Reactivity series: K to Au
Metals rank by decreasing reactivity: K, Na, Ca, Mg, Al, then (C), Zn, Fe, then (H), Cu, Ag, Au. Carbon and hydrogen are non-metal reference points. A metal higher up reacts more vigorously. Any metal above hydrogen reacts with dilute acids to release hydrogen; any metal above carbon must be extracted by electrolysis, because carbon cannot reduce its oxide. K, Na and Ca react with cold water, while Cu, Ag and Au resist water, steam and acid.
Reactions with water, steam and acid
Each reaction needs a minimum reactivity. Cold water reacts only with K, Na and Ca, giving a metal hydroxide, hydrogen and an alkaline solution. Steam reacts with Mg, Al, Zn and Fe to give a metal OXIDE plus hydrogen (not a hydroxide — it decomposes at that heat); iron gives Fe₃O₄. Dilute acids react with every metal above hydrogen, so Cu, Ag and Au do not react. Watch for fizzing and a squeaky pop from hydrogen.
Displacement and electron transfer
A more reactive metal displaces a less reactive one from its salt solution; a less reactive metal gives no reaction. Adding zinc to copper(II) sulfate deposits pink-brown copper, fades the blue colour and warms the mixture. (Extended) By electron transfer, the reactive metal is oxidised (loses electrons) and is the reducing agent, while the metal ion is reduced (gains electrons) and is the oxidising agent. For zinc: Zn → Zn²⁺ + 2e⁻; Cu²⁺ + 2e⁻ → Cu.
Drawn from real examiner reports.
Observations, not product names
Asked what is SEEN in a displacement, candidates write product names ("zinc sulfate forms") and score zero. Observation marks need sensory changes: the solution colour fading, a coloured solid coating the metal, and the mixture warming. The same holds for metal + acid (fizzing, solid dissolving). Never give a formula or product name as an observation.
W22 P31 Q6(a); W22 P32 Q6(a) — product names given where observable changes are required.
Oxidation vs reduction defined
A frequent trap is the opposite definition — reduction when oxidation is asked, or vice versa — and vague answers like "oxygen is produced" earn nothing. Core uses oxygen: oxidation is gain of oxygen, reduction is loss. (Extended) also accepts electrons: oxidation is loss of electrons, reduction is gain. State it precisely for the tier; OIL RIG is only a memory aid.
W22 P31 Q4(b)(ii); W22 P32 Q4(b)(ii) — opposite or vague oxidation/reduction definitions.
Rank by displacement, not speed
When deducing an order, candidates rank metals by how fast one fizzes in acid instead of by displacement. Displacement is the decisive test: if metal A deposits metal B from a solution of B's salt, then A is more reactive than B. Check the direction each time — "no reaction" means the added metal is the LESS reactive one. State each conclusion as a comparison.
Hydroxide (water) vs oxide (steam)
The product depends on the reactant. A metal in COLD water gives a metal hydroxide plus hydrogen; the SAME metal in steam gives a metal OXIDE plus hydrogen, because hydroxides decompose at steam temperatures. So MgO from magnesium in cold water is wrong, and Mg(OH)₂ from magnesium in steam is wrong. The word "hydroxide" signals cold water; "oxide" signals steam.
Check which metal is higher first
Before predicting a displacement, confirm the added metal is ABOVE the salt's metal in the series. A copper rod in zinc sulfate gives NO reaction, because copper is below zinc. Candidates often assume any two metals react, or reverse the direction. If the added metal is lower, record "no reaction" rather than inventing products.
(Extended) Reducing vs oxidising agent
Candidates swap the two agent labels. The metal that is OXIDISED (loses electrons) is the REDUCING agent — it reduces the other species. The ion that is REDUCED (gains electrons) is the OXIDISING agent. So in zinc plus copper(II) sulfate, zinc is the reducing agent and Cu²⁺ is the oxidising agent.
S22 P41 Q7(d)(ii) — candidates struggle to identify which species is oxidised in a displacement.
Earn observation marks first
Reactivity questions usually ask for observations before the equation, so bank those marks first — they need no calculation: state the colour change, the solid forming on the metal and the temperature rise. Then write the balanced equation, adjusting coefficients not formulae.
Write the series, then locate both metals
Begin every reactivity question by writing the series from memory: K, Na, Ca, Mg, Al, (C), Zn, Fe, (H), Cu, Ag, Au. Mark where the two species sit, then read off the outcome: an acid reaction needs a metal above H; a displacement needs the added metal above the salt's metal.
(Extended) Half-equations, then add
Write the oxidation half-equation (metal → ion + electrons) and the reduction half-equation (ion + electrons → metal), then add so electrons cancel to give the ionic equation. The oxidised species is the reducing agent; the reduced species the oxidising agent.
The reactivity series is a list of metals (and the non-metal references carbon and hydrogen) ranked from most reactive to least reactive.
Order to memorise (Core):
Mnemonic: Knife Not Carefully Mgindles Always Cuts Znipping Fern Heads; Cut Ageing Audience.
Key definitions:
Displacement reaction: A chemical reaction in which a more reactive element takes the place of a less reactive element in a compound. The more reactive metal pushes the less reactive metal out of its salt solution.
Oxidation (Core definition): Gain of oxygen (or loss of hydrogen). Reduction (Core definition): Loss of oxygen (or gain of hydrogen).
(Extended) Oxidation: Loss of electrons. (Extended) Reduction: Gain of electrons.
Reducing agent (Extended): A substance that causes reduction in another substance, and is itself oxidised. Oxidising agent (Extended): A substance that causes oxidation in another substance, and is itself reduced.
Write a balanced symbol equation for the reaction of zinc with dilute sulfuric acid.
Write a balanced symbol equation, including state symbols, for the reaction of iron with copper(II) sulfate solution. [3]
