CDS (II) 2024 General Knowledge · Question 13 of 120

Correct — D, Lithium. All four options are alkali metals, and every one of them is far too reactive to be left in the open — they attack the oxygen and moisture of the air, and they react vigorously with water. The usual protection is to keep them submerged under kerosene or another mineral oil, which excludes both air and water. That works for sodium, potassium and rubidium, but not for lithium, and the reason is density rather than chemistry. Lithium is the lightest of all metals, at about 0.534 grams per cubic centimetre, so it is not dense enough to sink in mineral oil; it floats, leaving its upper surface exposed to exactly the air the oil was meant to keep out. Lithium is therefore stored in a hydrocarbon sealant such as petroleum jelly, or sealed under an inert gas such as argon. Lithium is also the one alkali metal that reacts appreciably with nitrogen, so ordinary air attacks it in two ways rather than one.

• (a)Sodium — Sodium is the textbook case of a metal kept under kerosene. It is denser than the oil, so it stays fully submerged, and without that cover it would react with moist air and could catch fire.
• (b)Rubidium — Rubidium is heavier and even more reactive than potassium — it ignites on contact with air — so it too is kept out of contact with air, under oil or sealed in glass under an inert gas. Being dense, it submerges without difficulty.
• (c)Potassium — Potassium is stored under oil for the same reason as sodium, and with more urgency, since it reacts with water violently enough for the hydrogen released to ignite on its own.

The alkali metals form Group 1 of the periodic table — lithium, sodium, potassium, rubidium, caesium and francium. Each has a single loosely held outer electron, which it gives up readily, and that makes the group the most reactive family of metals. Reactivity increases down the group as the outer electron gets further from the nucleus, so caesium is more violent than lithium. All of them tarnish in air and react with water to give a hydroxide and hydrogen gas.

The item looks like a chemistry question and is settled by a physical property. Because every alkali metal needs protection from air, the answer cannot be found by asking which is least reactive; it is found by asking which one the oil fails to cover. Lithium floats on the lightest hydrocarbon oils, so submerging it is impossible and a different method has to be used. Two related facts are worth carrying. Lithium, sodium and potassium are the only three metals light enough to float on water, and lithium is the least dense solid element of all. And because lithium alone among the alkali metals combines readily with nitrogen, an inert cover for it has to be argon rather than nitrogen, which would do for many other reactive materials.

• Lithium is the least dense metal, about 0.534 grams per cubic centimetre, and the least dense solid element.
• Lithium is not dense enough to submerge in mineral oil, so it is stored in a hydrocarbon sealant such as petroleum jelly, or under an inert gas.
• The heavier alkali metals — sodium, potassium, rubidium — can be and are stored under mineral oil or kerosene.
• Lithium, sodium and potassium are the only three metals that float on water.
• Lithium is the one alkali metal that reacts appreciably with nitrogen as well as with oxygen and water.

• Reading the item as asking which metal is least reactive; all four need protection, and the answer turns on density.
• Assuming an inert cover can always be nitrogen; lithium reacts with nitrogen, so argon is used.
• Confusing kerosene storage with a chemical reaction — the oil simply keeps air and moisture away.

Usually as a which-metal-is-stored-under-kerosene item, or through the alkali-metal reactivity trend; assertion-and-reason versions on sodium and kerosene recur in UPSC.

• practice — not a real PYQ

  Sodium metal is kept immersed in kerosene chiefly because

  • (a)kerosene dissolves the impurities on its surface
  • (b)kerosene keeps air and moisture away from the metal
  • (c)kerosene reacts with sodium to form a protective layer
  • (d)kerosene keeps the metal cool

  Answer(b) kerosene keeps air and moisture away from the metal — sodium reacts readily with oxygen and water, so the oil acts as a barrier and nothing more.

• practice — not a real PYQ

  Which one among the following is the least dense metal?

  • (a)Sodium
  • (b)Lithium
  • (c)Aluminium
  • (d)Magnesium

  Answer(b) Lithium — at about 0.534 grams per cubic centimetre it is the least dense metal and the least dense solid element, which is why it floats even on oil.