Metals and materials · Classify
The reactivity series
Potassium attacks cold water hard enough to set fire to its own hydrogen. Copper carries cold water through houses for fifty years and does not change. Both are metals — so what is the difference made of?
Start here
Three beakers of cold water. A pea-sized piece of potassium, a lump of calcium, and a coil of copper wire.
The potassium fizzes so hard it skids across the surface and the gas it gives off catches fire. The calcium sinks, streams bubbles and turns the water cloudy. The copper does nothing — and copper pipes carry cold water in houses for fifty years without changing at all.
All three are metals. So why does one of them attack water and another ignore it?
Because reactivity is a property of the element, like melting point, and it differs from metal to metal. It is not about how hard, how heavy or how shiny the metal is. Test enough metals against the same thing and the differences line up into one order — and that order holds for every reaction, not just the one you tested.
A metal's reactivity is how readily it takes part in a chemical reaction. Put the metals in order, most reactive first, and you have the reactivity series.
The order is settled by evidence, and two tests do most of the work: cold water, and dilute acid. A metal high in the series manages both. One in the middle does nothing in water but reacts with acid. One at the bottom does neither.
Reference · keep this one open
Most reactive at the top, least reactive at the bottom
1 · Potassium (K)
stored under oil
2 · Sodium (Na)
stored under oil
3 · Calcium (Ca)
fizzes in cold water
4 · Magnesium (Mg)
barely touches cold water; fizzes in acid
5 · Aluminium (Al)
high in the order, but an oxide layer hides it
6 · Carbon (C)
not a metal — and it belongs here anyway, because it takes oxygen from the oxides of everything below it
7 · Zinc (Zn)
needs acid to get going
8 · Iron (Fe)
slow even in acid
9 · Lead (Pb)
barely reacts with acid
10 · Copper (Cu)
neither water nor acid touches it
11 · Silver (Ag)
used for contacts and mirrors because it stays as it is
12 · Gold (Au)
found in the ground as the metal itself
Nobody was told this order. It was assembled by doing what you are about to do — putting metals into the same liquids and recording what happened. The bench below covers six of these twelve; the same method extended down the list produces the rest of it. Carbon is in the list because it can take oxygen away from the oxides of every metal below it, which is how those metals are got out of the ground.
Your turn · twelve tubes
Six metals, two liquids. Say what will happen, then look.
0 of 12 read
| Metal | Cold water | Dilute acid |
|---|---|---|
| PotassiumA piece the size of a pea, freshly cut | ||
| CalciumA small lump | ||
| MagnesiumA cleaned ribbon | ||
| ZincA few granules | ||
| IronIron filings | ||
| CopperA coil of wire |
Say it before you look.
Pick a tube.
Potassium in a beaker of cold water
It reacts. Violent.
It fizzes hard enough to skid across the surface, and the hydrogen it releases catches fire with a lilac flame.
potassium + water potassium hydroxide + hydrogen
Teacher demonstration only, behind a safety screen, with the smallest piece that can be cut.
Potassium in a test tube of dilute hydrochloric acid
This one is not done.
This one is not done, at any concentration. Potassium is violent with water alone, and dilute acid is mostly water, so the reaction would be faster still. Potassium’s place at the top of the order is already settled by the water test — there is nothing left for the acid to tell you.
Calcium in a beaker of cold water
It reacts. Vigorous.
Bubbles stream off the whole surface and the water turns cloudy white as calcium hydroxide forms.
calcium + water calcium hydroxide + hydrogen
Calcium in a test tube of dilute hydrochloric acid
It reacts. Vigorous.
It fizzes hard enough to warm the tube, and the bubbles come too fast to count.
calcium + hydrochloric acid calcium chloride + hydrogen
Magnesium in a beaker of cold water
Nothing worth watching.
Almost nothing. After several minutes a few tiny bubbles cling to the ribbon — the reaction is real but far too slow to watch.
Magnesium in a test tube of dilute hydrochloric acid
It reacts. Vigorous.
A fast stream of bubbles, the tube becomes warm to hold, and the ribbon disappears.
magnesium + hydrochloric acid magnesium chloride + hydrogen
Zinc in a beaker of cold water
Nothing worth watching.
Nothing. The granules sit on the bottom unchanged for as long as you care to watch.
Zinc in a test tube of dilute hydrochloric acid
It reacts. Steady.
A steady stream of small bubbles from every granule.
zinc + hydrochloric acid zinc chloride + hydrogen
Iron in a beaker of cold water
Nothing worth watching.
