Types of reaction · Process
Displacement
An iron nail in blue solution comes out plated with copper. Two metals swapped places — and which way round they swap is never a matter of chance.
Start here
Leave an iron nail in blue copper sulfate. Come back to a copper-plated nail and a pale green solution.
Nothing was added and nothing was heated. The nail is now furry and orange-brown. The blue has drained out of the solution and been replaced by a faint green. Weigh the nail: it has gained mass.
Where did the copper on the nail come from?
Out of the solution. The copper was dissolved in it all along — that is what made it blue — and the iron has taken its place, pushing the copper out as solid metal and dissolving in its stead. The green is iron sulfate, which was not there before. Two metals swapped positions, and the more reactive one won.
A displacement reaction is one metal taking the place of another in its compound. It happens in one direction only: a more reactive metal displaces a less reactive one, and never the other way round.
Which means displacement is also a measuring instrument. Put every metal into every other metal's solution and the results put them in order of reactivity — an order nobody had to be told.
Reference · keep this one open
The reactivity series
Most reactive at the top, least reactive at the bottom. A metal higher in this list will displace any metal below it, and will never be displaced by one below it. The four metals on the bench below are marked — and two non-metals are in the list because they take part in exactly the same way.
Your turn · sixteen combinations
Four metals, four solutions. Pick a cell, predict, then run it.
0 of 16 run
The four cells down the diagonal put a metal in its own solution, and they are worth running too — a test that cannot give a result is still worth having done once.
| Metal added to | magnesium sulfate | zinc sulfate | iron sulfate | copper sulfate |
|---|---|---|---|---|
| Magnesium | ||||
| Zinc | ||||
| Iron | ||||
| Copper |
Magnesium in magnesium sulfate
A piece of magnesium in a colourless solution of its own sulfate.
Magnesium in zinc sulfate
A piece of magnesium in a colourless solution of zinc sulfate.
Magnesium in iron sulfate
A piece of magnesium in a pale green solution of iron sulfate.
Magnesium in copper sulfate
A piece of magnesium in a blue solution of copper sulfate.
Zinc in magnesium sulfate
A piece of zinc in a colourless solution of magnesium sulfate.
Zinc in zinc sulfate
A piece of zinc in a colourless solution of its own sulfate.
Zinc in iron sulfate
A piece of zinc in a pale green solution of iron sulfate.
Zinc in copper sulfate
A piece of zinc in a blue solution of copper sulfate.
Iron in magnesium sulfate
A piece of iron in a colourless solution of magnesium sulfate.
Iron in zinc sulfate
A piece of iron in a colourless solution of zinc sulfate.
Iron in iron sulfate
A piece of iron in a pale green solution of its own sulfate.
Iron in copper sulfate
A piece of iron in a blue solution of copper sulfate.
Copper in magnesium sulfate
A piece of copper in a colourless solution of magnesium sulfate.
Copper in zinc sulfate
A piece of copper in a colourless solution of zinc sulfate.
Copper in iron sulfate
A piece of copper in a pale green solution of iron sulfate.
Copper in copper sulfate
A piece of copper in a blue solution of its own sulfate.
Predict before you run it.
Nothing, and nothing could have happened.
The metal and the dissolved metal are the same element, so there is nobody to displace. Blank cells down the diagonal are not failed experiments; they are the shape of the question.
A reaction. Grey zinc appears on the metal.
There is no colour to watch — magnesium sulfate is as colourless as zinc sulfate — but the magnesium dissolves in place of the zinc and the mixture warms slightly. Magnesium is more reactive than zinc, so it takes its place.
magnesium + zinc sulfatemagnesium sulfate + zinc
A reaction. Dark grey iron appears on the metal.
The pale green solution fades as the magnesium dissolves in place of the iron, and the mixture warms slightly. Magnesium is more reactive than iron, so it takes its place.
magnesium + iron sulfatemagnesium sulfate + iron
A reaction. Orange-brown copper appears on the metal.
The blue solution fades as the magnesium dissolves in place of the copper, and the mixture warms slightly. Magnesium is more reactive than copper, so it takes its place.
magnesium + copper sulfatemagnesium sulfate + copper
Nothing, after twenty minutes and after a week.
Zinc is less reactive than magnesium, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, and nothing could have happened.
The metal and the dissolved metal are the same element, so there is nobody to displace. Blank cells down the diagonal are not failed experiments; they are the shape of the question.
A reaction. Dark grey iron appears on the metal.
The pale green solution fades as the zinc dissolves in place of the iron, and the mixture warms slightly. Zinc is more reactive than iron, so it takes its place.
zinc + iron sulfatezinc sulfate + iron
A reaction. Orange-brown copper appears on the metal.
The blue solution fades as the zinc dissolves in place of the copper, and the mixture warms slightly. Zinc is more reactive than copper, so it takes its place.
zinc + copper sulfatezinc sulfate + copper
Nothing, after twenty minutes and after a week.
Iron is less reactive than magnesium, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, after twenty minutes and after a week.
Iron is less reactive than zinc, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, and nothing could have happened.
