CDS (I) 2024 General Knowledge · Question 35 of 120

Correct — A, Mg > Al > Zn > Fe. The activity series settles this without any calculation. NCERT's table of relative reactivities runs K, Na, Ca, Mg, Al, Zn, Fe, Pb, then hydrogen, then Cu, Hg, Ag and Au. Strike out the metals not named in the question and the four that remain hold their positions in exactly that order: magnesium first, then aluminium, then zinc, then iron. All four sit above hydrogen in the series, which is why all four displace hydrogen from dilute hydrochloric acid; the series tells you how vigorously each does so. Magnesium fizzes briskly, iron reacts slowly and needs warming.

• (b)Mg > Al > Fe > Zn — Swaps the last two. Zinc stands above iron in the activity series, which is also why a zinc coating protects iron from rusting rather than the other way round.
• (c)Zn > Fe > Mg > Al — Puts the two most reactive metals of the four at the bottom. Magnesium and aluminium both stand above zinc and iron in the series.
• (d)Fe > Al > Zn > Mg — Reverses the series almost exactly, making the least reactive of the four the most reactive.

The activity series arranges metals in order of decreasing reactivity, and it is built from displacement experiments — a metal higher in the series displaces one lower down from its salt solution. Its most useful landmark is hydrogen. Metals above hydrogen displace it from dilute acids and so liberate hydrogen gas; metals below it, such as copper, silver and gold, do not. The order runs potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold.

Nothing here requires reasoning beyond recall, but the series pays for itself far beyond this item. It explains why the most reactive metals must be obtained by electrolysis while the middle group can be reduced with carbon and the bottom group turns up in the native state. It explains galvanisation — zinc sits above iron, so zinc corrodes in preference to the iron it coats. And it explains why aluminium looks unreactive in everyday life despite its high position: a tough oxide film forms on the surface and shields the metal underneath, so freshly exposed aluminium reacts far more readily than a weathered surface. That gap between position in the series and everyday appearance is the one place where a candidate's intuition can pull against the correct answer.

• NCERT activity series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au.
• Metals above hydrogen in the series displace hydrogen from dilute acids; those below it do not.
• Zinc above iron is the basis of galvanisation — the zinc corrodes in preference to the iron.
• Aluminium's surface oxide film makes it appear less reactive than its position in the series suggests.
• Position in the series also decides the extraction route — electrolysis at the top, carbon reduction in the middle, native metal at the bottom.

• Ranking aluminium low because aluminium articles do not corrode visibly; the protective oxide film masks a genuinely reactive metal.
• Placing iron above zinc, which contradicts the logic of galvanising iron with zinc.
• Assuming any of these four fails to react with dilute HCl; all four lie above hydrogen.

Asked as a straight ordering item on four metals from the middle of the series, sometimes with water and sometimes with dilute acid as the reagent.

• practice — not a real PYQ

  Which one of the following metals will NOT displace hydrogen from dilute hydrochloric acid?

  • (a)Zinc
  • (b)Iron
  • (c)Copper
  • (d)Magnesium

  Answer(c) Copper — it lies below hydrogen in the activity series, so it cannot displace hydrogen from a dilute acid.

• practice — not a real PYQ

  Iron articles are galvanised with zinc rather than the reverse because

  • (a)zinc is cheaper than iron in every market
  • (b)zinc lies above iron in the activity series and corrodes in preference to it
  • (c)zinc has a higher melting point than iron
  • (d)zinc does not react with any acid

  Answer(b) zinc lies above iron in the activity series and corrodes in preference to it — sacrificial protection.