Enforcement Officer / Accounts Officer (EO/AO) 2020 recruitment test, advertisement 421. Transcribed from the Commission's own booklet (series A, NMJX-U-GTA); answer as per the official UPSC final answer key. This question is in Part B. Answer last reviewed 24 August 2026.

Which one of the following will not be reduced by metallic zinc ?

(a)Cu^2+
(b)H^+
(c)Ag^+
(d)Al^3+

Answer

Why

Correct — D, (d) Al^3+. The options are ionic formulae: Cu^2+ is a copper ion carrying two positive charges, H^+ a hydrogen ion carrying one, Ag^+ a silver ion carrying one, and Al^3+ an aluminium ion carrying three. Reducing any of them means handing it electrons so that it becomes the free metal, or free hydrogen gas, and a metal can only do that to an ion whose parent metal is LESS reactive than itself. Aluminium is more reactive than zinc — it sits above zinc in the reactivity series and its standard reduction potential is about -1.66 volts against zinc's -0.76 volt — so aluminium holds on to its electrons more strongly than zinc does. Zinc cannot force electrons on to Al^3+, and the emphasised 'not' in the stem is asking for exactly that failure. The other three lie BELOW zinc in the series, so zinc reduces all of them readily: dropping a zinc strip into copper sulphate solution deposits red-brown copper as the blue fades, zinc with dilute hydrochloric or sulphuric acid gives off hydrogen gas, and zinc in silver nitrate solution deposits silver.

Why the others are wrong

(a)Cu^2+ — Zinc reduces the copper ion easily, so this is not the exception. Copper lies well below zinc in the reactivity series, with a standard reduction potential of about +0.34 volt against zinc's -0.76 volt, and the displacement Zn + CuSO4 giving ZnSO4 + Cu is the single most demonstrated reaction of this kind in Indian classrooms: a zinc strip left in blue copper sulphate solution acquires a red-brown coating while the blue colour fades. The same pair, separated into two half-cells, is the Daniell cell, in which zinc is the anode giving up electrons and copper the cathode receiving them.
(b)H^+ — Zinc reduces hydrogen ions, so this cannot be the answer. Hydrogen is placed in the reactivity series precisely so that this comparison can be made, and every metal above it — including zinc — displaces hydrogen from dilute acids. Zinc with dilute sulphuric or hydrochloric acid gives a zinc salt and hydrogen gas, and that reaction is the standard laboratory preparation of hydrogen, complete with the pop test for the gas collected. Since the reaction is so familiar, this option is unlikely to trap a candidate who has read the stem's negation, and it mainly serves to fix hydrogen's place in the series.
(c)Ag^+ — Zinc reduces the silver ion, so this is not the exception either. Silver is one of the least reactive common metals, near the bottom of the series with a standard reduction potential of about +0.80 volt — the highest of the four options — which makes its ion the EASIEST of them to reduce, not the hardest. Zinc placed in silver nitrate solution displaces silver, which deposits as a grey or crystalline coating on the zinc. Silver's low reactivity is also why it, like gold, is found in the free state in nature and resists corrosion, tarnishing only slowly in sulphur-bearing air.

Concept

The reactivity series arranges metals in order of how readily they give up electrons to form positive ions. Running downwards it is potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, then HYDROGEN, then copper, mercury, silver, platinum and gold. Hydrogen is inserted, though it is not a metal, so that the series predicts which metals displace it from dilute acids: everything above hydrogen does, everything below does not, which is why copper and silver do not fizz in dilute hydrochloric acid. The general rule for displacement is that a metal higher in the series displaces the ion of any metal lower down from its solution or its compound, because the higher metal is more willing to lose electrons and so more able to hand them over. The quantitative version of the same idea is the electrochemical series of standard reduction potentials: the more negative the potential, the stronger the metal is as a reducing agent and the harder its own ion is to reduce. Aluminium at about -1.66 volts sits below zinc at -0.76, which is above hydrogen at 0.00, copper at +0.34 and silver at +0.80 — so zinc reduces the last three and not the first. This is not academic. It is why aluminium cannot be extracted by reduction with an ordinary metal or with carbon and must instead be obtained by ELECTROLYSIS of alumina dissolved in molten cryolite, and why aluminium itself is a powerful reducing agent — the thermite reaction, in which aluminium reduces iron oxide to molten iron, is used to weld railway tracks. It also underlies galvanising: zinc protects the iron beneath it because zinc, being the more reactive of the two, corrodes in preference to it.

This is the third negatively worded item in a row in the chemistry block, and it is the one that most rewards a memorised sequence. Nothing in the question can be reasoned out from first principles under exam conditions; either the order of the reactivity series is available in memory or it is not. Note also how the options are printed — as ionic formulae with true superscript charges, written here with a caret — so Al^3+ means an aluminium ion carrying three positive charges. The charge number is not what decides the answer, and a candidate who picks Al^3+ because three is the largest charge has arrived at the right place by the wrong road, which will not survive a rephrasing of the question.

