CDS (II) 2022 General Knowledge · Question 13 of 120

Correct — A, Silver and Copper. NCERT settles this in one line of the Class 10 chapter on metals and non-metals: 'The best conductors of heat are silver and copper. Lead and mercury are comparatively poor conductors of heat.' Every other option in this question pairs one of those two poor conductors with something else, so the sentence disposes of the whole item. The physical reason is that heat travels through a metal mainly by its free electrons rather than by lattice vibrations, and silver and copper have the most mobile electron gas of the common metals — which is why they lead on electrical conductivity too, and why copper is the metal used for cooking vessels, heat sinks and household wiring while silver is reserved for contacts where cost allows.

• (b)Silver and Lead — Half right. Silver is the best conductor there is, but lead is named by NCERT as one of the two comparatively poor conductors among metals.
• (c)Copper and Mercury — Same flaw from the other side. Copper belongs, mercury does not — it is the second of the two poor conductors, and being liquid at room temperature it also lacks a rigid lattice to help carry heat.
• (d)Lead and Mercury — The exact pair the textbook names as comparatively poor conductors of heat, so it is the reverse of what is asked.

The physical properties that define a metal are lustre, malleability, ductility, sonority, high melting point and good conduction of heat and electricity. The last one comes from the sea of delocalised electrons in a metallic lattice: electrons at the hot end pick up energy, move, and deliver it at the cold end far faster than atomic vibrations alone could. The same electrons carry electric current, so a good thermal conductor among the metals is usually a good electrical conductor as well.

This question is answered by remembering a list of four names in two pairs — silver and copper at the top, lead and mercury at the bottom. Once you hold that, three options fall away mechanically because each of them contains a member of the bottom pair. If you have not memorised it, everyday evidence points the same way: cooking pans get copper bases, electrical wiring is copper, and lead was used for plumbing and roofing precisely because it is soft and inert rather than because it moves heat. Mercury is worth a separate thought — it is the one metal that is liquid at room temperature, and it is used in thermometers for its uniform expansion, not for any excellence in conduction.

• NCERT names silver and copper as the best conductors of heat, and lead and mercury as comparatively poor conductors.
• Conduction in metals is carried mainly by free, delocalised electrons.
• Silver has the highest electrical conductivity of all metals, with copper immediately behind it.
• Mercury is the only metal that is liquid at ordinary room temperature.
• Metals are also lustrous, malleable, ductile and sonorous, and generally have high melting points.

• Assuming a heavy or expensive metal must conduct well — lead and mercury are both poor.
• Reading conduction of heat and conduction of electricity as unrelated properties; in metals both ride on the same free electrons.
• Picking gold because it is precious; silver outranks it for both heat and electricity.

As a pair-selection question like this one, as a single best-or-worst conductor question, or as a property-to-metal match.

• practice — not a real PYQ

  Which one of the following metals is the best conductor of electricity?

  • (a)Copper
  • (b)Gold
  • (c)Silver
  • (d)Aluminium

  Answer(c) Silver — it has the highest electrical conductivity of any metal, with copper next; cost is why copper is used for wiring.

• practice — not a real PYQ

  Which one of the following metals is a liquid at ordinary room temperature?

  • (a)Lead
  • (b)Mercury
  • (c)Sodium
  • (d)Zinc

  Answer(b) Mercury — it is the only metal liquid at room temperature, and it is also a comparatively poor conductor of heat.