Metal reactivity

The reactivity series is a list of metals arranged in order of decreasing reactivity — the most reactive metal at the top, the least reactive at the bottom. "Reactivity" means how readily a metal reacts (with water, acid and oxygen). The more easily a metal reacts, the higher it sits.

Two non-metals, carbon (C) and hydrogen (H), are slotted into the list as reference points. They are not metals, but knowing where they sit helps you decide how a metal is extracted from its ore (covered in the extraction subtopic).

The standard 4WSD0-1C order:

K, Na, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au

A memory mnemonic. Take the first letters — K, Na, Ca, Mg, Al, C, Zn, Fe, H, Cu, Ag, Au — and learn a sentence such as:

"Please Send Cats Monkeys And Cute Zebras In Cages Securely Guarded."

(Potassium, Sodium, Calcium, Magnesium, Aluminium, Carbon, Zinc, Iron, Hydrogen, Copper, Silver, Gold.)

Why bother with an order? Once you know the order you can predict what a metal will do — whether it reacts with cold water, whether it fizzes in acid, how easily it tarnishes — without testing it. That is what makes the series so useful in exams.

How vigorously a metal reacts with water tells you how high up the series it is. There are three bands.

Band 1 — react with COLD water (the most reactive metals).

K, Na and Ca react with cold water to form a metal hydroxide + hydrogen gas:

• 2K + 2H₂O → 2KOH + H₂ (very violent; the hydrogen often ignites with a lilac flame)
• 2Na + 2H₂O → 2NaOH + H₂ (vigorous; the metal melts into a ball and fizzes across the surface)
• Ca + 2H₂O → Ca(OH)₂ + H₂ (steady stream of bubbles; cloudy solution)

The fizzing is the hydrogen being released, and the solution becomes alkaline (a hydroxide forms).

Band 2 — react only with STEAM (the middle metals).

Magnesium reacts only very slowly with cold water but reacts quickly with steam. Zinc and iron do not react with cold water at all but do react with steam, forming a metal oxide + hydrogen:

• Mg + H₂O → MgO + H₂ (with steam)
• Zn + H₂O → ZnO + H₂ (with steam)
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (with steam)

Band 3 — NO reaction with water (the least reactive metals).

Cu, Ag and Au do not react with cold water or steam. This is exactly why copper is used for water pipes and gold and silver survive in nature.

Watch the product. Cold-water reactions give a hydroxide; steam reactions give an oxide. Both give off hydrogen.

Dropping a metal into dilute acid (such as dilute hydrochloric acid or dilute sulfuric acid) is the most common school test for reactivity.

The general reaction. A reactive metal reacts with a dilute acid to give a salt + hydrogen:

$\text{metal} + \text{acid} \rightarrow \text{salt} + \text{hydrogen}$

Examples:

• Mg + 2HCl → MgCl₂ + H₂ (vigorous fizzing)
• Zn + 2HCl → ZnCl₂ + H₂ (steady fizzing)
• Fe + 2HCl → FeCl₂ + H₂ (slow, gentle fizzing; pale green solution)

You see bubbles of hydrogen gas — and the rate of fizzing shows the reactivity:

Metal	Reaction with dilute acid
K, Na, Ca	Dangerously violent — too reactive to test safely
Mg	Vigorous fizzing — fast stream of bubbles
Zn	Steady fizzing
Fe	Slow, gentle fizzing
Cu, Ag, Au	No reaction — these sit below hydrogen in the series

The rule. A metal reacts with dilute acid only if it is above hydrogen in the series. Cu, Ag and Au are below H, so they do not react — no fizzing at all. This is a key dividing line.

Confirming the gas. The gas given off is hydrogen: collect it and hold a lighted splint to it — it burns with a squeaky pop.

Metals also react with oxygen to form metal oxides — and again the more reactive the metal, the more readily it reacts.

$\text{metal} + \text{oxygen} \rightarrow \text{metal oxide}$

Metal	Behaviour with oxygen
K, Na	Burn very readily; tarnish almost instantly in air (stored under oil)
Mg	Burns with a brilliant white flame → white magnesium oxide, MgO
Al	Surface oxidises quickly to form a protective oxide layer
Zn, Fe	Oxidise slowly; burn only when heated/powdered
Cu	Goes black slowly on heating (copper(II) oxide, CuO)
Ag	Tarnishes very slowly
Au	Does not react with oxygen — stays shiny

Why gold is used for jewellery. Gold is at the very bottom of the series — it is unreactive. It does not react with water, acid or oxygen, so it does not tarnish or corrode and stays bright and shiny for thousands of years. That is exactly why gold (and to a lesser extent silver) is prized for jewellery, coins and electrical contacts.

Putting the three tests together. No single test orders every metal — you combine all three:

• Water separates the top band (K, Na, Ca react with cold water; Mg/Zn/Fe need steam; Cu/Ag/Au nothing).
• Acid draws the line at hydrogen (everything above H fizzes; Cu/Ag/Au below H do not).
• Oxygen ranks how readily metals burn or tarnish.

The metal that reacts most vigorously across all three goes highest; the one that barely reacts goes lowest. That gives the full order: K, Na, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au.