Copper

Copper (Cu) is a transition element valued for its conductivity, malleability, and corrosion resistance. It features heavily in electrolysis and metal reactivity topics.

Where it appears in 0620

Copper is the worked example whenever the syllabus needs an unreactive, coloured transition metal:

• Transition elements: copper shows the typical properties — coloured compounds, catalytic behaviour, and a variable oxidation state (Cu compounds are usually Cu2+).
• Electrolysis: electrolytic purification of copper is a named process, and copper is a common metal in electroplating.
• Reactivity series: copper lies below hydrogen, so it does not react with dilute acids — a favourite “explain why nothing happens” question.
• Tests for ions: Cu2+ gives a blue precipitate with sodium hydroxide, and anhydrous copper(II) sulfate is the standard test for water.

Position in the periodic table

• Symbol: Cu (from Latin cuprum)
• Atomic number: 29
• Transition metal, Period 4
• Electron configuration: 2, 8, 18, 1

Physical properties

Property	Detail
Appearance	Reddish-brown, lustrous
Melting point	1085 C
Conductivity	Excellent electrical and thermal conductor
Malleability	Can be hammered into sheets
Ductility	Can be drawn into wires

Reactivity

Copper is low in the reactivity series, below hydrogen:

• Does not react with dilute acids (it is below hydrogen)
• Does not react with water or steam
• Is displaced by more reactive metals: Fe(s) + CuSO4(aq) -> FeSO4(aq) + Cu(s)
• Copper(II) oxide is reduced by hydrogen or carbon: CuO(s) + H2(g) -> Cu(s) + H2O(g)

Because copper is below carbon, it could in principle be extracted by carbon reduction, but electrolysis is used to reach the very high purity that wiring demands.

Electrolytic purification of copper

A key application of electrolysis:

Component	Detail
Anode	Impure copper
Cathode	Pure copper (thin strip)
Electrolyte	Copper(II) sulfate solution, CuSO4(aq)

At the anode (oxidation): Cu -> Cu2+ + 2e- The impure copper dissolves. Insoluble impurities drop off as “anode sludge” (contains silver and gold).

At the cathode (reduction): Cu2+ + 2e- -> Cu Pure copper is deposited and the cathode gains mass.

The concentration of CuSO4 stays constant: Cu2+ ions are removed at the cathode as fast as they enter solution at the anode.

Compounds and colours

Compound	Formula	Colour
Copper(II) sulfate (hydrated)	CuSO4.5H2O	Blue crystals
Copper(II) sulfate (anhydrous)	CuSO4	White powder
Copper(II) oxide	CuO	Black solid
Copper(II) hydroxide	Cu(OH)2	Blue precipitate
Copper(II) carbonate	CuCO3	Green solid

Green copper(II) carbonate decomposes on heating to a black solid: CuCO3(s) -> CuO(s) + CO2(g). The colour change from blue CuSO4.5H2O to white anhydrous CuSO4 is the test for water and is reversible.

Uses

Use	Property exploited
Electrical wiring	Excellent conductor, ductile
Water pipes / plumbing	Does not react with water, malleable
Cooking bases	Good thermal conductor
Alloys (brass, bronze)	Strength, appearance, corrosion resistance

Test for Cu2+ ions

Add sodium hydroxide solution to a solution containing Cu2+ ions: a blue precipitate of copper(II) hydroxide forms, insoluble in excess NaOH.

Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s)

Key facts at a glance

Fact	Detail
Symbol	Cu
Proton number	29
Position	Transition element, Period 4
Reactivity	Below hydrogen — no reaction with dilute acids
Signature compound	Blue Cu2+ (e.g. CuSO4.5H2O)
Main use	Electrical wiring (excellent conductor)

Common exam mistakes

• Claiming copper reacts with dilute acid to give hydrogen. It does not — copper is below hydrogen in the reactivity series, so there is no reaction.
• Swapping the electrodes in purification. Impure copper is the anode (positive); pure copper is the cathode (negative). Getting them the wrong way round loses both electrode marks.
• Saying the electrolyte gets used up. The CuSO4 concentration stays constant, because copper is added to solution at the anode at the same rate it is removed at the cathode.
• Writing copper hydroxide as CuOH. Cu2+ needs two hydroxide ions: the formula is Cu(OH)2.

Exam-style questions

Impure copper can be purified by electrolysis. Name the electrolyte, describe what happens at each electrode, and write both half-equations. (4 marks)

Mark scheme

• electrolyte is copper(II) sulfate solution [1]
• at the anode (impure copper) the copper dissolves: Cu -> Cu2+ + 2e- (oxidation) [1]
• at the cathode (pure copper) copper is deposited: Cu2+ + 2e- -> Cu (reduction) [1]
• impurities collect as sludge below the anode / the cathode gains mass [1]

Examiner note: label which electrode is which — an unlabelled “copper dissolves” cannot be credited if it isn’t tied to the anode.

A student adds sodium hydroxide solution to an unknown solution and sees a blue precipitate that does not dissolve in excess. Identify the ion present and write an ionic equation. (2 marks)

Mark scheme

• the ion is Cu2+ (copper(II)) [1]
• Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s) [1]

Examiner note: a blue precipitate points to Cu2+; do not confuse it with the green precipitate given by Fe2+.