminium oxide marks the switchover point.

Step-by-step reasoning

To show that an oxide is amphoteric:

1. React it with a dilute acid such as hydrochloric acid; if it dissolves to form a salt, it can act as a base. 2. React a fresh sample with sodium hydroxide solution; if it dissolves to form a salt, it can act as an acid. 3. Only an oxide that does both is amphoteric.

Visual explanation

Picture two beakers, one of acid and one of alkali, each with white zinc oxide powder at the bottom. On stirring, the powder disappears in both, leaving two clear solutions. A basic oxide would have vanished only in the acid; an acidic oxide only in the alkali.

Real-world analogy

An amphoteric oxide is like a bilingual person at a meeting of two groups who speak different languages. With one group it speaks one language, with the other group it switches — it adapts to whichever partner it meets.

Real-world example

In the extraction of aluminium, bauxite ore (impure aluminium oxide) is treated with hot sodium hydroxide solution. Aluminium oxide dissolves as sodium aluminate because it is amphoteric, while basic impurities such as iron(III) oxide do not dissolve and can be filtered off. The pure aluminium oxide is then recovered for electrolysis.

Why?

Why does iron(III) oxide stay undissolved when bauxite is treated with alkali? Iron(III) oxide behaves as a basic oxide under these conditions: it reacts with acids but not with alkali. Only the amphoteric aluminium oxide reacts with both, which is what makes the separation work.

Common misconception

"An amphoteric oxide is neutral." A neutral oxide, such as carbon monoxide, reacts with neither acids nor bases. An amphoteric oxide is the opposite: it reacts with both.

Worked example

Question: Write equations to show that lead(II) oxide is amphoteric, using nitric acid and sodium hydroxide.

Reasoning: As a base: PbO + 2HNO₃ gives lead(II) nitrate and water. As an acid: PbO + 2NaOH gives sodium plumbate(II) and water.

Answer: PbO(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + H₂O(l) and PbO(s) + 2NaOH(aq) → Na₂PbO₂(aq) + H₂O(l)

Quick check

1. Name two amphoteric oxides. Answer: Aluminium oxide and zinc oxide (lead(II) oxide is another).

Exam focus

Define amphoteric precisely — reacts with both acids and bases — and give equations for both reactions of Al₂O₃ or ZnO. Be ready to explain why a white hydroxide precipitate of Al³⁺ or Zn²⁺ dissolves in excess sodium hydroxide.

Advanced insight

Water itself is amphoteric in the Brønsted–Lowry sense: it donates H⁺ to ammonia and accepts H⁺ from hydrogen chloride. Amino acids are amphoteric too, with an acidic carboxyl group and a basic amino group in the same molecule, which lets proteins resist changes in pH.

Summary

Amphoteric oxides, such as Al₂O₃, ZnO and PbO, react with acids to form ordinary salts with the metal as a positive ion, and with alkalis to form aluminates, zincates or plumbates with the metal in a negative ion. They lie between basic metal oxides and acidic non-metal oxides, a fact used in purifying bauxite and identifying metal ions.

Practice questions

1. What does amphoteric mean? Answer: Able to react both as an acid (with bases) and as a base (with acids). 2. Write an equation for zinc oxide reacting with sulfuric acid. Answer: ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) 3. Explain how to distinguish Al³⁺ ions from Mg²⁺ ions using sodium hydroxide solution. Answer: Both give white precipitates, but the aluminium hydroxide precipitate dissolves in excess sodium hydroxide while magnesium hydroxide does not. 4. Where in Period 3 does the amphoteric oxide appear? Answer: At aluminium, between the basic oxides of sodium and magnesium and the acidic oxides of silicon, phosphorus and sulfur.