The metal and the dissolved metal are the same element, so there is nobody to displace. Blank cells down the diagonal are not failed experiments; they are the shape of the question.
A reaction. Orange-brown copper appears on the metal.
The blue solution fades as the iron dissolves in place of the copper, and the mixture warms slightly. Iron is more reactive than copper, so it takes its place.
iron + copper sulfateiron sulfate + copper
Nothing, after twenty minutes and after a week.
Copper is less reactive than magnesium, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, after twenty minutes and after a week.
Copper is less reactive than zinc, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, after twenty minutes and after a week.
Copper is less reactive than iron, so it cannot push it out of its compound. No amount of waiting, warming or stirring changes that — the reaction has no way to run.
Nothing, and nothing could have happened.
The metal and the dissolved metal are the same element, so there is nobody to displace. Blank cells down the diagonal are not failed experiments; they are the shape of the question.
Now look at the shape of the table.
Every reaction is on one side of the diagonal and every blank is on the other. That is not a coincidence about these four metals — it is what an order looks like when you draw it as a grid. Read the rows: magnesium displaced everything, zinc displaced two, iron displaced one, copper displaced nothing.
magnesium · zinc · iron · copper — most reactive to least
Compare that with the reference list above: your four metals sit in exactly that order. The list is not something to be believed — sixteen test tubes have just reproduced the part of it you can reach, and the same method extended down the list is where the whole of it came from.
Key fact
A more reactive metal displaces a less reactive metal from its compound — and never the reverse. Which metal wins tells you their order of reactivity.
Three consequences
One rule, three places it decides the answer
A works stores copper sulfate solution. Would a galvanised steel tank do the job?
No. Zinc and iron are both more reactive than copper, so the tank itself would displace copper out of the solution — plating the tank, weakening it, and ruining the solution. Copper sulfate is stored in plastic or glass.
Iron is obtained from iron oxide by heating it with carbon in a blast furnace. Would the same trick get aluminium out of aluminium oxide?
No. Carbon can displace iron because carbon sits above iron in the reactivity order — but aluminium is above carbon, so carbon cannot displace it. Aluminium needs electricity, which is why it was the last common metal to be discovered.
An unknown metal X is put into copper sulfate and copper appears. It is then put into magnesium sulfate and nothing happens. Can you place X in the order?
Yes — between the two. X displaced copper, so it is above copper. It failed to displace magnesium, so it is below magnesium. Two tubes and X is bracketed, which is exactly how the reactivity series was assembled.
Think again
“The copper on the nail came out of the nail — the iron turned into copper on the outside.”
The copper is on the nail, and nothing else was put in the tube. Commit before you read on.
Iron atoms cannot become copper atoms — reactions rearrange atoms and never turn one element into another, and no beaker of blue solution overturns it. The copper came from the solution, where it was dissolved and invisible. The evidence is the colour: the blue drained away because the dissolved copper left, and the new green is dissolved iron that was not there before.
And the swap is exact. Weigh the nail and the mass gained is copper deposited; weigh what is left and iron has gone into solution. Two metals changed places. Neither changed identity.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What happens in a displacement reaction?
Rung 2 · The one that catches people
A copper wire is left in zinc sulfate solution for a week and nothing happens. What does that tell you?
Rung 3 · Explain
An iron nail is left in blue copper sulfate solution. Describe everything you would observe, and explain each observation in terms of which metal is where.
Rung 4 · Take it somewhere new
You are given an unknown metal and solutions of magnesium sulfate, zinc sulfate, iron sulfate and copper sulfate. Design the smallest set of tests that would place the unknown metal in the reactivity order, and say how you would know when you had finished.
Key note
In a displacement reaction a more reactive metal takes the place of a less reactive metal in its compound, and never the other way round. The displaced metal appears as a solid, and the solution changes colour whenever the two dissolved metals are different colours. Testing every metal against every solution puts them in order of reactivity — which is how the reactivity series was built.
Going further
Displacement is how railway track is welded in the middle of nowhere. A crucible of iron oxide mixed with aluminium powder is lit; aluminium is more reactive than iron, so it takes the oxygen and the iron is displaced as a white-hot liquid that pours straight into the gap between the rails. It reaches around 2500 °C, needs no power supply, and is the same rule you have just measured with test tubes and a wire.
It is also why some metals were known to the ancient world and others were not. Gold and copper are unreactive enough to be found as the metal, so they were used thousands of years before anyone understood chemistry. Iron needs displacing from its ore with carbon in a furnace, which is a Bronze-Age-ending piece of technology. Aluminium is more reactive than carbon, so no furnace will do it — it stayed undiscovered until electricity could be used to break the ore apart, which is why aluminium was once more valuable than silver.
Before this lesson
Next in this unit
At GCSE this becomes
- The reactivity series in full, extracting metals with carbon and by electrolysis, and displacement as electron transfer.
Where to next
Ask Mr Badmus AI
Still not sure why half the grid is empty?
Thermite is an industrial process, not a bench reaction. It reaches temperatures no school equipment can hold and is never set up in a school laboratory.
Lesson content © MrBadmusAI.