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General ScienceEasyAsked in: CDS · CDS-II 2020, GK

Which one of the following does not form oxide on reaction with oxygen?

  1. A.Magnesium
  2. B.Lead
  3. C.Tin
  4. D.Silver

Correct answer

D. Silver

Explanation

The correct answer is D, Silver. Silver sits near the bottom of the reactivity series and does not combine with oxygen even when heated in air.

How readily a metal reacts with oxygen depends on its place in the reactivity series: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, copper, mercury, silver and gold. Metals at the top burn easily, those in the middle form oxides slowly or only on heating, and silver and gold stay unaffected. That is why silver and gold are used for jewellery and coins. Silver articles do turn black with time, but that layer is silver sulphide, formed by the hydrogen sulphide in air, not an oxide.

Option A is wrong because magnesium burns in air with a dazzling white flame and forms magnesium oxide, MgO. Option B is wrong because lead forms lead oxide, PbO, when heated in air. Option C is wrong because tin, when heated strongly in air, forms tin oxide, SnO2.

Exam tip: silver and gold do not react with oxygen even on heating; tarnished silver is Ag2S, not an oxide.

Practice Questions

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Q1.General ScienceAsked in: CDS · CDS-II 2020, GKEasy

The valency of phosphorus is

  1. A.2, 3
  2. B.3, 4
  3. C.4, 5
  4. D.3, 5
Show answer
Correct answer: D. 3, 5

Phosphorus has five electrons in its outer shell and shows two common valencies, 3 and 5. Phosphorus, atomic number 15, has the electronic configuration 2, 8, 5.

Q2.General ScienceAsked in: CDS · CDS-II 2026, GKMedium

Which one of the following statements is NOT correct ?

  1. A.Single celled eukaryotes belong to Kingdom Protista
  2. B.Yeast, a fungus is multicellular
  3. C.The fungi constitute a unique kingdom of heterotrophs
  4. D.Ascomycetes, a saprophytic fungi is multicellular or rarely unicellular
Show answer
Correct answer: B. Yeast, a fungus is multicellular

This statement is false, because yeast is one of the few fungi that are unicellular. In Whittaker's five-kingdom system, fungi form a separate kingdom.

Q3.General ScienceAsked in: CDS · CDS-II 2017, GKMedium

Desalination of seawater is done by using reverse osmosis. The pressure applied to the solution is

  1. A.larger than osmotic pressure
  2. B.smaller than osmotic pressure
  3. C.equal to osmotic pressure
  4. D.equal to atmospheric pressure
Show answer
Correct answer: A. larger than osmotic pressure

Only a pressure greater than the osmotic pressure of seawater can push pure water backwards through the membrane. In ordinary osmosis, water moves through a semipermeable membrane from the dilute side to the concentrated side.

Q4.General ScienceAsked in: CDS · CDS-II 2017, GKEasy

The foul smell of urine of a healthy man having healthy food, when spilled on floor, is mainly due to the bacterial decomposition of

  1. A.urea into sulphur dioxide
  2. B.sugar into carbon dioxide
  3. C.lipids into methane
  4. D.urea into ammonia
Show answer
Correct answer: D. urea into ammonia

Bacteria on the floor break down the urea in urine and release ammonia, which has a sharp, pungent smell. Urea, CO(NH2)2, is the main nitrogenous waste of humans.

Q5.General ScienceAsked in: CDS · CDS-II 2020, GKEasy

Which one of the following is used for storing biological tissues?

  1. A.Liquid nitrogen
  2. B.Liquid helium
  3. C.Liquid argon
  4. D.Liquid bromine
Show answer
Correct answer: A. Liquid nitrogen

It is very cold, chemically inert and cheap, which makes it the standard coolant for preserving cells and tissues. Liquid nitrogen boils at about −196 °C. At such low temperatures the chemical and enzyme activity inside cells almost stops, so tissues can be kept unchanged for years.