Metals

What you'll learn

Metals form a substantial portion of the CIE IGCSE Chemistry specification, testing knowledge of physical and chemical properties, the reactivity series, extraction methods, and alloys. Exam questions frequently combine these concepts, requiring analysis of experimental data, prediction of reactions, and explanation of industrial processes. Mastery of this topic is essential for both Paper 2 (Core) and Paper 4 (Extended).

Key terms and definitions

Ore — a naturally occurring rock containing metal compounds in concentrations high enough to make extraction economically viable.

Reactivity series — an ordered list of metals arranged by their tendency to lose electrons and form positive ions, from most reactive (potassium) to least reactive (gold).

Reduction — the removal of oxygen from a compound, or the gain of electrons; the process by which most metals are extracted from their ores.

Displacement reaction — a reaction in which a more reactive metal removes a less reactive metal from its compound.

Alloy — a mixture of two or more elements, where at least one element is a metal, designed to enhance properties such as strength or corrosion resistance.

Electrolysis — the decomposition of an ionic compound when molten or in aqueous solution by the passage of electricity; used to extract reactive metals.

Oxidation — the addition of oxygen to a substance, or the loss of electrons; metals undergo oxidation when they corrode or react with oxygen.

Native metal — a metal found uncombined in the Earth's crust, typically unreactive metals like gold.

Core concepts

Physical properties of metals

Metals share characteristic physical properties that distinguish them from non-metals:

• High melting and boiling points — metals have strong metallic bonding between atoms, requiring significant energy to overcome. Tungsten has a melting point of 3422°C, making it suitable for light bulb filaments.
• Good electrical conductivity — delocalised electrons in the metallic structure move freely, carrying charge through the metal.
• Good thermal conductivity — energy transfers rapidly through the metallic lattice via delocalised electrons, making metals ideal for cooking utensils and radiators.
• Malleability and ductility — metals can be hammered into shape (malleable) or drawn into wires (ductile) because layers of atoms can slide over each other whilst maintaining metallic bonding.
• Lustre — polished metal surfaces reflect light, giving a characteristic shine.
• High density — most metals have closely packed atoms resulting in high density, though sodium and potassium are exceptions.

The reactivity series

The reactivity series ranks metals by their tendency to form positive ions. For CIE IGCSE Chemistry, the required order is:

Potassium (most reactive)
Sodium
Calcium
Magnesium
Aluminium
Carbon (non-metal, included for extraction methods)
Zinc
Iron
Hydrogen (non-metal, included for displacement reactions)
Copper
Silver
Gold (least reactive)

Metals above carbon in the series must be extracted by electrolysis; those below can be extracted by reduction with carbon. Metals above hydrogen displace it from dilute acids, producing hydrogen gas and a salt.

Reactions of metals

Reaction with oxygen

Metals react with oxygen to form metal oxides. Reactivity determines the vigour of the reaction:

• Potassium and sodium burn vigorously with characteristic flame colours (lilac and yellow respectively)
• Magnesium burns with an intense white flame: 2Mg + O₂ → 2MgO
• Iron reacts slowly at room temperature (rusting) but burns in pure oxygen as iron wool
• Copper requires heating and forms black copper(II) oxide: 2Cu + O₂ → 2CuO
• Gold does not react with oxygen even when heated

Reaction with water

Reactivity determines whether metals react with water or steam:

• Potassium, sodium and calcium react vigorously with cold water, producing metal hydroxide and hydrogen gas: 2Na + 2H₂O → 2NaOH + H₂
• Magnesium reacts very slowly with cold water but vigorously with steam: Mg + H₂O → MgO + H₂
• Zinc and iron react with steam but not cold water
• Copper, silver and gold do not react with water or steam

Reaction with dilute acids

Metals above hydrogen in the reactivity series displace hydrogen from dilute hydrochloric acid or dilute sulfuric acid:

Mg + H₂SO₄ → MgSO₄ + H₂
Zn + 2HCl → ZnCl₂ + H₂

The rate of reaction indicates relative reactivity. Magnesium reacts vigorously with rapid effervescence; zinc reacts at a moderate rate; iron reacts slowly. Copper does not react as it is below hydrogen in the series.

Displacement reactions

A more reactive metal displaces a less reactive metal from a solution of its salt:

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc atoms lose electrons (oxidation) to form Zn²⁺ ions; Cu²⁺ ions gain electrons (reduction) to form copper atoms. The blue copper sulfate solution fades as Cu²⁺ ions are removed, and brown copper metal deposits on the zinc. Iron displaces copper from copper sulfate solution; copper cannot displace iron from iron(II) sulfate solution.

Extraction of metals

The extraction method depends on the metal's position in the reactivity series and the stability of its ore.

