What you'll learn
Chemical changes: the reactivity series and extraction of metals is the topic in AQA GCSE Chemistry that explains why gold is found as a nugget while aluminium had to wait until the nineteenth century to be isolated at all. The organising idea is simple: metals react by losing electrons to form positive ions, and the reactivity series ranks how readily each metal does this. Once you have that order, you can predict reactions with water and acid, predict displacement reactions, and determine how any metal must be extracted from its ore. By the end of this guide you should be able to recall the reactivity series including carbon and hydrogen, deduce the order from experimental observations, explain displacement in terms of electron transfer, explain why the extraction method depends on position relative to carbon, and describe biological methods of extraction.
Key terms and definitions
Reactivity series — metals arranged in order of how readily they react, reflecting the ease with which they form positive ions
Ore — a rock containing enough of a metal or its compound to make extraction economically worthwhile
Native metal — a metal found in the Earth as the element itself rather than as a compound
Reduction — loss of oxygen, or gain of electrons
Oxidation — gain of oxygen, or loss of electrons
Displacement reaction — a reaction in which a more reactive metal takes the place of a less reactive one in a compound
Electrolysis — decomposition of a molten or dissolved ionic compound using an electric current
Phytomining — using plants to absorb metal compounds from low-grade soil, which are then recovered by burning the plants
Bioleaching — using bacteria to produce solutions containing metal compounds from low-grade ores
Leachate — the solution produced by bioleaching, from which the metal is recovered
Low-grade ore — an ore containing only a small proportion of the metal
Core concepts
The reactivity series
Metals react by losing their outer electrons to form positive ions. The more readily a metal does this, the higher it sits in the reactivity series.
The order you need, from most to least reactive, is potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, silver and gold.
Carbon and hydrogen are included as reference points even though they are non-metals, because their positions determine how each metal can be extracted and whether it reacts with acid. Carbon sits between zinc and iron for extraction purposes, and hydrogen sits between iron and copper.
Deducing reactivity from observations
The order can be established experimentally, and questions regularly give observations and ask you to rank unknown metals.
Reaction with cold water separates the top of the series. Potassium, sodium, lithium and calcium react with cold water to produce hydrogen and a metal hydroxide. Potassium reacts most violently, igniting with a lilac flame; sodium melts into a ball and moves rapidly across the surface; lithium fizzes steadily.
Reaction with dilute acid separates the middle. Magnesium reacts vigorously, zinc steadily, and iron slowly, each producing a salt and hydrogen. The rate of effervescence gives the order directly.
No reaction with either identifies the bottom. Copper, silver and gold do not react with water or with dilute acid, because they are below hydrogen in the series and cannot displace it from the acid.
Displacement reactions refine the order further, since a metal will displace any metal below it from a solution of that metal's salt.
Displacement reactions and electron transfer
When a more reactive metal is placed in a solution of a less reactive metal's salt, the more reactive metal takes its place.
Placing iron in copper sulfate solution produces iron sulfate and copper metal. The blue colour of the solution fades and a brown deposit of copper appears on the iron.
In terms of electrons, the iron atoms lose electrons to become iron ions, so the iron is oxidised. The copper ions gain those electrons to become copper atoms, so the copper ions are reduced. Since oxidation and reduction occur together, displacement is a redox reaction.
The sulfate ions take no part and appear unchanged on both sides, making them spectator ions. Removing them gives the ionic equation, which shows the electron transfer clearly and is what examination questions usually request.
Metal ores and native metals
Most metals are found in the Earth combined with other elements, chiefly as oxides, and these compounds must be reduced to obtain the metal.
Unreactive metals such as gold are found as the metal itself, because they have so little tendency to form compounds. This is why gold was among the first metals known to humanity despite being difficult to extract from anything.
The general principle is that the less reactive a metal, the easier it is to extract, and the earlier in history it was isolated.
Extraction by reduction with carbon
Metals less reactive than carbon can be extracted by heating their oxide with carbon. Carbon is more reactive, so it takes the oxygen from the metal oxide.
The metal oxide loses oxygen and is therefore reduced. The carbon gains oxygen and is therefore oxidised, acting as the reducing agent.
Iron is the important industrial example. Iron oxide is reduced in a blast furnace using carbon, and the process also makes use of carbon monoxide as a reducing agent. Zinc, lead, tin and copper are also extracted this way.
The reason this method is used wherever possible is cost: carbon in the form of coke is cheap and the process, though it requires high temperatures, uses far less energy than electrolysis.
Extraction by electrolysis
Metals more reactive than carbon cannot be extracted by carbon reduction, because carbon is not reactive enough to remove the oxygen from them. They must be extracted by electrolysis of the molten compound.
Aluminium is the standard example. Aluminium oxide is melted and electrolysed, with aluminium forming at the negative cathode as aluminium ions gain electrons, and oxygen forming at the positive anode.
Two practical details are examined. The aluminium oxide is mixed with cryolite to lower its melting point, which reduces the energy needed and therefore the cost. And the carbon anodes must be replaced regularly, because the oxygen produced reacts with them at the high temperature to form carbon dioxide.