Nothing you can see. Left damp for days it rusts — but that is a slow reaction with water and air together, not with water alone.
Iron in a test tube of dilute hydrochloric acid
It reacts. Slow.
Sparse bubbles that take a while to get going, and the tube barely warms.
iron + hydrochloric acid iron chloride + hydrogen
Copper in a beaker of cold water
Nothing worth watching.
Nothing. Copper carries cold water in houses for fifty years and comes out the same colour.
Copper in a test tube of dilute hydrochloric acid
Nothing worth watching.
Nothing. No bubbles, no warming, no change in the acid — and no amount of waiting will start it.
Fizzes in cold water
Potassium, Calcium
Water alone is enough. Potassium and calcium are the top of the series, and both have to be kept away from air and damp.
Needs acid before much happens
Magnesium, Zinc, Iron
Cold water gives nothing worth watching; acid gets a real reaction. Magnesium, zinc and iron are the middle of the series, and the fizzing gets weaker the further down you go.
Neither liquid touches it
Copper
Nothing in water, nothing in acid. Copper is the bottom of the series here — the kind of metal we leave outdoors, run water through and make coins from.
Twelve tubes, three answers.
Nothing on this bench sorted the metals by how hard or how heavy they were. Magnesium bends between your fingers and beats iron; copper is soft and beats nothing. Reactivity is its own property, and the only way to find it is to put metals into the same liquid and watch.
Read the three bands as one list and they give an order: potassium, calcium, magnesium, zinc, iron, copper. The same method run down the rest of the list produces the whole series.
Key fact
The reactivity series is the metals in order of how readily they react. It is one order, fixed by evidence, and it predicts every reaction they take part in — not just the test that established it.
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
“A metal that does nothing in cold water is unreactive.”
One test, one result, one conclusion. Commit before you read on.
Zinc did nothing in cold water. In dilute acid the same granules fizz steadily, and zinc will pull the oxygen off copper oxide if you heat the two together. Unreactive is not a word you can earn from one liquid.
This is why the series needs more than one test. Water separates the top of the list; acid separates the middle; the bottom three ignore both and have to be sorted by other reactions altogether. “Nothing happened” tells you where a metal is not, and that is still evidence.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these lists the six metals from the bench in order, most reactive first?
Rung 2 · The one that catches people
A steel bicycle frame holds up a rider. A strip of magnesium bends between your fingers. Which is the more reactive metal?
Rung 3 · Explain
Zinc granules sit unchanged in a beaker of cold water. Explain why this does not show that zinc is unreactive, and describe one test that would place zinc properly.
Rung 4 · Take it somewhere new
You are given an unknown metal M. It does not react with cold water, it fizzes gently in dilute acid, and it is displaced from its sulfate by zinc. Say where M sits in the series and justify each part of your answer.
Key note
The reactivity series runs potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, lead, copper, silver, gold. Potassium, sodium and calcium react with cold water. Magnesium, zinc and iron need dilute acid before much happens. Copper, silver and gold react with neither. Carbon is a non-metal placed in the same order, below aluminium and above zinc, because that is where its power to take oxygen from a metal oxide puts it.
Going further
Potassium and sodium are kept in jars under oil, and the oil is doing something specific: it keeps air and water vapour off the surface. Left out on a bench, a cut piece of sodium goes dull within seconds as it reacts with the air. A bottle of copper turnings needs no oil at all. How a substance has to be stored is a clue to where it sits in the series.
Aluminium is the awkward one. Its position is high — just below magnesium — but a saucepan made of it can be filled with cold water and boiled with nothing happening at all. The moment aluminium meets air it grows a tough, invisible layer of aluminium oxide, and the water never reaches the metal underneath. The position is about the element; the everyday behaviour is about the coating. This is worth remembering, because it is the reason aluminium is used for things that get wet.
Before this lesson
Next in this unit
At GCSE this becomes
- Reactivity explained as how readily a metal atom loses electrons, oxidation and reduction written as half equations, and the series extended to include hydrogen so that reactions with acid can be ordered too.
Where to next
- Next: Predicting displacement
- Previous: Metal and non-metal oxides
The periodic table
Ask Mr Badmus AI
Still not sure why “nothing happened” counts as evidence?
This bench is a simulation of a demonstration and is not a method. Potassium and calcium in water, and the four metals in dilute hydrochloric acid, all need a written risk assessment before anything is run in a room. Potassium is a teacher demonstration only, behind a screen, with the smallest piece that can be cut — and it is never put into acid at any concentration.
Lesson content © MrBadmusAI.