CDS (II) 2021 General Knowledge · Question 23 of 120

Correct — A, Mg > Al > Zn > Fe. The four metals are being ranked by how readily they react with dilute hydrochloric acid, and the answer is read straight off the activity series, which lists metals in order of decreasing reactivity: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, then hydrogen, then copper, mercury, silver and gold. Every metal above hydrogen in that list can push hydrogen out of a dilute acid, and the higher it stands the more vigorously it does so. Take the four in the question in the order the series prints them and magnesium comes first, then aluminium, then zinc, then iron — which is option (a) exactly. The laboratory picture matches: magnesium ribbon fizzes hard in dilute hydrochloric acid and the tube warms noticeably, zinc gives a steady stream of bubbles, iron reacts slowly, and aluminium behaves erratically at first because a tough oxide film has to be broken through before the metal underneath is reached. Copper, sitting below hydrogen, gives no hydrogen with dilute hydrochloric acid at all.

• (b)Mg < Al < Zn < Fe — The correct sequence with the inequality signs reversed, so it claims iron is the most reactive of the four. Iron sits lowest of them in the activity series.
• (c)Mg > Zn > Fe > Al — Right at the top and wrong in the middle. Aluminium stands above zinc and iron in the series; its sluggish start in the laboratory comes from the surface oxide layer, not from low reactivity.
• (d)Fe > Mg > Al > Zn — Puts iron first, which reverses the two ends of the series. Iron is the least reactive of the four listed metals.

The reactivity series is a list of metals arranged in order of decreasing activity, built up from how each behaves with air, with water and with dilute acids, and from which metals displace which from solution. Its most useful single feature is the position of hydrogen in the middle: metals above hydrogen displace it from dilute acids, and metals below it do not. The same ordering explains where metals are found in nature — those at the top occur only as compounds, while gold and platinum at the bottom are found free.

Three of the four options are permutations of the same four symbols, so nothing can be guessed and the series has to be recalled in order. Aluminium is where candidates usually stumble, because everyday experience of aluminium vessels suggests an unreactive metal. The opposite is true: aluminium is highly reactive and immediately grows a thin, tightly bonding oxide film that shields the metal beneath, which is why aluminium cookware survives and why anodising deliberately thickens that film. Reading the inequality signs is the other hazard, since option (b) is the right sequence written backwards — a candidate who recalls the order correctly and skims the symbols can still lose the mark.

• The activity series in decreasing order of reactivity runs potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold.
• Metals placed above hydrogen displace it from dilute acids; copper, silver and gold, which sit below, do not.
• Magnesium reacts most vigorously of the four listed metals with dilute hydrochloric acid, and the reaction is markedly exothermic.
• Aluminium's apparent sluggishness comes from a protective oxide layer on its surface, not from a low position in the series.
• Metals at the top of the series are never found free in nature, while those at the bottom, such as gold and platinum, occur as free elements.

• Placing aluminium below zinc or iron because aluminium articles resist corrosion; the oxide film is doing that work.
• Reading the inequality signs the wrong way round when every option uses the same four symbols.
• Expecting copper to release hydrogen from dilute hydrochloric acid; it sits below hydrogen and does not.

As an ordering item like this one, as a displacement-reaction question asking which metal displaces which, or as a which-metal-does-not-react item with dilute acid or with water.

• practice — not a real PYQ

  Which one of the following metals does not liberate hydrogen gas on reacting with 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

  An iron nail dipped in copper sulphate solution slowly turns brownish and the blue colour fades. This happens because

  • (a)copper is more reactive than iron and displaces it
  • (b)iron is more reactive than copper and displaces it
  • (c)the solution is decomposing on its own
  • (d)iron dissolves without any reaction

  Answer(b) iron is more reactive than copper and displaces it — iron stands above copper in the activity series, so it pushes copper out of its salt.