Electrolysis is expensive because large amounts of energy are required both to melt the compound and to drive the current, which is why it is used only where carbon reduction is impossible. This explains why aluminium, despite being one of the most abundant elements in the Earth's crust, remained an expensive novelty until electrical power became cheap.
Biological methods of extraction
Copper-rich ores are becoming scarce, so alternative methods are used to obtain copper from low-grade ores that traditional extraction could not process economically. Both methods are examinable on the separate Chemistry course.
Phytomining uses plants grown on soil containing low concentrations of copper compounds. The plants absorb the compounds and concentrate them in their tissues. The plants are harvested and burned, and the ash contains the metal compounds in a much higher concentration than the original soil.
Bioleaching uses bacteria to break down low-grade ores, producing a solution called a leachate that contains metal compounds.
From either route, the copper can be obtained by displacement using scrap iron, which is cheap and effective since iron is more reactive than copper, or by electrolysis of the solution.
The advantages of these methods are that they avoid the need to mine, move and process very large quantities of rock, and they use less energy. The disadvantages are that both are slow processes, and phytomining depends on growing conditions.
Worked examples
Example 1: Deducing an order from observations (4 marks)
Three metals are tested. Metal A reacts vigorously with cold water. Metal B does not react with cold water but fizzes steadily with dilute acid. Metal C does not react with water or with acid. Place the metals in order of reactivity, most reactive first, and state which could be copper.
Metal A reacts with cold water, which only the most reactive metals do, so A is the most reactive. Metal B does not react with cold water but does react with dilute acid, placing it in the middle of the series above hydrogen. Metal C reacts with neither, placing it below hydrogen and therefore at the bottom.
The order is A, then B, then C. Metal C could be copper, since copper lies below hydrogen and reacts with neither cold water nor dilute acid.
Example 2: Explaining an extraction method (4 marks)
Explain why iron is extracted by heating its oxide with carbon but aluminium must be extracted by electrolysis.
Iron is less reactive than carbon. Carbon is therefore able to remove the oxygen from iron oxide, reducing it to iron while the carbon itself is oxidised. This method is used because it is relatively cheap, since carbon is inexpensive and the process uses less energy than electrolysis.
Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide. Aluminium must therefore be extracted by electrolysis of the molten oxide, in which aluminium ions gain electrons at the cathode. This is expensive because large quantities of energy are needed both to melt the compound and to pass the current.
Example 3: Writing the electron transfer in displacement (3 marks)
Zinc is added to copper sulfate solution. Describe what happens in terms of electrons and state which species is oxidised.
Each zinc atom loses two electrons to form a zinc ion with a charge of plus 2, so the zinc is oxidised. Each copper ion gains those two electrons to form a copper atom, so the copper ions are reduced and copper metal is deposited. The sulfate ions are spectator ions and take no part in the reaction. Because zinc is more reactive than copper, it loses electrons more readily, which is why the reaction proceeds in this direction and not the reverse.
Common mistakes and how to avoid them
The most frequent error is stating that a metal is extracted by electrolysis because it is very reactive, without referring to carbon. The comparison with carbon is the whole basis of the decision and must be stated.
Students often forget that carbon and hydrogen belong in the reactivity series. Without them, questions about extraction method and about reaction with acid cannot be answered.
In displacement questions, many candidates describe the colour change without explaining the electron transfer. Both are usually required, and the electron account carries more marks.
Another common slip is saying that copper does not react with acid because it is unreactive. The precise reason is that copper lies below hydrogen in the series, so it cannot displace hydrogen from the acid.
Finally, candidates frequently confuse phytomining with bioleaching. Phytomining uses plants that are burned to ash; bioleaching uses bacteria to produce a leachate solution.
Exam technique for "Chemical changes: reactivity series and extraction of metals"
Write out the reactivity series, including carbon and hydrogen, at the start of any question on this topic. It takes fifteen seconds and makes every subsequent part straightforward.
For extraction questions, state the metal's position relative to carbon first, then name the method, then give the reason. That order produces a complete answer every time.
When observations are given for unknown metals, sort them by the most demanding test first — reaction with cold water — then work down. Building the order step by step is more reliable than trying to see it whole.
For displacement, always identify which species loses electrons and which gains, and name the spectator ions if an ionic equation is requested.
Quick revision summary
Metals react by losing electrons to form positive ions, and the reactivity series ranks how readily they do so: potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper, silver, gold, with carbon and hydrogen included as reference points. Reaction with cold water identifies the most reactive, reaction with dilute acid the middle, and reaction with neither the least reactive, which lie below hydrogen. A more reactive metal displaces a less reactive one from solution, being oxidised as it loses electrons while the less reactive metal's ions are reduced; the anion is a spectator ion. Unreactive metals such as gold occur native, while most metals occur as compounds that must be reduced. Metals below carbon are extracted by heating the oxide with carbon, which is cheap; metals above carbon require electrolysis of the molten compound, which is expensive because of the energy needed, with cryolite added to lower aluminium oxide's melting point and carbon anodes replaced as they burn away. Low-grade copper ores are processed by phytomining, in which plants are grown and burned to ash, or bioleaching, in which bacteria produce a leachate, with the copper then recovered by displacement with scrap iron or by electrolysis.