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IGCSE Chemistry 0620 · Chapter 9 notes · ~14 min read

Metals

What makes a metal a metal, why alloys beat pure metals, how the reactivity series predicts everything, how to stop rusting — and how iron and aluminium are extracted.

PropertiesAlloysReactivity seriesCorrosionExtraction of iron & aluminium
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Metals are shiny, good conductors, malleable and ductile, with high melting points. They form positive ions and basic oxides.
  • Alloys are mixtures of a metal with other elements. They are harder and stronger than the pure metal.
  • The reactivity series (K Na Ca Mg Al C Zn Fe H Cu Ag Au) predicts reactions with water, acid and other metals — and how a metal is extracted.
  • Rusting needs oxygen and water. Galvanising and sacrificial protection use zinc. EXT

By the end you can

  • Compare metals and non-metals, and give uses of aluminium and copper.
  • Define an alloy and explain why alloys are harder than pure metals.
  • Order metals by reactivity using their reactions, and predict displacement reactions.
  • Describe the conditions for rusting and the methods of preventing it.
  • Describe the extraction of iron in the blast furnace and aluminium by electrolysis.

1Properties and uses of metals

Metals and non-metals

PropertyMetalsNon-metals
appearanceshinydull
electrical conductivitygoodpoor (graphite is an exception)
thermal conductivitygoodpoor
malleable and ductileyes — can be hammered or drawn into wiresno — brittle if solid
melting and boiling pointsusually highusually low
densityusually highusually low
soundring ("sonorous")dull sound
oxidesbasicacidic
  • Exceptions: mercury is a liquid at room temperature; Group I metals are soft with low melting points and low density; graphite is a non-metal that conducts.

Reactions of metals

  • With oxygen: metal + oxygen → metal oxide (basic). 2Mg + O₂ → 2MgO (burns with a bright white flame).
  • With water: the very reactive metals give hydroxide + hydrogen; the moderately reactive give oxide + hydrogen with steam. Mg + H₂O(g) → MgO + H₂.
  • With dilute acids: metal + acid → salt + hydrogen.

Uses

MetalUsesProperty that explains the use
aluminiumaircraftlow density, and strong when made into an alloy
aluminiumelectricity cables (overhead power lines)good electrical conductor and low density
aluminiumfood containersresists corrosion, malleable and non-toxic
copperelectrical wiringvery good electrical conductor, ductile, does not corrode
copperpans (base of cooking pans)very good thermal conductor

2Alloys

Stronger than pure metals

Alloy — a mixture of a metal with other elements (usually other metals).

pure metalalloylayers slide easilylayers are blockedall atoms the same size→ soft, bends easilydifferent sized atoms→ harder and strongerbrass:copper + zinc(door handles, taps)stainless steel:iron + chromium +nickel (+ carbon)(cutlery, sinks:does not rust)alloy = a mixture of a metal with other elements (usually other metals)
Left: in a pure metal all the atoms are the same size and the layers slide easily. Right: in an alloy, atoms of other sizes block the layers from sliding, so it is harder.
  • Pure metals are often too soft for most uses because the layers of identical atoms slide over each other.
  • EXT In an alloy, atoms of a different size disrupt the regular layers, so the layers cannot slide as easily. The alloy is harder and stronger.
AlloyMade ofUses
brasscopper + zincdoor handles, taps, musical instruments
stainless steeliron + chromium + nickel (+ some carbon)cutlery, kitchen sinks, surgical tools
mild steeliron + a little carboncar bodies, buildings
  • Stainless steel is used for cutlery because it is hard and does not rust.
Exam tipTo spot an alloy in a diagram, look for atoms of different sizes mixed in the lattice. A pure metal has all the atoms the same size.
Common mistake"An alloy is a compound." It is a mixture. The elements are not chemically bonded in fixed amounts.

