Displacement reactions for GCSE Chemistry
A displacement reaction happens when a more reactive element pushes a less reactive element out of its compound. The classic example: A piece of zinc dropped into a blue copper sulfate solution turns the solution colourless and coats the zinc with pink-brown copper. Zinc is more reactive, so it takes the sulfate from copper and the copper drops out as a solid.
This guide covers the AQA reactivity series, metal and halogen displacement, the ionic equations examiners want, and the practical observations you need for the higher-tier paper.
More reactive wins
A more reactive element always displaces a less reactive one from its compound. This rule works for metals and halogens alike.
Use the reactivity series
Memorise the metal reactivity series and the halogen group order. Both are non-negotiable for the AQA exam.
Ionic equations matter
Higher tier requires you to write the ionic equation, identify spectator ions and explain electron transfer.
What is a displacement reaction?
A displacement reaction is a chemical reaction in which a more reactive element takes the place of a less reactive element in a compound. The general pattern looks like this: A + BC → AC + B, where A is more reactive than B.
Displacement reactions are a type of redox reaction. The more reactive element loses electrons (oxidation) and the less reactive element gains electrons (reduction). At GCSE you mostly meet two flavours: Metal displacement and halogen displacement.
Displacement vs decomposition A displacement reaction needs two reactants and produces two products. A decomposition reaction needs only one reactant which breaks down into simpler products. Mixing them up is a common Paper 1 trap. Look for the second reactant before deciding.
The reactivity series
The reactivity series is the order of metals from most to least reactive. Memorising it is the fastest way to predict whether a displacement reaction will work. AQA expects you to know the standard list of metals plus the non-metals carbon and hydrogen as comparison points.
| Position | Metal | Memory phrase fragment |
|---|---|---|
| Most reactive | Potassium (K) | Please |
| Sodium (Na) | Stop | |
| Calcium (Ca) | Calling | |
| Magnesium (Mg) | Maybe | |
| Aluminium (Al) | Aunty | |
| (Carbon – non-metal) | C | Can |
| Zinc (Zn) | Zap | |
| Iron (Fe) | Five | |
| (Hydrogen – non-metal) | H | Hungry |
| Copper (Cu) | Cats | |
| Silver (Ag) | Sweet | |
| Least reactive | Gold (Au) | Goldfish |
How to use the series in an exam For any displacement question, find the two metals in the list. If the free metal is higher up than the metal in the compound, displacement will happen. If it is lower, no reaction. Write this reasoning out fully because examiners reward the logic, not just the answer.
Metal displacement reactions
When a more reactive metal is added to a solution of a less reactive metal's salt, the more reactive metal goes into solution and the less reactive metal forms as a solid. The classic AQA practical uses iron, copper, zinc and magnesium in their sulfate solutions.
Magnesium added to copper sulfate solution is a favourite exam question. The blue solution fades, a pink-brown solid coats the magnesium, and the test tube warms up because the GCSE metal and halogen displacement reactions you study are exothermic.
| Reaction | Observation | Word equation |
|---|---|---|
| Mg + CuSO₄ | Blue solution fades to colourless, pink-brown solid forms, heat released | Magnesium + copper sulfate → magnesium sulfate + copper |
| Zn + CuSO₄ | Blue fades, copper coats the zinc, mild warming | Zinc + copper sulfate → zinc sulfate + copper |
| Fe + CuSO₄ | Blue slowly fades, copper deposit on iron nail | Iron + copper sulfate → iron sulfate + copper |
| Cu + MgSO₄ | No reaction | Copper is less reactive than magnesium |
| Cu + ZnSO₄ | No reaction | Copper is less reactive than zinc |
Halogen displacement reactions
Halogens (Group 7) also displace each other from solutions of their salts. Reactivity decreases down the group, so chlorine is more reactive than bromine, which is more reactive than iodine. Fluorine is the most reactive of all but is not used in school labs.
The rule is the same: A more reactive halogen pushes a less reactive halogen out of its compound. Chlorine added to potassium bromide solution turns the colourless solution orange because bromine has been displaced.
| Reaction | Observation | Outcome |
|---|---|---|
| Cl₂ + KBr | Colourless to orange | Chlorine displaces bromine |
| Cl₂ + KI | Colourless to brown | Chlorine displaces iodine |
| Br₂ + KI | Orange to brown | Bromine displaces iodine |
| Br₂ + KCl | No change | Bromine is less reactive than chlorine |
| I₂ + KCl | No change | Iodine is less reactive than chlorine |
Ionic and half equations (higher tier)
Higher-tier AQA expects ionic equations. Take magnesium plus copper sulfate. The full equation is: Mg + CuSO₄ → MgSO₄ + Cu. The sulfate ion appears on both sides, so it is a spectator. Cancel it and you get the ionic equation: Mg + Cu²⁺ → Mg²⁺ + Cu.
The two half equations are: Mg → Mg²⁺ + 2e⁻ (oxidation, magnesium loses electrons) and Cu²⁺ + 2e⁻ → Cu (reduction, copper gains electrons). The mnemonic OIL RIG helps: Oxidation Is Loss, Reduction Is Gain (of electrons).
Spotting the redox The more reactive element is always oxidised (it loses electrons). The less reactive element in the compound is always reduced. If you can identify which is which, you have already answered any "explain in terms of electrons" question.
Worked example: Will it react?
Question: Iron filings are added to silver nitrate solution. Will a reaction occur? If yes, write the word equation and the ionic equation.
Step 1: Locate iron and silver in the reactivity series. Iron is higher (more reactive) than silver.
Step 2: Yes, displacement will happen. Iron pushes silver out of the nitrate solution.
Step 3: Word equation. Iron + silver nitrate → iron nitrate + silver.
Step 4: Ionic equation. Nitrate is a spectator, so: Fe + 2Ag⁺ → Fe²⁺ + 2Ag. Iron has been oxidised, silver has been reduced. Observation: A grey solid (silver) coats the iron filings.
Where students lose marks
Examiner reports flag a handful of repeated mistakes. Most are about description and not chemistry. Always state the colour change, the solid that forms, and whether the test tube warms up. One-line answers like "a reaction happens" rarely score full marks.
Mark-scheme traps to avoid Forgetting the direction of reactivity (writing the wrong element as more reactive). Missing the observation marks by not naming the colour change. Writing the wrong colour for halogens (chlorine is pale green, not yellow). Failing to balance the ionic equation (silver needs a 2 in front because iron is 2+). Saying displacement reactions are endothermic (they are exothermic).
Displacement reactions revision checklist
Tick each off before your AQA Chemistry paper.
- Definition: A more reactive element displaces a less reactive element from its compound
- Pattern: A + BC → AC + B (where A is more reactive than B)
- Reactivity series: Potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold
- Halogen order: Fluorine, chlorine, bromine, iodine (reactivity decreases down the group)
- Halogen colours: Chlorine pale green, bromine orange, iodine brown
- Ionic equation example: Mg + Cu²⁺ → Mg²⁺ + Cu
- Typically exothermic at GCSE: Test tube warms up
- OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons
