NDA (II) 2022 General Ability Test · Question 68 of 150

Correct — A, Zinc > Iron > Lead > Copper. This is the order these four metals occupy in the activity series. Zinc is the most reactive of them and reacts with steam to give zinc oxide and hydrogen; iron reacts with steam too, but only when red hot and more slowly; lead, sitting just above hydrogen, reacts with water essentially not at all; copper lies below hydrogen and cannot displace hydrogen from water or steam at any temperature.

• (b)Copper > Lead > Zinc > Iron — This puts copper — the least reactive of the four — at the top and zinc near the bottom. Copper does not react with water or steam at all, so it cannot head a reactivity-with-water list.
• (c)Copper > Zinc > Iron > Lead — It gets the middle pair right but again crowns copper. Copper lies BELOW hydrogen in the activity series, which is exactly why copper vessels do not corrode in water the way iron does.
• (d)Zinc > Copper > Iron > Lead — It starts correctly with zinc, then promotes copper above iron and lead. Iron rusts in moist air and reacts with steam; copper does neither, so copper must come last, not second.

The reactivity or activity series arranges metals by how readily they lose electrons and form positive ions. Reading downwards it runs potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, then hydrogen, then copper, mercury, silver and gold. Where a metal sits decides how it behaves with water, with dilute acids and with the salt solutions of other metals — one order governs all three reactions.

The stem says 'with water', but the ordering asked for is the general activity series; behaviour with water is just the series graded by severity. Potassium, sodium and calcium react with cold water; magnesium needs hot water; zinc and iron need steam; lead and everything below it essentially do not react with water. The decisive test for all four options here is where copper goes — copper sits below hydrogen and therefore cannot release hydrogen from water, which eliminates every option that ranks it above iron or lead and leaves only (a).

• The activity series runs K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au.
• Only metals above hydrogen can displace hydrogen from water or dilute acids — copper, silver and gold cannot.
• Potassium, sodium and calcium react with cold water; magnesium with hot water; zinc and iron only with steam.
• A more reactive metal displaces a less reactive one from its salt solution, which is why an iron nail dipped in copper sulphate picks up a brown copper coating.
• The same order explains extraction — highly reactive metals need electrolysis, while metals low in the series can be won by simple heating of their ores.

• Placing copper high because it is a familiar, useful metal — usefulness is not reactivity.
• Forgetting that hydrogen sits inside the series and marks the cut-off for reaction with water and acids.
• Assuming a metal that reacts with acid must also react with water; zinc and iron react with dilute acids but need steam for water.

As an ordering question like this one, as a single pick — which metal reacts with cold water — or through a displacement scenario such as an iron nail in copper sulphate solution.

• practice — not a real PYQ

  Which one of the following metals reacts with cold water?

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

  Answer(c) Sodium — the alkali metals sit at the top of the activity series and react vigorously even with cold water.

• practice — not a real PYQ

  An iron nail is left in copper sulphate solution. What happens?

  • (a)Nothing at all
  • (b)The nail gets a reddish-brown coating of copper and the blue colour fades
  • (c)The solution turns a deeper blue
  • (d)The nail dissolves with rapid evolution of oxygen

  Answer(b) The nail gets a reddish-brown coating of copper and the blue colour fades — iron is above copper in the activity series and displaces it.