IGCSE Chemistry 0620: Amphoteric Oxides. Aluminium Oxide and Zinc Oxide

A metal-recycling plant recovers aluminium from crushed drink cans. Before melting a batch, technicians test flakes of a white oxide coating removed from the aluminium surface, to confirm what it is. Small samples of the coating are added separately to dilute hydrochloric acid and to dilute sodium hydroxide solution. In both tests, the white solid dissolves completely, forming a colourless solution.

(a) State the term used to describe an oxide that behaves in this way, and name one other metal oxide (other than aluminium oxide) that behaves the same way. [2]

(b) Write a word equation for the reaction of aluminium oxide with hydrochloric acid, and identify the two types of product formed when aluminium oxide reacts with sodium hydroxide. [2]

(c) Explain why this same test result would not have been observed if the coating had been magnesium oxide instead of aluminium oxide. [2]

Part (a): Naming the type of oxide

An oxide that reacts with both acids and bases to form a salt and water is called an amphoteric oxide.

Aluminium oxide, $\text{Al}_2\text{O}_3$ , is one example. Another metal oxide that behaves the same way is zinc oxide, $\text{ZnO}$ .

Part (b): Reactions of the amphoteric oxide

With hydrochloric acid (acting as a base. An amphoteric oxide can neutralise an acid, just like a basic oxide would):

$\text{aluminium oxide} + \text{hydrochloric acid} \rightarrow \text{aluminium chloride} + \text{water}$

With sodium hydroxide (acting as an acidic oxide would. Reacting with a base/alkali):

Aluminium oxide reacts with the alkali to form a salt (sodium aluminate) and water. The same two types of product (a salt, plus water) as in the reaction with the acid, even though the identity of the salt is different in each case.

Part (c): Why magnesium oxide would give a different result

Magnesium oxide, $\text{MgO}$ , is a basic oxide, not an amphoteric one.

With hydrochloric acid, magnesium oxide would react and dissolve, forming magnesium chloride and water, a normal acid + base reaction.
With sodium hydroxide, magnesium oxide would not react or dissolve, because basic oxides only neutralise acids, they do not react with other bases (alkalis).

So only one of the two tests (the acid test) would show the solid dissolving if the coating were magnesium oxide, not both, unlike the amphoteric aluminium oxide, which dissolves in both.

Final answers

(a) Amphoteric; another example is zinc oxide.
(b) Aluminium oxide + hydrochloric acid → aluminium chloride + water; with sodium hydroxide, the products are a salt (sodium aluminate) and water.
(c) Magnesium oxide is basic, not amphoteric: it would dissolve in the acid but not in the sodium hydroxide.