Metals and materials · Process
Getting metals out of rocks
Half of a lump of malachite is copper, by mass, and no amount of melting will pour any of it out. So how do you get a metal out of a rock that is not letting go?
Start here
A lump of green-blue stone from a Cornish mine, and a length of copper pipe.
The stone is malachite. Roughly half of it, by mass, is copper. None of that copper looks like copper, behaves like copper, or conducts like copper — and no amount of melting the stone will pour any out.
The copper is in there. Why can it not simply be melted out?
Because the copper is chemically joined to other elements — in this stone, to oxygen and carbon. Melting changes a solid into a liquid and joins nothing and separates nothing. Getting the copper out means taking the oxygen off it, and that is a reaction, not a temperature.
A rock with enough of a metal compound in it to be worth digging up is an ore. Most ores are, or can be roasted into, the metal's oxide.
So extraction is nearly always the same job: take the oxygen away. Removing oxygen from a compound is called reduction, and there are only a few ways to do it. Which one works is decided by the reactivity series.
Reference · keep this one open
The carbon line
Above carbon
Potassium, sodium, calcium, magnesium, aluminium. These hold oxygen more tightly than carbon does, so carbon cannot take it. They need electricity.
Carbon
The line itself. Not a metal, and its place in the series is what decides every row above and below.
Below carbon
Zinc, iron, lead, copper. Carbon takes the oxygen and the metal is left. This is a furnace, and it is cheap.
Far below carbon
Silver and gold. Their oxides fall apart on heating alone, and gold is mostly found as the metal in the first place.
Carbon is a non-metal with a place in the series, and this is what the place is for. It can pull oxygen away from the oxide of anything below it, and from nothing above it.
Your turn · six deliveries
Six things arrive at the works. Find the method that frees the metal.
0 of 6 found
Pick a delivery, then pick a method.
A tray of river gravel with dull yellow flecks in it, from a claim in the Klondike.
It works.
The gold is already gold. Crushing the rock and washing it away leaves the flecks behind, because gold is much denser than the sand. Nothing has reacted — this is separation, not chemistry.
A tray of river gravel with dull yellow flecks in it, from a claim in the Klondike.
It does nothing.
There is no compound here to break up. Heating gold-bearing gravel gives you hot gravel.
A tray of river gravel with dull yellow flecks in it, from a claim in the Klondike.
It does nothing.
Carbon takes oxygen out of oxides. This gold is not joined to oxygen, so the carbon simply burns.
A tray of river gravel with dull yellow flecks in it, from a claim in the Klondike.
It does nothing.
Electricity splits compounds into their elements. Gold in gravel is already an element, so there is nothing to split.
A jar of dark powder, silver joined to oxygen.
It does nothing.
Washing separates one solid from another. It cannot separate silver from the oxygen it is chemically joined to — you end up with finer silver oxide.
A jar of dark powder, silver joined to oxygen.
It works.
Silver is so unreactive that it barely holds its oxygen at all: heat the oxide and it comes apart on its own into silver and oxygen gas. No second ingredient is needed.
['silver oxide', 'silver + oxygen']
A jar of dark powder, silver joined to oxygen.
It works — and it is the wrong tool.
Carbon does take the oxygen. But heat alone had already finished the job, and now there is spare carbon to clean out of the silver. Choosing this is not a mistake in chemistry; it is a mistake in cost.
A jar of dark powder, silver joined to oxygen.
It works — and nobody would pay for it.
Electricity would split the oxide. It is the most expensive method available, spent on the compound that needs the least.
A black powder, roasted from the green malachite in the hook.
It does nothing.
You get finer black powder. The copper is joined to oxygen and no amount of grinding parts them.
A black powder, roasted from the green malachite in the hook.
It does not work.
Copper oxide is untroubled by a strong flame. It glows while it is hot, and when it cools it is still black copper oxide.
A black powder, roasted from the green malachite in the hook.
It works.
Carbon is above copper in the series, so it takes the oxygen. Specks of pink-brown copper appear in the black mixture, and the gas coming off turns limewater cloudy — carbon dioxide.
['copper oxide + carbon', 'copper + carbon dioxide']
A black powder, roasted from the green malachite in the hook.
It works — and it is the wrong tool.
Electricity would free the copper. Carbon does the same job with a Bunsen burner and a test tube, and carbon is cheap.
A truckload of rusty-red rock: iron joined to oxygen, with sand and clay mixed in.
It does nothing.
Crushing is done first at every ironworks — it makes the rock easier to feed in. It frees no iron, because the iron is joined to oxygen.
A truckload of rusty-red rock: iron joined to oxygen, with sand and clay mixed in.
It does not work.
Iron holds its oxygen far too well for heat alone. Furnaces reached this temperature for centuries without producing iron from ore.
A truckload of rusty-red rock: iron joined to oxygen, with sand and clay mixed in.
It works.
Carbon takes the oxygen and molten iron collects at the bottom. On an industrial scale the carbon arrives as coke, and this reaction is what a blast furnace is for.
['iron oxide + carbon', 'iron + carbon dioxide']
A truckload of rusty-red rock: iron joined to oxygen, with sand and clay mixed in.
It works — and it is the wrong tool.
Electricity can free iron, and the world makes iron by the hundred million tonnes. Paying for electricity instead of coke would change the price of everything made of steel.
A white powder, roasted from zinc blende ore.
It does nothing.
A finer white powder, still zinc oxide. The oxygen is chemically joined on.
A white powder, roasted from zinc blende ore.
It does not work.