3The reactivity series

The order

reactivitywith water, steam, acidextractionKpotassiumNasodiumCacalciumMgmagnesiumAlaluminiumCcarbonZnzincFeironHhydrogenCucopperAgsilverAugoldmost reactive at the topcold water:hydroxide + H₂steam and dilute acidoxide layer protects itsteam and dilute acid(Zn faster than Fe)no reaction with water,steam or dilute acidelectrolysis ofthe molten orereduce the ore byheating with carboneasy to extract; goldand silver are foundas the metal itselfcarbon and hydrogen are non-metals: they are shown for comparisonmemory: Please Stop Calling Me A Careless Zebra; I Hate Copper, Silver & Gold
The reactivity series. The most reactive metals are at the top. Carbon and hydrogen are non-metals shown for comparison. The mnemonic gives the order.
Memory trickPlease Stop Calling Me A Careless Zebra; I Hate Copper, Silver & Gold = Potassium Sodium Calcium Magnesium Aluminium Carbon Zinc Iron Hydrogen Copper Silver Gold.

Reactions with water and acid

MetalCold waterSteamDilute acid
potassium, sodiumviolent → hydroxide + H₂—explosive
calciumsteady fizzing → hydroxide + H₂—vigorous
magnesiumvery slowburns brightly → MgO + H₂fast fizzing
aluminiumno reaction (oxide layer)reacts only if the oxide layer is removedslow at first, then faster
zincno reactionreacts → ZnO + H₂steady fizzing
ironno reactionreacts slowlyslow fizzing
copper, silver, goldno reactionno reactionno reaction
  • Ordering metals by experiment: use the same-size pieces of each metal in the same acid. The more vigorous the fizzing (and the greater the temperature rise), the more reactive the metal.

Displacement reactionsEXTENDED

beforeafter (a few minutes)copper in FeSO₄iron nail in bluecopper(II) sulfatecopper coats the nailsolution: blue → pale greenno reaction: copper isless reactive than ironiron + copper(II) sulfate → iron(II) sulfate + copperFe + Cu²⁺ → Fe²⁺ + Cu (iron is more reactive, so it displaces copper)
Iron nail in copper(II) sulfate. Iron is more reactive than copper, so it displaces copper. Copper cannot displace iron from iron(II) sulfate.
  • A more reactive metal displaces a less reactive metal from a solution of its salt.
  • Iron + copper(II) sulfate → iron(II) sulfate + copper. Ionic equation: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s). The blue solution turns pale green.
  • Copper + iron(II) sulfate → no reaction (copper is less reactive).
  • Displacement is a redox reaction: iron loses electrons (oxidised) and copper ions gain them (reduced).
Worked example

EXT Metal P is added to a solution of Q²⁺ and a reaction occurs. Metal Q reacts with R²⁺. Metal R does not react with P²⁺. Put P, Q and R in order of reactivity.

  1. P displaces Q²⁺, so P is more reactive than Q
  2. Q displaces R²⁺, so Q is more reactive than R
  3. R does not displace P²⁺, which fits
  4. order: P > Q > R (most reactive first)

Aluminium — the odd one outEXTENDED

  • Aluminium is high in the series but seems unreactive. Its surface is covered with a thin, tough layer of aluminium oxide (Al₂O₃) that stops water and acid reaching the metal. That is why aluminium is used for aircraft and food containers.
Common mistakeBelieving that aluminium is unreactive because it is low in the reactivity series. It is high in the series — the oxide layer is protecting it.

4Corrosion

Rusting

123air + waterRUSTSboiled water + oil(no dissolved air)no rustdry air (drying agentremoves the water)no rustrust = hydrated iron(III) oxide: iron + oxygen + waterboth oxygen AND water must be present
Three test tubes. Rust forms only where iron has both air (oxygen) and water.
  • Rusting needs both oxygen and water. Rust is hydrated iron(III) oxide.
  • Test tube 1: iron nail in air and water → rusts. Tube 2: boiled water (no dissolved air) with a layer of oil → no rust. Tube 3: dry air (calcium chloride absorbs the water) → no rust.
  • Rust is flaky, so it falls off and exposes fresh iron to rust further. Salt and acid speed up rusting.
  • Corrosion is the slow destruction of a metal by reaction with substances in the environment. Only iron "rusts" — other metals corrode in other ways.

