NDA (II) 2023 General Ability Test · Question 57 of 146

Correct — D, Steel. Steel is iron with a small proportion of carbon dissolved in it, and carbon is a non-metal — so steel is the one option in this set that pairs a metal with a non-metal. NCERT Class 10 Science makes both halves of that explicit. It defines an alloy as a homogeneous mixture of two or more metals, or of a metal and a non-metal, and it explains that pure iron is very soft and stretches easily when hot, but mixing in a small amount of carbon, about 0.05 per cent, makes it hard and strong. The other three named alloys are combinations of metals only.

• (a)Brass — Brass is an alloy of copper and zinc, and both of those are metals. NCERT names it in exactly that form when explaining why alloys conduct electricity less well than the pure metals they are made from.
• (b)Bronze — Bronze is an alloy of copper and tin, again two metals. It is the classic swap-trap with brass, so fix the pair — copper with zinc gives brass, copper with tin gives bronze.
• (c)Amalgam — An amalgam is not a fixed recipe at all but a category — NCERT says that if one of the metals in an alloy is mercury, the alloy is known as an amalgam. Mercury and its partner metal are both metals, so no non-metal is involved.

An alloy is a homogeneous mixture made by melting a primary metal and dissolving other elements in it in definite proportions before cooling. The added element need not be a metal — NCERT's definition explicitly allows a metal combined with a non-metal, and iron with carbon is the everyday example. Alloying is done to get properties the pure metal lacks, such as hardness, strength, corrosion resistance or a lower melting point.

Students go wrong here by carrying an unstated assumption that alloy means a mixture of metals only. The question is testing exactly that assumption. Learn the standard set as pairs — brass is copper and zinc, bronze is copper and tin, solder is lead and tin, an amalgam is anything alloyed with mercury, steel is iron and carbon, and stainless steel is iron with nickel and chromium. Only the steel entries bring in a non-metal.

• NCERT Class 10 Science defines an alloy as a homogeneous mixture of two or more metals, or of a metal and a non-metal.
• Brass is an alloy of copper and zinc; bronze is an alloy of copper and tin; solder is an alloy of lead and tin.
• If one of the metals in an alloy is mercury, the alloy is called an amalgam.
• Pure iron is soft, but mixing in a small amount of carbon, about 0.05 per cent, makes it hard and strong; adding nickel and chromium instead gives stainless steel, which is hard and does not rust.

• Assuming every alloy must be a mixture of metals only — the standard definition allows a metal plus a non-metal.
• Treating amalgam as a fixed pair of metals when it names any alloy containing mercury.
• Swapping brass, which is copper and zinc, with bronze, which is copper and tin.

NDA asks for the composition of a named alloy, or which alloy is the odd one out on some property — memorise the standard pairs and note which of them brings in a non-metal.

• practice — not a real PYQ

  Brass is an alloy of

  • (a)copper and tin
  • (b)copper and zinc
  • (c)lead and tin
  • (d)iron and carbon

  Answer(b) copper and zinc — bronze is the one made of copper and tin, and solder is lead and tin.

• practice — not a real PYQ

  An alloy in which one of the constituent metals is mercury is known as

  • (a)solder
  • (b)amalgam
  • (c)bronze
  • (d)brass

  Answer(b) amalgam — the term covers any alloy of mercury with another metal, not one particular recipe.