Zinc oxide does not give up its oxygen to heat. It goes yellow while hot and white again as it cools, and that is a change in the crystals, not a reaction.
A white powder, roasted from zinc blende ore.
It works.
Carbon takes the oxygen. Zinc is unusual here: it boils at the temperature the furnace runs at, so the metal leaves as a vapour and is condensed somewhere cooler.
['zinc oxide + carbon', 'zinc + carbon dioxide']
A white powder, roasted from zinc blende ore.
It works — and it is a judgement call.
Zinc is close to carbon in the series, and both routes are used in industry depending on the ore and the price of power. Carbon is the one to know for the rule.
A white powder purified from bauxite, the ore that most of Australia and Guinea ship out by the boatload.
It does nothing.
Bauxite is crushed and purified before anything else happens, and at the end of it you have this white powder: aluminium still joined to oxygen.
A white powder purified from bauxite, the ore that most of Australia and Guinea ship out by the boatload.
It does not work.
Aluminium holds oxygen more tightly than almost anything. Heat alone does not come close.
A white powder purified from bauxite, the ore that most of Australia and Guinea ship out by the boatload.
It does not work — and this is the important one.
Aluminium is above carbon in the reactivity series, so aluminium holds the oxygen more strongly than carbon can pull. A hotter furnace changes the speed of reactions that can happen, and this one cannot.
A white powder purified from bauxite, the ore that most of Australia and Guinea ship out by the boatload.
It works.
A very large current passed through the molten oxide tears it apart into aluminium and oxygen. Every aluminium object you have ever held was made this way, and the works are built next to power stations for a reason.
['aluminium oxide', 'aluminium + oxygen']
Already the metal — just separate it
Gold-bearing gravel
The least reactive metals of all sit in the ground uncombined, so the work is physical: crush, wash, pick out. No reaction is needed and none is possible.
Heat alone is enough
Silver oxide
Very unreactive metals hold oxygen so weakly that their oxides fall apart when warmed. Nothing has to be added at all.
Heat with carbon
Copper oxide, Iron ore, Zinc oxide
For every metal below carbon in the series, carbon takes the oxygen and the metal is left. This is the route for the metals humanity has had for thousands of years.
Electricity
Aluminium oxide
For every metal above carbon, nothing chemical on the shelf will take the oxygen away, and electricity does the separating instead. It is the expensive route, and for these metals it is the only one.
Six deliveries, four routes, one thing deciding.
Nothing in that list is about how hard the rock is or how deep the mine is. It is the reactivity series, read against carbon, deciding how much energy it takes to break the metal free.
Key fact
Heating a metal oxide with carbon takes the oxygen away and leaves the metal — but only for metals below carbon in the reactivity series. Metals above carbon have to be split out with electricity.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Any oxide gives up its oxygen to carbon if the furnace is hot enough.”
Hotter fixes a great many things in chemistry. Commit before you read on.
Aluminium oxide is the counter-example, and it is not a rare or awkward one — it is most of the world's aluminium. Aluminium is above carbon in the series, so it holds oxygen more tightly than carbon does. Carbon has nothing to pull with, and a hotter furnace does not change which of the two wants the oxygen more.
Heat gets a possible reaction going faster. It cannot make an impossible one possible. That is why aluminium was a rarity for most of history and became ordinary within about thirty years of cheap electricity.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these metals can be obtained from its oxide by heating with carbon?
Rung 2 · The one that catches people
Aluminium is one of the commonest metals in the Earth's crust, and in the 1850s it cost more than gold. Which statement explains why?
Rung 3 · Explain
Explain why iron can be obtained from its oxide in a furnace with carbon, but aluminium cannot, using the reactivity series in your answer.
Rung 4 · Take it somewhere new
A new metal M is discovered. Its oxide is unchanged when heated alone and unchanged when heated with carbon. Describe where M sits in the reactivity series and how it would have to be extracted.
Key note
An ore is a rock containing enough of a metal compound to be worth extracting, and most ores are metal oxides. Removing the oxygen is reduction. A metal below carbon in the reactivity series can be reduced by heating its oxide with carbon, which takes the oxygen and leaves the metal — this is how iron, zinc, copper and lead are obtained. A metal above carbon holds oxygen too tightly for carbon to remove, and is obtained by passing electricity through its molten compound. The few metals below carbon that are least reactive of all, such as gold, are found in the ground as the metal itself.
Going further
A blast furnace does not mostly work the way the bench above suggests. Coke burns to carbon dioxide, the carbon dioxide meets more hot coke and becomes carbon monoxide, and it is the carbon monoxide that does most of the work of taking oxygen off the iron oxide as it falls through. Solid carbon touching solid ore is a small part of it. The rule you have learned is right about WHICH metals and slightly simple about HOW.
Aluminium is expensive to win from its ore and cheap to use again. Melting down cans and remaking them takes roughly a twentieth of the electricity that extracting the same aluminium from bauxite would need, because the hard part — prising the oxygen off — has already been paid for once. It is the clearest case in chemistry of recycling being an energy decision rather than a tidiness one.
Before this lesson
Next in this unit
At GCSE this becomes
- Reduction and oxidation defined by electron transfer, the blast furnace equations, and electrolysis of aluminium oxide dissolved in molten cryolite.
Where to next
Ask Mr Badmus AI
Still not sure why a hotter furnace cannot help?
The bench is a simulation and is not a method. Reducing copper oxide with carbon is a standard school practical and the one thing in this unit a class is most likely to run: it needs a written risk assessment covering strong heating, a reducing mixture and hot residues that stay dangerous long after the flame is out.
Lesson content © MrBadmusAI.