Displacement Reactions and Metal Reactivity Series: O-Level Chemistry Practical

The ionic equation strips out the spectator sulfate ions and shows the electron transfer directly:

$$\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \to \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$$

In redox terms:

• Zinc is oxidised - it loses two electrons and its oxidation state rises from 0 to +2.
• Copper(II) ion is reduced - it gains two electrons and its oxidation state falls from +2 to 0.

The general rule: a metal will displace another metal from its salt solution only if the displacing metal is higher in the reactivity series. If both metals occupy the same position, or if the metal added is lower, no observable reaction occurs.

2 | Apparatus

3 | Preparation - why emery paper matters

All common metals develop a thin oxide or hydroxide layer on their surfaces when exposed to air. This layer is chemically inert and physically blocks contact between the reactive metal and the solution. If you place an uncleaned strip of iron into copper(II) sulfate, the iron oxide coating may prevent any visible reaction even though iron is reactive enough to displace copper.

Rub each metal strip briskly with emery paper immediately before placing it in solution. The scratching action removes the oxide layer and exposes the shiny, reactive metal beneath. Rinse the strip with distilled water to remove any metal debris from the abrasion, then blot gently. Do not touch the cleaned surface with your fingers - skin oils deposit a fresh contaminating layer.

This preparation step is frequently tested as a planning question: "Give one reason why metal strips should be cleaned with emery paper before the experiment." The answer is not "to make them shiny" - it is to remove the oxide coating that would otherwise prevent metal-to-ion contact and produce a false negative result.

4 | The 4×4 observation grid

Set up the 16 tubes in a 4×4 arrangement. Each row represents the metal strip placed into the tube; each column represents the solution in the tube. The combination where the same metal matches its own salt (e.g. copper in copper sulfate) will never show displacement - these are the control comparisons.

Every cell must be filled. Blank cells score zero regardless of whether a reaction occurred.

5 | Expected observations for each reactive combination

The observations below give the detail that earns full marks. Notice the pattern: each description names the change in solution colour, identifies what appears on the metal surface, and states the temperature change.

Mg + CuSO₄(aq)

The blue colour of the copper(II) sulfate solution fades progressively and becomes colourless (or very pale blue at the edges). A pink-brown solid deposits on the surface of the magnesium strip. The tube feels warm when held - the reaction is exothermic. If a large excess of magnesium is used, the blue colour disappears completely.

Ionic equation:

$$\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \to \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}$$

Zn + CuSO₄(aq)

The blue solution fades to colourless. A pink-brown solid deposits on the zinc strip. The tube feels slightly warm. This is a less vigorous reaction than Mg + CuSO₄ because the potential difference in reactivity between zinc and copper is smaller than between magnesium and copper.

Ionic equation:

$$\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \to \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$$

Fe + CuSO₄(aq)

The blue solution fades to a very pale green (because Fe²⁺ ions entering the solution impart a faint green tint). A pink-brown solid deposits on the iron strip. Slight warming. The colour change is subtler than for zinc or magnesium - candidates who look for a dramatic fading to colourless will miss the pale green endpoint.

Ionic equation:

$$\text{Fe(s)} + \text{Cu}^{2+}\text{(aq)} \to \text{Fe}^{2+}\text{(aq)} + \text{Cu(s)}$$

Mg + FeSO₄(aq)

The pale green iron(II) sulfate solution fades and becomes colourless (Mg²⁺ ions are colourless). A grey or grey-black solid deposits on the magnesium strip (iron metal). The tube warms. Note: iron metal is not pink-brown - that colour is specific to copper. Iron deposits are dark grey to black.

Ionic equation:

$$\text{Mg(s)} + \text{Fe}^{2+}\text{(aq)} \to \text{Mg}^{2+}\text{(aq)} + \text{Fe(s)}$$

Mg + ZnSO₄(aq)

Zinc sulfate solution is colourless, and magnesium sulfate solution is also colourless - so no colour change in the solution occurs. The evidence of reaction is the grey solid that deposits on the magnesium strip (zinc metal). This combination frequently catches candidates who only look for colour changes in the solution and record "no observable reaction" when a deposit is clearly visible.

