Zinc

Zinc (Zn) is a moderately reactive metal, and in 0620 it is examined most through rust prevention: galvanising and sacrificial protection both depend on zinc being more reactive than iron. Its amphoteric oxide and its cation test also appear regularly.

Where zinc appears in 0620

• Rusting and prevention: galvanising and sacrificial protection of iron and steel.
• Reactivity series: zinc sits above iron and below magnesium.
• Oxides: zinc oxide is amphoteric (reacts with both acids and alkalis).
• Qualitative analysis: the Zn2+ test with sodium hydroxide and with aqueous ammonia.
• Transition elements: zinc is often used to show what an atypical transition metal looks like.

Position in the periodic table

• Symbol: Zn
• Atomic number: 30
• Transition metals block, Period 4
• Electron configuration: 2, 8, 18, 2
• Forms Zn2+ ions

Zinc is atypical for a transition element: it has only one oxidation state (+2) and its compounds are white or colourless rather than coloured.

Physical properties

Property	Detail
Appearance	Bluish-white, moderately lustrous
Melting point	420 C
Density	7.13 g/cm3
Conductivity	Reasonable conductor
Hardness	Brittle at room temperature

Reactivity

Zinc sits above iron and below magnesium in the reactivity series.

With dilute acids

Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)

Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)

Observations: steady effervescence, the zinc dissolves, a colourless solution forms, and the gas gives a squeaky pop with a lighted splint.

Displacement reactions

Zinc displaces less reactive metals from their salt solutions:

Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)

Observations: the blue colour fades as brown copper deposits on the zinc.

With oxygen

2Zn(s) + O2(g) -> 2ZnO(s)

Zinc burns with a blue-green flame to form zinc oxide, a white solid. ZnO is amphoteric: it reacts with both acids and alkalis. (Zinc oxide is yellow when hot and white when cold.)

Galvanising

Galvanising is coating iron or steel with zinc to prevent rusting. It works two ways:

1. The zinc layer is a physical barrier, keeping water and oxygen off the iron.
2. If the coating is scratched, the zinc still corrodes first because it is more reactive than iron — sacrificial protection: Zn -> Zn2+ + 2e-.

Sacrificial protection

Blocks of zinc are bolted to ship hulls, underground pipes and bridges. Being more reactive than iron, the zinc is oxidised in preference and corrodes away, leaving the iron intact. The blocks are replaced as they dissolve.

Test for Zn2+ ions

Add sodium hydroxide solution to a Zn2+ solution: a white precipitate of Zn(OH)2 forms that dissolves in excess NaOH to give a colourless solution.

Zn2+(aq) + 2OH-(aq) -> Zn(OH)2(s)

Aluminium behaves the same way with NaOH, so to tell Zn2+ from Al3+ use aqueous ammonia: Zn(OH)2 redissolves in excess ammonia, but Al(OH)3 does not.

Key facts

• Symbol: Zn — proton number: 30
• Position: transition-metal block, Period 4
• Electron configuration: 2, 8, 18, 2 (forms Zn2+ only)
• Key property: more reactive than iron — basis of galvanising
• Oxide: ZnO, white and amphoteric
• Cation test: white Zn(OH)2, soluble in excess NaOH and in excess ammonia

Compounds

Compound	Formula	Notes
Zinc oxide	ZnO	White, amphoteric
Zinc chloride	ZnCl2	White solid, soluble
Zinc sulfate	ZnSO4	Colourless solution
Zinc carbonate	ZnCO3	White, decomposes on heating
Zinc hydroxide	Zn(OH)2	White precipitate, dissolves in excess NaOH

Common exam mistakes

• Calling zinc a “typical” transition metal. Its compounds are white/colourless and it shows only the +2 state, unlike the coloured, variable-oxidation-state transition metals.
• Confusing galvanising with electroplating or tin-plating. Galvanising gives sacrificial protection because zinc is more reactive than iron; tin is less reactive, so scratched tin-plate rusts faster.
• Forgetting Zn(OH)2 dissolves in excess NaOH. That “white precipitate, soluble in excess” behaviour is the whole point of the Zn2+ test.
• Saying zinc protects iron just by “covering” it. The key idea is that zinc is more reactive, so it is oxidised in preference even when the coating is broken.

Worked exam questions

Explain how galvanising protects iron from rusting, even when the zinc coating is scratched. (3 marks)

Mark scheme

• The zinc coating acts as a barrier, keeping water and oxygen off the iron [1]
• If it is scratched, zinc is more reactive than iron / higher in the reactivity series [1]
• So zinc is oxidised (corrodes) in preference to the iron — sacrificial protection [1]

Examiner note: the “even when scratched” mark needs the reactivity comparison, not just “the zinc covers the iron”. Barrier alone does not explain scratched protection.

Two colourless solutions each give a white precipitate with sodium hydroxide that dissolves in excess. One contains Zn2+ and one contains Al3+. Describe how to tell them apart. (2 marks)

Mark scheme

• Add aqueous ammonia to each solution [1]
• With Zn2+ the white precipitate dissolves in excess ammonia (colourless solution); with Al3+ it does not dissolve [1]

Examiner note: NaOH alone cannot separate these two ions — both dissolve in excess. Aqueous ammonia is the distinguishing reagent.