Metals more reactive than carbon (potassium to aluminium)

These metals are extracted by electrolysis of molten compounds. Aluminium extraction from purified aluminium oxide (alumina) proceeds in the Hall-Héroult process:

1. Aluminium oxide (melting point 2072°C) is dissolved in molten cryolite to reduce the operating temperature to approximately 950°C, saving energy costs
2. Carbon (graphite) electrodes are used; the cathode and anode reactions are:
   • Cathode (negative electrode): Al³⁺ + 3e⁻ → Al (reduction)
   • Anode (positive electrode): 2O²⁻ → O₂ + 4e⁻ (oxidation)
3. Molten aluminium sinks to the bottom of the cell and is tapped off
4. Oxygen produced at the anode reacts with the carbon electrodes, forming carbon dioxide; anodes require regular replacement

Metals less reactive than carbon (zinc to copper)

These metals are extracted by reduction with carbon in a blast furnace or similar apparatus. Iron extraction from haematite (iron(III) oxide) in the blast furnace:

1. Iron ore (haematite, Fe₂O₃), coke (carbon), and limestone (calcium carbonate) are added at the top
2. Hot air is blown in at the bottom; coke burns to produce carbon dioxide and heat: C + O₂ → CO₂
3. Carbon dioxide is reduced to carbon monoxide: CO₂ + C → 2CO
4. Carbon monoxide reduces iron(III) oxide to iron: Fe₂O₃ + 3CO → 2Fe + 3CO₂
5. Molten iron runs to the bottom; limestone removes acidic impurities as slag (calcium silicate)
6. The product is cast iron, containing approximately 4% carbon, making it hard but brittle

Copper extraction from copper-containing ores involves heating with carbon, though increasingly phytoextraction (using plants to absorb copper ions) and bacterial extraction are employed for low-grade ores.

Metals less reactive than hydrogen (silver and gold)

These native metals occur uncombined in the Earth's crust and require only physical separation methods.

Recycling and sustainability

Recycling metals reduces environmental impact and conserves finite ore reserves:

• Aluminium recycling uses only 5% of the energy required for extraction from bauxite
• Iron and steel are separated magnetically from waste and remelted
• Copper can be recycled indefinitely without loss of properties
• Recycling reduces mining activity, habitat destruction, and carbon dioxide emissions from extraction processes

Alloys and their uses

Pure metals often lack the necessary properties for practical applications. Alloys are mixtures containing at least one metal, designed to improve properties:

Steel — iron alloyed with small amounts of carbon (typically 0.1-1.5%) and sometimes other metals

• Mild steel (low carbon content): easily shaped, used for car bodies and machinery
• High-carbon steel: harder and stronger, used for cutting tools and bridges
• Stainless steel: contains chromium (and often nickel), resists corrosion, used for cutlery and surgical instruments

Brass — copper alloyed with zinc; harder than copper, does not corrode, used for musical instruments, ornaments, and door fittings

Bronze — copper alloyed with tin; hard, resists corrosion, historically used for tools and weapons, now for statues and church bells

Solder — tin alloyed with lead (or other metals in lead-free solder); low melting point, used for joining electrical components

Aluminium alloys — aluminium mixed with small amounts of other metals; low density but much stronger than pure aluminium, used for aircraft construction

Alloys are stronger than pure metals because different-sized atoms disrupt the regular arrangement, preventing layers sliding over each other easily.

Corrosion of metals

Rusting is the corrosion of iron in the presence of both oxygen and water, forming hydrated iron(III) oxide (Fe₂O₃·xH₂O). Rusting is an oxidation reaction. Experiments demonstrate both oxygen and water are necessary:

• Iron nails in dry air (with a desiccant) do not rust
• Iron nails in boiled water (with oil preventing oxygen dissolving) do not rust
• Iron nails exposed to both air and water rust rapidly

Rust prevention methods:

• Barrier methods: painting, oiling, greasing, or plastic coating exclude oxygen and water
• Galvanising: coating iron with zinc; even if scratched, zinc (more reactive than iron) corrodes preferentially, protecting the iron
• Sacrificial protection: attaching blocks of a more reactive metal (zinc or magnesium) to iron structures; the more reactive metal corrodes instead
• Alloying: stainless steel contains chromium, which forms a protective oxide layer preventing further corrosion

Worked examples

Example 1: Displacement reactions

Question: A student adds excess zinc powder to 25 cm³ of copper(II) sulfate solution. Describe what the student would observe and explain the observation using the reactivity series. Write a balanced equation including state symbols. [4 marks]

Answer:

Observation: The blue colour of the solution fades / becomes colourless [1 mark]. Brown/reddish solid (copper) forms on the zinc powder / in the solution [1 mark].

Explanation: Zinc is more reactive than copper / zinc is higher in the reactivity series [1 mark], so zinc displaces copper from copper sulfate solution.