Ionic equation:

$$\text{Mg(s)} + \text{Zn}^{2+}\text{(aq)} \to \text{Mg}^{2+}\text{(aq)} + \text{Zn(s)}$$

Zn + FeSO₄(aq)

The pale green solution fades slightly as Fe²⁺ is removed from solution and replaced with colourless Zn²⁺. A grey solid deposits on the zinc strip. Slight warming.

6 | Observation language

The distinction between acceptable and unacceptable observation language is examined every year. The following table shows the contrast.

Three rules for observation language in displacement practicals:

1. Name the deposit. State the colour and that it is a solid on the surface of the named metal strip. Do not write "a precipitate" - precipitates form in solution; deposits form on metal surfaces.
2. State the solution colour change. Name the initial colour and the final colour. "Blue fades to colourless" is complete. "The solution changed" is not.
3. Note exothermicity if detectable. "Tube feels warm" earns a third observation mark when it is present. It is never appropriate to record this for combinations that show no reaction.

For deeper practice with observation phrasing across all practical topics, see the O-Level Chemistry Observation Language Drills.

7 | Ordering the reactivity series from the grid

Once the 4×4 grid is complete, the reactivity series can be deduced by counting and ranking.

Counting reactions per metal (as displacing agent):

This count gives the initial ranking: Mg is most reactive (reacted with 3 out of 3 others), Cu is least reactive (reacted with none). Zinc and iron both reacted with only CuSO₄, so they cannot be separated by this count alone.

Resolving the zinc/iron tie:

Look at the Zn + FeSO₄ combination and the Fe + ZnSO₄ combination.

• Zn + FeSO₄: pale green fades and grey solid forms on zinc. Reaction occurred. Zinc displaced iron.
• Fe + ZnSO₄: no observable reaction.

Therefore zinc is more reactive than iron.

Full deduction from evidence:

$$\text{Mg} > \text{Zn} > \text{Fe} > \text{Cu}$$

Write the deduction as a chain of inequalities, not as a list. The examiner mark scheme awards the mark for the correct order in the correct format.

8 | The annual mark-trap - "no observable reaction"

Every year, a significant proportion of candidates score zero on cells in the observation grid because they leave blank spaces. The examiner mark scheme awards a mark for every correctly completed cell, including cells where no reaction occurs.

Writing nothing in a cell communicates ambiguity: did you not look, or did nothing happen? The examiner cannot award a mark.

The correct response for a non-reactive combination is:

No observable reaction - or equivalently - No change in solution colour; no deposit on metal strip; no temperature change felt.

Both phrasings earn the mark. A blank earns nothing.

The combinations that require "no observable reaction" in this practical:

• Cu + MgSO₄(aq)
• Cu + ZnSO₄(aq)
• Cu + FeSO₄(aq)
• Fe + ZnSO₄(aq)
• Fe + MgSO₄(aq)
• Zn + MgSO₄(aq)
• Any metal in its own matching salt solution (Mg in MgSO₄, Zn in ZnSO₄, Fe in FeSO₄, Cu in CuSO₄)

That is nine cells out of sixteen where "no observable reaction" is the full and complete answer. Candidates who treat silence as acceptable lose nearly half the observation marks.

9 | Extension - adding hydrogen and carbon to the series

The 4×4 displacement grid places Mg, Zn, Fe, and Cu in order. Two additional elements frequently appear in O-Level questions: hydrogen and carbon. Neither can be placed in a test tube and dropped into a salt solution, so their positions are established by different evidence.

Hydrogen - positioned between lead and copper

Hydrogen's position in the reactivity series is determined by the acid test: metals above hydrogen in the series react with dilute hydrochloric acid to produce hydrogen gas. Metals below hydrogen do not.

• Magnesium, zinc, and iron all react with dilute HCl to produce hydrogen gas (confirmed by the squeaky pop test with a lighted splint).
• Copper does not react with dilute HCl; no gas is produced.
• Lead reacts very slowly with dilute HCl (the reaction is quickly passivated by an insoluble lead chloride coating).

