Four Mystery Metals: Ranking Reactivity Without a Chart
A school laboratory issues four metal strips labelled A, B, C and D and forbids the use of any printed reactivity series. The students must rank the metals from their own observations.
With dilute hydrochloric acid at room temperature: A gives a steady stream of bubbles; B shows no change even after ten minutes; C fizzes briskly for about fifteen seconds and then stops completely, although the strip was rubbed with sandpaper immediately before use; D gives slow, sparse bubbles.
Displacement tests, all with freshly cleaned strips:
- A in D-sulphate solution: a grey deposit forms on A and the blue colour fades.
- C in D-sulphate solution: a deposit forms on C.
- D in B-nitrate solution: a shiny deposit forms on D.
- A in C-nitrate solution: no change.
- B in every solution: no change.
The teacher adds one more requirement: the same metal must be used to make a tank that will hold D-sulphate solution for a week; the solution is faintly acidic (pH about 4).
1. Arrange A, B, C and D in decreasing order of reactivity. Justify every link in the chain with one specific observation, and identify the metal most likely to be C.
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Order: C > A > D > B.
Building the chain, one observation at a time:
- A turns D-sulphate grey and discharges its blue colour, so A is above D (A displaces D).
- C also deposits D, so C is above D - but this alone does not place C relative to A.
- A produces no change in C-nitrate, so A cannot displace C. Hence C is above A.
- D deposits B from B-nitrate, so D is above B.
- B shows no reaction with acid or with any salt solution, so B is at the bottom.
Combining: C > A > D > B.
Identity of C: aluminium. Its very short burst of fizzing followed by complete stoppage, despite sandpapering, is the classic signature of a thin, adherent Al₂O₃ layer sealing the surface. Aluminium is in fact highly reactive - the displacement result (it displaces D but is not displaced by A) proves it sits near the top of this set. Zinc (A), iron (D) and copper (B) fit the remaining observations, including copper's inertness towards dilute acid.
2. Explain why strip C stops fizzing, describe one practical way to make it react again, and explain why the first fifteen seconds of brisk fizzing must not be read as proof that C is the most reactive metal here.
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Why it stops: aluminium exposed to air instantly forms a thin, hard, unreactive oxide film. Dilute HCl dissolves this film within seconds, which is the initial brisk fizzing; once the film is gone and the acid is in contact with metal, fresh oxide re-forms and the metal is passivated, so gas evolution almost ceases. Sandpapering does not help because the new surface is oxidised within moments.
How to restore reaction: amalgamate the strip with a drop of mercury (the amalgam prevents a coherent oxide film), or scrape the strip under the surface of the acid so the fresh metal is never exposed to air, or use hot concentrated alkali, which dissolves the oxide layer.
Why the burst is misleading: reaction rate over a few seconds is not the same as position in the reactivity series. The short burst is simply the oxide layer dissolving; after that the barrier throttles the reaction. The reliable evidence is displacement chemistry, which is thermodynamically controlled: C displaces D while A cannot displace C, so C > A regardless of which strip bubbles faster in a given minute.
3. Which of the four metals should be used to make the storage tank, and why are the other three unsuitable? Use both the acid data and the displacement data.
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Choice: B.
Why B works: B shows no reaction with dilute hydrochloric acid, so it lies below hydrogen in the reactivity series and a weakly acidic solution (pH 4) will not corrode it. It also fails to displace every metal tested, so it cannot displace D from D-sulphate: the tank would remain chemically untouched and the solution would keep its composition for the week.
Why the others fail:
- A is above D (it deposits D from D-sulphate) and it bubbles steadily in acid, so it would both contaminate the solution with A ions and be eaten away by the slight acidity.
- C is above even A and reacts with acid, so it is doubly unsuitable.
- D is the same metal as the dissolved salt, so no displacement occurs, but D does give slow bubbles with dilute HCl - it is above hydrogen and will be slowly attacked by the pH 4 solution, thinning the tank wall and introducing hydrogen.
The judgement rests on two independent criteria being satisfied together: must not displace D, and must not displace hydrogen from a weak acid. Only B meets both.