NDA (II) 2023 General Ability Test · Question 64 of 146

Correct — B, Iron. Iron sits far enough down the activity series that cold water leaves it alone — an iron nail in a glass of water simply rusts slowly over days, and no hydrogen comes off. Iron will only pull oxygen out of water when the water is supplied as steam, and even then it needs strong heating, giving 3Fe + 4H₂O → Fe₃O₄ + 4H₂. The other three metals in the list are all above iron in the series and do liberate hydrogen from cold water.

• (a)Potassium — Potassium is the most reactive metal offered here. It reacts violently with cold water to give potassium hydroxide and hydrogen, and the reaction is so exothermic that the escaping hydrogen catches fire on its own.
• (c)Calcium — Calcium does react with cold water, only more gently. It gives calcium hydroxide and hydrogen, and the bubbles of hydrogen sticking to the metal make the lump rise to the surface — a standard classroom observation.
• (d)Sodium — Sodium reacts vigorously with cold water to give sodium hydroxide and hydrogen, releasing enough heat to melt itself into a ball and ignite the gas. It is stored under oil precisely to keep it away from water and air.

The activity (reactivity) series ranks metals by how readily they give up electrons. Where a metal sits decides what it can attack. The most reactive metals — potassium, sodium, calcium — displace hydrogen from cold water. Magnesium needs hot water. Aluminium, zinc and iron need steam. Copper, silver and gold do not displace hydrogen from water at all.

Read the stem carefully — it asks which metal does NOT react with cold water, so the answer must be the least reactive name on the list. Potassium, sodium and calcium are all standard cold-water metals, which leaves iron alone. The trap is remembering that iron does react with water in some form (steam) and therefore ticking it as reactive; the qualifier 'cold' is doing all the work in this question.

• Metals that liberate hydrogen from cold water include potassium, sodium and calcium.
• Magnesium shows no visible reaction with cold water but reacts with hot water.
• Iron reacts only with steam, forming iron oxide and hydrogen, 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
• Copper, silver and gold do not displace hydrogen from water or from dilute acids.

• Ignoring the word 'cold' and marking iron as reactive because it reacts with steam.
• Assuming every metal that rusts or tarnishes must also liberate hydrogen from water.
• Confusing reaction with dilute acid (which iron does undergo) with reaction with water.

Asked as an odd-one-out on cold-water reactivity, as an ordering item on the activity series, or as an equation-completion item on the steam reaction.

• practice — not a real PYQ

  Which one of the following metals reacts with steam but not with cold water?

  • (a)Sodium
  • (b)Iron
  • (c)Gold
  • (d)Calcium

  Answer(b) Iron — it is unreactive towards cold water but reduces steam to hydrogen on strong heating.

• practice — not a real PYQ

  When calcium is dropped into cold water, the lump is seen to float mainly because

  • (a)calcium is lighter than water
  • (b)bubbles of hydrogen stick to its surface
  • (c)calcium hydroxide forms a buoyant film
  • (d)the water boils and lifts it

  Answer(b) bubbles of hydrogen stick to its surface — the adhering gas makes the metal buoyant.