CDS (II) 2022 General Knowledge · Question 14 of 120

Correct — B, Sonorous. This is NCERT's own sentence turned into a question. The Class 10 chapter on metals and non-metals says that 'the metals that produce a sound on striking a hard surface are said to be sonorous', and then asks the reader directly: 'Can you now say why school bells are made of metals?' Sonority is the property of ringing when struck, and it follows from the metallic bond — a metal is elastic enough for a blow to set up a long-lived vibration in it, and stiff enough for that vibration to be at an audible frequency and to be passed on to the air. The other three properties in the option list are all genuine metallic properties, but none of them has anything to do with making a sound.

• (a)Malleable — Malleability is the ability to be beaten into thin sheets, which is how gold leaf and aluminium foil are made. It explains how a bell can be shaped, not why it rings.
• (c)Ductile — Ductility is the ability to be drawn into wire — the property that matters for cables and filaments. A bell is not drawn, and a wire is not what produces the sound.
• (d)Lustrous — Lustre is the shine of a fresh metal surface, an optical property arising from free electrons reflecting light. It affects how a bell looks and nothing about how it sounds.

Sonority is one of the standard physical properties of metals, listed alongside lustre, malleability, ductility, high melting point and good conduction of heat and electricity. All of them trace back to the same structure — positive ions in a regular lattice held together by a shared cloud of delocalised electrons. That bonding is non-directional, which lets the layers slip without the solid breaking, and it is elastic, which lets a struck metal keep vibrating instead of absorbing the blow.

A question of this shape is really asking which technical word matches an everyday observation, so translate the observation first. A school bell is struck and it makes a sound. Of the four words offered, only one is about sound. Bells in practice are cast from bell metal, a bronze of roughly four parts copper to one part tin, because the alloy rings longer and more cleanly than either parent metal — a useful aside, because it shows that alloying is done to sharpen a property, not only to increase strength or resist corrosion. It is also worth noticing how directly CDS lifts from the textbook here: the stem is barely more than NCERT's own rhetorical question rearranged.

• Metals that produce a sound on striking a hard surface are said to be sonorous.
• Malleability is the ability to be hammered into thin sheets; ductility is the ability to be drawn into wire.
• Lustre is the shine of a freshly cut metal surface, caused by free electrons reflecting light.
• Bells are usually cast in bell metal, a bronze of about four parts copper to one part tin.
• Gold and silver are the most malleable and the most ductile of the metals.

• Choosing malleability because a bell is shaped by casting or beating; the question asks about the sound, not the shaping.
• Treating sonorous as a rare or special property; it belongs to metals generally.
• Assuming pure copper makes the best bell — the alloy bell metal is used precisely because it rings better.

As a which-property question tied to a familiar object, or in reverse, giving the property and asking for the object or the definition.

• practice — not a real PYQ

  The property of a metal by which it can be drawn into thin wires is called

  • (a)malleability
  • (b)ductility
  • (c)sonority
  • (d)lustre

  Answer(b) ductility — malleability is beating into sheets, sonority is ringing when struck, and lustre is the shine of the surface.

• practice — not a real PYQ

  Bell metal, used for casting bells, is an alloy of

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

  Answer(b) copper and tin — bell metal is a bronze of roughly four parts copper to one part tin; copper with zinc is brass and lead with tin is solder.