Equation: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) [1 mark for correct equation with state symbols]

Example 2: Extraction of metals

Question: Zinc oxide can be reduced to zinc by heating with carbon. Titanium oxide cannot be reduced by carbon and must be extracted using a more reactive metal.

(a) Write a balanced equation for the reduction of zinc oxide with carbon. [2 marks]
(b) Explain why carbon cannot reduce titanium oxide. [2 marks]
(c) Suggest why titanium is more expensive than zinc. [1 mark]

Answer:

(a) 2ZnO + C → 2Zn + CO₂ [1 mark for correct equation, 1 mark for balancing]
(Alternative: ZnO + C → Zn + CO)

(b) Titanium is more reactive than carbon / titanium is above carbon in the reactivity series [1 mark], so titanium has a greater affinity for oxygen than carbon / carbon cannot remove oxygen from titanium oxide [1 mark].

(c) The extraction method for titanium is more expensive / requires more energy / uses expensive reactive metals / involves more complex processes [1 mark].

Example 3: Rusting experiment

Question: A student sets up three test tubes containing iron nails:

• Tube A: iron nail in air and water
• Tube B: iron nail in boiled water with oil layer on top
• Tube C: iron nail in dry air with anhydrous calcium chloride

After one week, only Tube A shows rust.

(a) Explain why the nail in Tube A rusted. [2 marks]
(b) Explain why the nail in Tube B did not rust. [2 marks]
(c) State one industrial method of preventing rusting and explain how it works. [2 marks]

Answer:

(a) Both oxygen and water were present [1 mark]. Iron reacts with oxygen and water to form hydrated iron(III) oxide / rust [1 mark].

(b) Water was boiled to remove dissolved oxygen [1 mark], and the oil layer prevented oxygen from the air dissolving in the water / only water present, no oxygen [1 mark].

(c) Method: Galvanising / coating with zinc [1 mark].

Explanation: Zinc is more reactive than iron, so zinc corrodes instead of iron / zinc acts as a sacrificial metal / even if the coating is scratched, zinc protects iron [1 mark].

(Alternative acceptable answers: painting/oiling creates a barrier to exclude oxygen and water; stainless steel contains chromium which forms a protective layer)

Common mistakes and how to avoid them

• Mistake: Writing that rusting requires oxygen OR water. Correction: Rusting requires both oxygen AND water simultaneously; experiments must show both conditions are necessary.

• Mistake: Stating that copper reacts with dilute acids to produce hydrogen. Correction: Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from acids; no reaction occurs with dilute hydrochloric or sulfuric acid.

• Mistake: Confusing oxidation and reduction in extraction. Correction: During extraction, metal ions in the ore gain electrons (reduction) to form metal atoms; this is reduction because oxygen is removed or electrons are gained.

• Mistake: Claiming alloys are compounds. Correction: Alloys are mixtures, not compounds; the elements retain their individual properties and are not chemically bonded in fixed ratios.

• Mistake: Writing that aluminium is extracted by reduction with carbon. Correction: Aluminium is more reactive than carbon (above it in the reactivity series), so it must be extracted by electrolysis of molten aluminium oxide dissolved in cryolite.

• Mistake: Stating that galvanising works only by providing a barrier. Correction: Galvanising provides barrier protection, but more importantly, zinc acts as a sacrificial metal; even if scratched, zinc (being more reactive) corrodes preferentially, protecting the underlying iron.

Exam technique for Metals

• Command word 'Explain' requires both a statement and a reason. For displacement reactions, state which metal is more reactive AND the consequence (one metal displaces the other). For extraction methods, link the metal's position in the reactivity series to the method used.

• State symbols matter in equations involving displacement and extraction. Solid metals are (s), aqueous solutions are (aq), molten substances are (l). Check the mark scheme; equations may require state symbols for full marks.

• Rust prevention questions often award separate marks for naming the method and explaining the mechanism. Distinguish between barrier methods (exclude oxygen/water) and sacrificial protection (more reactive metal corrodes instead).

• Data interpretation questions frequently test the reactivity series. When given experimental results (temperature change, rate of gas production, observations), use them to deduce the order of reactivity. More reactive metals produce faster reactions, greater temperature increases, and more vigorous effervescence.

Quick revision summary

Metals have characteristic physical properties including high melting points, electrical conductivity, and malleability due to metallic bonding with delocalised electrons. The reactivity series orders metals from potassium (most reactive) to gold (least reactive). More reactive metals displace less reactive metals from solutions and react more vigorously with oxygen, water, and acids. Metals above carbon require electrolysis for extraction; those below are reduced with carbon. Rusting requires both oxygen and water; prevention methods include barrier protection and sacrificial metals. Alloys are mixtures designed to enhance properties, with examples including steel, brass, and bronze. Recycling metals conserves resources and reduces energy consumption compared with extraction from ores.