UPPSC 2020 Prelims GS Paper-I · Question 107 of 150

Correct — C, A-4, B-2, C-1, D-3. Sodium (A) is a soft alkali metal that can be cut with a knife → 4. Mercury (B) is the only metal that is liquid at ordinary room temperature (melting point about −39 °C) → 2. Silver (C) has the highest electrical conductivity of all metals, so it is the good conductor of electricity → 1. Lead (D) has a comparatively low thermal conductivity, making it a poor conductor of heat → 3. That gives A-4, B-2, C-1, D-3 = option (c).

• (a)A-2, B-3, C-1, D-4 — Makes sodium the liquid and mercury a poor heat conductor; in fact mercury is the liquid metal and sodium is the knife-soft one, so A and B are both misassigned.
• (b)A-1, B-4, C-3, D-2 — Calls sodium the best electrical conductor and lead a liquid; silver is the top conductor and only mercury is liquid, so every pair here is off.
• (d)A-4, B-1, C-2, D-3 — Right for sodium (4) and lead (3), but swaps mercury and silver — mercury is the liquid (2) and silver the conductor (1), so B and C are reversed.

The item tests each metal's most distinctive physical property. Alkali metals such as sodium are so soft they can be cut with a knife; mercury is unique among metals in being liquid at room temperature; silver leads all metals in electrical conductivity; and lead has a relatively low thermal conductivity, so among these four it is the poor conductor of heat.

Lock the two one-of-a-kind clues first: 'liquid at room temperature' can only be mercury, and 'can be cut with a knife' can only be the soft alkali metal sodium. That fixes B-2 and A-4, after which silver (best conductor) must take 1 and lead takes 3 — uniquely giving option (c).

• Mercury (Hg) — the only metal liquid at room temperature (m.p. ≈ −39 °C)
• Sodium (Na) — soft alkali metal that can be cut with a knife
• Silver (Ag) — highest electrical (and thermal) conductivity of all metals
• Lead (Pb) — low thermal conductivity → a poor conductor of heat

• Remembering silver only for jewellery, not that it is the best conductor
• Mixing up the thermal- and electrical-conductivity rankings

As a metal-to-property match — anchor the unique clues (liquid = mercury, knife-soft = sodium) and the remaining pairs fall out.

No directly related past PYQ was found.

• practice — not a real PYQ

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

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

  Answer(b) Mercury — the only metal that is liquid at ordinary room temperature (gallium melts just above it, at ~30 °C).

• practice — not a real PYQ

  Among the following, which metal is the best conductor of electricity?

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

  Answer(c) Silver — it has the highest electrical conductivity of all metals.