Preventing rust

galvanisingsacrificial protectionironzinc coatingscratchzinc is more reactive than iron:it loses electrons firstZn → Zn²⁺ + 2e⁻so the iron is protected,even where it is scratchedsteel ship hullzinc (or magnesium) blockthe block is more reactive thansteel, so it corrodes insteadand is replaced regularlybarrier methods: paint · oil or grease · plastic coating · galvanisingthey keep oxygen and water away from the iron
Left: galvanising. A zinc coating protects iron even when it is scratched. Right: sacrificial protection with a zinc or magnesium block on a ship's hull.
  • Barrier methods keep oxygen and water away from the iron: paint, oil or grease, plastic coating, and a coating of another metal (zinc, tin or chromium).
  • EXT Galvanising = coating iron with zinc. Zinc is a barrier and more reactive than iron. If scratched, zinc corrodes first: Zn → Zn²⁺ + 2e⁻. The electrons flow to the iron and protect it.
  • EXT Sacrificial protection: a block of a more reactive metal (zinc or magnesium) is attached to steel (ship hulls, pipelines). The block loses electrons and corrodes instead of the steel. It is replaced regularly.
  • EXT Tin plating is only a barrier. If it is scratched, the iron (more reactive than tin) rusts faster.
Exam tipA barrier stops working when scratched. Sacrificial protection still works when scratched, because the zinc is more reactive than iron and is oxidised first.

5Extraction of metals

How reactivity decides the method

Ore — a rock that contains enough of a metal compound to make it worth extracting the metal.

MetalsMethodWhy
K, Na, Ca, Mg, Alelectrolysis of the molten oretoo reactive to be reduced by carbon
Zn, Fe (and Cu)heat with carbon (reduction)less reactive than carbon, so carbon can remove the oxygen
Au, Agfound native (as the metal itself)very unreactive
  • Rule: carbon can reduce the oxides of metals below it in the series (zinc, iron, copper). It cannot reduce those above it.

Iron — the blast furnace

ore, coke, limestonewaste gaseshot airmolten ironslag≈ 200 °C≈ 1500 °C1 C + O₂ → CO₂coke burns: heat2 CO₂ + C → 2COmakes the reducing agent3 Fe₂O₃ + 3CO → 2Fe + 3CO₂iron(III) oxide is reduced4 CaCO₃ → CaO + CO₂limestone breaks down5 CaO + SiO₂ → CaSiO₃slag (calcium silicate)
The blast furnace. Iron ore, coke and limestone go in at the top; hot air is blown in at the bottom. Molten iron and slag are tapped off from the base.
  • Raw materials: iron ore (haematite, Fe₂O₃), coke (carbon), limestone (CaCO₃) and hot air.
  • 1. Coke burns: C + O₂ → CO₂ (releases heat).
  • 2. CO₂ is reduced by coke: CO₂ + C → 2CO. Carbon monoxide is the reducing agent.
  • 3. CO reduces the ore: Fe₂O₃ + 3CO → 2Fe + 3CO₂. The iron(III) oxide is reduced to molten iron.
  • 4. Limestone breaks down: CaCO₃ → CaO + CO₂.
  • 5. CaO reacts with the silica (SiO₂) impurity: CaO + SiO₂ → CaSiO₃ — this is the molten slag, which floats on the iron. Both are tapped off at the bottom.
Worked example

EXT What mass of iron can be made from 800 kg of pure iron(III) oxide? (Ar: Fe = 56, O = 16)

  1. Fe₂O₃ + 3CO → 2Fe + 3CO₂
  2. Mr(Fe₂O₃) = 2 × 56 + 3 × 16 = 160
  3. 160 g of Fe₂O₃ gives 2 × 56 = 112 g of iron
  4. 800 kg × 112 ÷ 160 = 560 kg
Common mistakeSaying coke is "just a fuel". It is both the fuel and the source of the reducing agent, carbon monoxide.

