Zinc Oxide

Zinc oxide is one of only three oxides the 0620 syllabus labels amphoteric (with Al2O3 and PbO). Almost every ZnO question turns on that single idea: it behaves as a base towards acids and as an acid towards alkalis, so it sits in neither the basic nor the acidic column.

Key facts

Property	Detail
Formula	ZnO
Relative formula mass (Mr)	81 (Zn 65, O 16)
Bonding / structure	Giant ionic lattice (Zn2+ and O2- ions)
Appearance	White solid; reversibly turns yellow when hot
Classification	Amphoteric oxide (Supplement)
Main exam roles	Amphoteric example, zinc salt preparation, Zn2+ analysis

Where this compound appears in 0620

• Oxides (Topic 8): classifying oxides as basic, acidic, neutral or amphoteric. ZnO is the standard amphoteric example alongside aluminium oxide.
• Preparation of salts: ZnO is an insoluble base, so zinc salts are made by adding excess solid oxide to warm acid, then filtering.
• Extraction of metals: zinc is below carbon in the reactivity series, so its oxide is reduced by carbon.
• Tests for ions: the amphoteric behaviour of zinc hydroxide is used to identify the Zn2+ ion.

Reactions and tests you must know

As a base (with acids):

ZnO(s) + 2HCl(aq) -> ZnCl2(aq) + H2O(l) ZnO(s) + H2SO4(aq) -> ZnSO4(aq) + H2O(l)

As an acid (with hot alkali):

ZnO(s) + 2NaOH(aq) -> Na2ZnO2(aq) + H2O(l) (sodium zincate)

Reacting with an alkali is what a basic oxide cannot do, so this equation is the proof of amphoteric character. Compare aluminium oxide, which behaves identically.

Reduction (extraction of zinc):

ZnO(s) + C(s) -> Zn(s) + CO(g)

Carbon removes the oxygen, so this is a redox reaction in which carbon is the reducing agent.

Identifying Zn2+ in solution: adding sodium hydroxide gives a white precipitate of Zn(OH)2 that dissolves in excess NaOH; adding aqueous ammonia also gives a white precipitate that dissolves in excess ammonia. Aluminium behaves the same way with sodium hydroxide, so the two are told apart with ammonia: Zn(OH)2 dissolves in excess aqueous ammonia, but Al(OH)3 does not. Zinc hydroxide is amphoteric, which is exactly why it redissolves in the alkali.

Zinc oxide itself has a few uses worth remembering: a white pigment, and the active ingredient of calamine lotion and some sun creams.

Colour change on heating: ZnO turns yellow when hot and white again when cool. No new substance forms — this is a physical (thermochromic) change, and the formula stays ZnO.

Worked exam question

Zinc oxide can be used to prepare zinc sulfate crystals and is also classified as an amphoteric oxide.

(a) Zinc oxide is added in excess to warm dilute sulfuric acid. Explain why an excess is used and how pure crystals are then obtained. (3 marks)

(b) Write an equation, with state symbols, for the reaction of zinc oxide with aqueous sodium hydroxide, and state what this reaction shows about zinc oxide. (2 marks)

Mark scheme

• (a) Excess is used so that all the acid reacts / no acid is left over [1]; the unreacted (excess) zinc oxide is removed by filtration [1]; the filtrate (zinc sulfate solution) is evaporated to the point of crystallisation and left to form crystals [1]
• (b) ZnO(s) + 2NaOH(aq) -> Na2ZnO2(aq) + H2O(l) [1]; it reacts with an alkali as well as with acids, so zinc oxide is amphoteric [1]

Examiner note: in (a) the “excess” marks depend on linking the leftover solid to filtration — answers that only say “to neutralise the acid” miss the salt-preparation points. Na2ZnO2 is the accepted formula for sodium zincate; equations that simply give “a salt + water” are not enough here.

Common exam mistakes

• Calling ZnO basic only. It reacts with sodium hydroxide as well as with acids — you must give both behaviours to earn the amphoteric marks.
• Treating the yellow colour as a reaction. The white-to-yellow change on heating is reversible and physical; no new product forms, so it is not thermal decomposition.
• Giving the alkali product as “zinc hydroxide” or “ZnOH”. With sodium hydroxide the product is sodium zincate, Na2ZnO2, not a hydroxide.
• Choosing titration to make a zinc salt. ZnO is insoluble, so the excess-solid-then-filter method is correct; titration is only for soluble bases and alkalis.