Metals and materials · Model
Predicting displacement
An iron nail goes into blue copper sulfate and comes out coated in copper. Nobody added any copper. So where did it come from — and could you have said so in advance?
Start here
An iron nail is dropped into blue copper sulfate solution and left for ten minutes.
It comes out coated in soft pink-brown copper, and the blue of the solution has faded towards pale green. Nobody added copper to the nail. Nobody took the nail out and dipped it in anything.
Where did the copper on the nail come from?
It came out of the solution. The blue colour was copper joined to sulfate; iron is the more reactive of the two metals, so the iron took the sulfate and the copper was left with nowhere to go but out, as the metal. The fading blue and the growing brown coat are the same event seen twice.
That is a displacement reaction: a more reactive metal takes the place of a less reactive one in its compound. The less reactive metal is pushed out — displaced — as the element.
The useful part is that it is not a surprise. Both metals have a place in the reactivity series, and comparing the two places tells you the answer before you run anything.
Reference · keep this one open
Higher displaces lower. Lower never displaces higher.
Find the metal you are adding, find the metal inside the compound, and see which one is nearer the top. Carbon counts here too, even though it is not a metal.
Your turn · eight proposals
Commit to an answer, then run it.
0 of 8 run
- Magnesium
- Carbonnon-metal
- Zinc
- Iron
- Copper
- Silver
Magnesium ribbon in copper sulfate solution
A cleaned magnesium ribbon stood in a tube of blue copper sulfate solution.
It happens.
The blue drains out of the solution within a minute, a soft brown-pink solid builds on the ribbon, and the tube becomes hot enough to notice.
['magnesium + copper sulfate', 'magnesium sulfate + copper']
Copper wire in magnesium sulfate solution
A coil of clean copper wire stood in a tube of colourless magnesium sulfate solution.
Nothing happens.
The wire stays bright and the solution stays colourless. It looks the same after a week on the windowsill.
Zinc granules in copper sulfate solution
A few zinc granules dropped into blue copper sulfate solution.
It happens.
The blue fades, the granules go dull brown-pink, and the tube warms.
['zinc + copper sulfate', 'zinc sulfate + copper']
Iron filings in zinc sulfate solution
Iron filings tipped into colourless zinc sulfate solution.
Nothing happens.
The filings stay grey and the solution stays colourless. Nothing collects, nothing fades.
Copper wire in silver nitrate solution
A coil of clean copper wire stood in colourless silver nitrate solution.
It happens.
Grey needles of silver grow along the wire, and the solution turns a faint blue as copper goes into it.
['copper + silver nitrate', 'copper nitrate + silver']
Silver wire in copper sulfate solution
A length of silver wire stood in blue copper sulfate solution.
Nothing happens.
The wire stays bright, the blue stays exactly as blue as it started.
Zinc granules in iron sulfate solution
Zinc granules dropped into pale green iron sulfate solution.
It happens.
The pale green fades and a dark grey deposit collects on the granules. It is slower and far less obvious than the copper ones, and it does happen.
['zinc + iron sulfate', 'zinc sulfate + iron']
Carbon powder heated with copper oxide
Black copper oxide mixed with carbon powder in a test tube and heated strongly.
It happens.
The mixture glows, and when it cools there are specks of pink- brown copper in the black. The gas given off turns limewater cloudy.
['carbon + copper oxide', 'copper + carbon dioxide']
Sort the eight by what happened and the rule is the only thing left.
Every one that happened had the added element above the one in the compound. Every one that did nothing had it below. There is no third column — no “slowly”, no “a bit”, no “eventually”.
Key fact
A more reactive metal displaces a less reactive one from its compound. The other way round does not happen — not slowly, not with heating, not eventually.
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 metal will displace any other if it is left long enough.”
It is a reasonable instinct — most slow things are just slow. Commit before you read on.
Time changes how fast a reaction goes. It cannot change whether the reaction goes at all. Magnesium holds sulfate more strongly than copper does, so a copper strip has nothing to offer and nothing happens — in ten minutes, in ten years.
This is the difference between slow and impossible, and it is worth being precise about. Iron in dilute acid is slow: sparse bubbles, but it is going. Copper in magnesium sulfate is not slow. It is not happening.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these is a displacement reaction?
Rung 2 · The one that catches people
Copper wire in silver nitrate grows grey needles. Silver wire in copper sulfate does nothing. Which statement explains the pair?
Rung 3 · Explain
A strip of zinc is left in iron sulfate solution and very little seems to happen. Explain whether a reaction is taking place, and how you would decide.
Rung 4 · Take it somewhere new
Steel ships carry blocks of zinc bolted to the hull below the waterline, and the blocks are replaced every few years. Explain what the zinc is doing and why zinc was chosen.
Key note
In a displacement reaction a more reactive metal takes the place of a less reactive metal in its compound, and the less reactive metal is released as the element. Comparing the two positions in the reactivity series predicts the result, including when the result is no reaction. Carbon obeys the same rule: it displaces the metals below it from their oxides, which is why it is placed in the series at all.
Going further
The most violent displacement in ordinary use is aluminium powder with iron oxide. Aluminium is well above iron, takes the oxygen, and leaves iron so hot that it runs as a liquid — which is how lengths of railway track are welded together in the field, with no power supply for miles. It is not a school experiment at any scale and nothing about it is worth improvising.
Ships carry blocks of zinc bolted to the hull below the waterline. Zinc is above iron in the series, so when seawater attacks the steel the zinc reacts instead — it is the more reactive metal and it goes first. The blocks are eaten away and replaced every few years, which is far cheaper than replacing a hull. The whole design rests on one fact about the order and no other chemistry at all.
Before this lesson
Next in this unit
At GCSE this becomes
- Displacement as electron transfer, oxidation and reduction, ionic half equations, and the series used to predict cell voltages.
Where to next
Ask Mr Badmus AI
Still not sure why waiting longer cannot help?
The deck is a simulation. The copper displacements are a standard class practical and need a written risk assessment; the aluminium reaction described under “Going further” is not done in schools at any scale.
Lesson content © MrBadmusAI.