Aluminium — electrolysis

+−molten Al₂O₃ in cryolitemolten aluminiumtapped offgraphite anodes (+) · graphite-lined steel tank (−)pure aluminium oxide(from bauxite) dissolved inmolten cryolite at about 950 °C(cryolite lowers the meltingpoint: it saves energy)cathode (−): Al³⁺ + 3e⁻ → Alanode (+): 2O²⁻ → O₂ + 4e⁻hot oxygen burns the carbonanodes (→ CO₂), so theyare replaced regularlyaluminium is more reactive thancarbon, so it cannot be extractedby heating with carbon
Extraction of aluminium. Pure aluminium oxide from bauxite is dissolved in molten cryolite and electrolysed. Aluminium forms at the cathode and oxygen at the anode.
  • Ore: bauxite, which is impure aluminium oxide (Al₂O₃). It is purified first.
  • Aluminium is above carbon in the series, so carbon cannot reduce it. It is extracted by electrolysis.
  • Al₂O₃ has a very high melting point (over 2000 °C). It is dissolved in molten cryolite (about 950 °C), which lowers the melting point and so saves energy.
  • EXT Cathode (−) (the graphite lining of the tank): Al³⁺ + 3e⁻ → Al. Molten aluminium collects at the bottom and is tapped off.
  • EXT Anode (+) (graphite blocks): 2O²⁻ → O₂ + 4e⁻.
  • The hot oxygen burns the carbon anodes to CO₂, so the anodes must be replaced regularly.
  • The process needs a lot of electricity, so smelters are often built near cheap hydroelectric power. Recycling aluminium saves most of this energy.
Worked example

EXT What mass of aluminium can be made from 102 kg of pure aluminium oxide? (Ar: Al = 27, O = 16)

  1. 2Al₂O₃ → 4Al + 3O₂, so 1 Al₂O₃ gives 2 Al
  2. Mr(Al₂O₃) = 2 × 27 + 3 × 16 = 102
  3. 102 g of Al₂O₃ gives 2 × 27 = 54 g of Al
  4. 54 kg
Exam tipIn an extraction question, give the ore, the reaction type (reduction or electrolysis), and why that method (position in the reactivity series).

6Test yourself

Test yourself — cover the answers first

Quick check

  1. Give two physical properties of metals that non-metals usually lack.
  2. Give one use of aluminium and the property that explains it.
  3. What is an alloy?
  4. Name the alloys of copper + zinc, and of iron + chromium + nickel.
  5. Which metal is more reactive: zinc or copper?
  6. What two things are needed for iron to rust?
  7. Name the ore of aluminium.
  8. EXT Why are alloys harder than pure metals?

Exam-style

  1. A student adds three metals to dilute hydrochloric acid. X fizzes quickly, Y fizzes slowly, Z does not fizz. Put them in order of reactivity and name the gas. [3]
  2. Describe how rusting can be prevented using a barrier. Explain why a scratched barrier stops working. [3]
  3. EXT Explain why a galvanised bucket does not rust even if it is scratched. [3]
  4. Describe the extraction of iron in the blast furnace. Include the raw materials and the main reactions. [6]
  5. EXT Explain why aluminium is extracted by electrolysis, and why cryolite is used. [3]
Show answers
  1. shiny, good conductors, malleable, ductile, sonorous (any two)
  2. aircraft/cables/food containers — low density/conducts/resists corrosion
  3. a mixture of a metal with other elements
  4. brass; stainless steel
  5. zinc
  6. oxygen and water
  7. bauxite
  8. atoms of different sizes stop the layers sliding
  9. X > Y > Z
    hydrogen
    test: squeaky pop with a lit splint
  10. paint / oil / plastic coating (or another metal coating)
    keeps out oxygen and water
    if scratched, air and water reach the iron again
  11. zinc is more reactive than iron
    so it loses electrons / corrodes first (Zn → Zn²⁺ + 2e⁻)
    so the iron is protected (sacrificial protection)
  12. haematite (Fe₂O₃), coke, limestone and hot air
    coke burns in the air to make CO₂: C + O₂ → CO₂
    CO₂ reacts with coke to make carbon monoxide: CO₂ + C → 2CO
    carbon monoxide reduces the iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂
    limestone decomposes to CaO, which reacts with silica to form slag
    molten iron and slag are tapped off at the bottom
  13. aluminium is more reactive than carbon, so carbon cannot reduce it
    Al₂O₃ has a very high melting point
    dissolving it in molten cryolite lowers the temperature needed, saving energy
Practise this chapter
Properties & uses of metalsAlloysReactivity seriesCorrosionExtraction of metals
Next chapterChemistry of the environment →
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