Key facts

The reactivity series in decreasing order: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Pt, Au.
A metal displaces from solution the ion of any metal placed below it in the series.
Aluminium lies above zinc, so zinc cannot reduce Al^3+ to aluminium.
Standard reduction potentials: Al^3+/Al about -1.66 V, Zn^2+/Zn about -0.76 V, H^+/H2 0.00 V by definition, Cu^2+/Cu about +0.34 V, Ag^+/Ag about +0.80 V.
The more negative the standard reduction potential, the stronger the metal is as a reducing agent and the harder its ion is to reduce.
Metals above hydrogen in the series displace hydrogen from dilute acids; those below it do not.
Zinc with dilute hydrochloric or sulphuric acid is the standard laboratory preparation of hydrogen gas.
Zinc placed in copper sulphate solution displaces copper — the classic displacement demonstration and the basis of the Daniell cell.
Aluminium is extracted by electrolysis of alumina in molten cryolite because no ordinary chemical reductant will reduce Al^3+.
In the thermite reaction aluminium reduces iron oxide to molten iron, a reaction used to weld railway tracks.
Galvanising works because zinc, being more reactive than iron, corrodes in preference to the iron it coats.

A metal reduces only the ion of a metal placed BELOW it in the series

Al^3+ — the one zinc cannot reduce→Aluminium sits ABOVE zinc in the reactivity series, with a standard reduction potential of about -1.66 V against zinc’s -0.76 V, so it holds on to its electrons more strongly than zinc does. Zinc cannot force electrons on to Al^3+, and that failure is exactly what the stem’s emphasised ‘not’ is asking for.

Cu^2+→Copper lies well below zinc, at about +0.34 V. Zn + CuSO4 gives ZnSO4 + Cu — a zinc strip left in blue copper sulphate solution acquires a red-brown coating while the blue fades. It is the most demonstrated displacement in Indian classrooms, and the basis of the Daniell cell.

H^+→Hydrogen is placed in the reactivity series, though it is not a metal, precisely so that this comparison can be made: every metal above it displaces hydrogen from dilute acids. Zinc with dilute sulphuric or hydrochloric acid gives a zinc salt and hydrogen gas — the standard laboratory preparation.

Ag^+→Silver is one of the least reactive common metals, near the bottom of the series at about +0.80 V — the highest potential of the four options, which makes its ion the EASIEST of them to reduce rather than the hardest. Zinc placed in silver nitrate solution displaces silver on to the strip.

The series, in decreasing reactivity→K, Na, Ca, Mg, Al, Zn, Fe, Pb, HYDROGEN, Cu, Hg, Ag, Pt, Au. The quantitative version is the electrochemical series of standard reduction potentials: the more negative the potential, the stronger the metal as a reducing agent and the harder its own ion is to reduce.

Why none of this is academic→Because no ordinary chemical reductant will reduce Al^3+, aluminium has to be extracted by ELECTROLYSIS of alumina dissolved in molten cryolite. Aluminium is itself a powerful reducing agent — in the thermite reaction it reduces iron oxide to molten iron, which is used to weld railway tracks. And galvanising works because zinc, being more reactive than iron, corrodes in preference to the iron it coats.

Study next

The reactivity series of metals and displacement reactionsOxidation and reduction, oxidising and reducing agents, and how to assign oxidation numbersThe electrochemical series and standard electrode potentialsElectrochemical cells: the Daniell cell, electrolysis, and electroplatingExtraction of metals — why the method used depends on the metal's position in the seriesCorrosion, rusting and its prevention by galvanising, painting and cathodic protectionOccurrence of metals: which are found free in nature and why

Common traps

Missing the emphasised 'not' and answering with the ion zinc reduces most readily.
Choosing by the size of the charge. A 3+ charge does not make an ion harder to reduce; the metal's position in the series does.
Inverting the rule and thinking a more reactive metal's ion is easier to reduce. It is harder, because the metal holds its electrons more tightly.
Forgetting that hydrogen has a place in the reactivity series and treating H^+ as a special case.
Reading a more negative standard reduction potential as meaning a weaker reducing agent. It means a stronger one.
Confusing the reactivity of aluminium metal, which is high, with its apparent inertness in air, which is due to a protective oxide layer.

Displacement and the reactivity series form one of the most heavily used corners of chemistry on general ability papers. The recurring shapes are 'which metal will displace X from its salt solution', 'which of the following will not react with dilute acid', 'arrange these metals in order of reactivity', and questions on why a particular metal is extracted by electrolysis. Aluminium, zinc, iron, copper and silver are the metals that appear most often, and hydrogen's position is the pivot the questions turn on.

Related PYQs

EPFO_EOAO_2020_Q51Open & attempt →

Which one of the following substances, when added to water, will not change the pH ?

(a) NaHCO3
(b) NH4Cl
(c) Na2CO3
(d) NaCl

Answer(d) NaCl

The first item of this chemistry block. Both questions are solved by classifying each option against a single remembered ordering — there the strength of the parent acid and base, here the reactivity series — rather than by reasoning about the individual substance.

Practice

practice — not a real PYQ
Which one of the following metals will NOT displace hydrogen from dilute hydrochloric acid ?
- (a)Zinc
- (b)Magnesium
- (c)Iron
- (d)Copper
Answer(d) Copper
practice — not a real PYQ
Aluminium is extracted from its oxide by electrolysis rather than by reduction with carbon because
- (a)aluminium oxide does not melt at any attainable temperature
- (b)aluminium is so reactive that ordinary chemical reducing agents cannot reduce its ion
- (c)aluminium metal reacts violently with carbon dioxide
- (d)aluminium occurs in nature in the free state
Answer(b) aluminium is so reactive that ordinary chemical reducing agents cannot reduce its ion

Why