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General ScienceEasyAsked in: NDA · NDA-I 2017, GAT

Which one of the following elements is used in pencil-lead ?

  1. A.Zinc
  2. B.Lead
  3. C.Carbon (Graphite)
  4. D.Tin

Correct answer

C. Carbon (Graphite)

Explanation

The correct answer is C, Carbon (Graphite). The so-called lead of a pencil is a mix of graphite and clay; it contains no lead metal.

Graphite is a crystalline form, or allotrope, of carbon. Its atoms are joined in flat layers of hexagons, and the layers are held together only weakly, so they slide off easily and leave a dark mark on paper. The name lead stuck because early users in sixteenth-century England thought the black mineral was a kind of lead ore. Mixing more clay makes a harder, lighter pencil (H grades), and more graphite a softer, darker one (B grades). Graphite is also soft, slippery and a good conductor of electricity, so it is used as a dry lubricant and for electrodes.

A is wrong: zinc is used for galvanising iron and in dry cells. B is wrong: lead metal is not used in pencils; it is a poisonous metal used in batteries and radiation shields. D is wrong: tin is used for tin-plating and in solder.

Exam tip: Pencil lead is graphite, an allotrope of carbon; diamond is the other well-known allotrope.

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.

Q5.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.