From this evidence, hydrogen sits between lead and copper:

$$\text{Mg} > \text{Zn} > \text{Fe} > \text{Pb} > \text{H} > \text{Cu}$$

Carbon - positioned between aluminium and zinc

Carbon's position is established by reduction experiments. Carbon (as coke) can reduce the oxides of metals below it in the reactivity series when heated strongly, but it cannot reduce the oxides of metals above it.

• Carbon reduces zinc oxide (ZnO) to zinc at high temperature - carbon sits above zinc.
• Carbon cannot reduce aluminium oxide (Al₂O₃) under laboratory conditions - aluminium sits above carbon.

Therefore:

$$\text{Al} > \text{C} > \text{Zn}$$

The extended series including all four practical metals, hydrogen, and carbon:

$$\text{Mg} > \text{Al} > \text{C} > \text{Zn} > \text{Fe} > \text{Pb} > \text{H} > \text{Cu}$$

10 | Planning-style variant - where does tin fit?

A common Paper 3 planning question asks candidates to design a practical to determine the position of an unfamiliar metal in the reactivity series. A typical version:

"Plan a practical to determine where tin (Sn) sits relative to zinc and iron in the reactivity series. You have access to tin strips, zinc strips, iron strips, tin(II) sulfate solution (SnSO₄), zinc sulfate solution (ZnSO₄), and iron(II) sulfate solution (FeSO₄)."

Model plan:

Aim: To determine whether tin is more reactive or less reactive than zinc and iron by observing displacement reactions.

Variables:

• Independent variable: the metal strip used (Sn, Zn, Fe)
• Dependent variable: presence of a deposit on the metal strip and/or colour change in the solution
• Controlled variables: concentration of all salt solutions (0.5 mol/dm³), volume of solution in each tube (5 cm³), size of metal strip, time of observation (5 minutes), temperature (room temperature)

Method:

1. Clean each metal strip with emery paper and rinse with distilled water immediately before use.
2. Set up six test tubes in a rack. Add 5 cm³ of solution to each tube according to the following table:

1. Place the appropriate cleaned metal strip into each tube. Observe and record the colour of the solution and the surface of the metal strip after 5 minutes.

Expected results and deduction:

If tin is more reactive than both zinc and iron:

• Tubes 1 and 2 will show a grey solid depositing on the zinc or iron strip; SnSO₄ solution will become colourless (Zn²⁺ and Fe²⁺ are pale coloured).
• Tubes 3 and 5 will show no observable reaction.

If tin sits between zinc and iron:

• Tube 2: grey solid on iron strip (tin deposits from SnSO₄ displaced by iron - but wait: if tin is above iron, iron cannot displace it; this needs careful logic). Specifically, if Zn greater than Sn greater than Fe: Zn displaces Sn (tube 3 shows reaction), Fe cannot displace Sn (tube 5 shows no reaction), Sn displaces Fe (tube 5 - correction: in tube 5, Sn is in FeSO₄, so Sn displaces Fe; grey deposit on Sn strip).

Conclusion: Compare the reaction outcomes in the six tubes to place tin in the correct order relative to zinc and iron.

Safety: Wear safety goggles throughout. Dispose of metal salt solutions in the designated waste container.

11 | Where this fits - related practical skills

The displacement reactions grid connects directly to several other Paper 3 skills:

• Observation language precision: O-Level Chemistry Observation Language Drills
• Salt preparation (making ZnSO₄ and CuSO₄ from metal-acid and neutralisation routes): O-Level Chemistry Salt Preparation Playbook
• Identifying metal ions from precipitate tests (connecting displacement to qualitative analysis): O-Level Chemistry Qualitative Analysis Toolkit
• Rate of reaction in context (why more reactive metals react faster): Rate of Reaction Experiments O-Level Chemistry

Sources

• SEAB 2026 O-Level Chemistry syllabus (5073): https://www.seab.gov.sg/docs/default-source/national-examinations/syllabus/2026/2026-o-level-syllabus/5073_y26_sy.pdf

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