NDA (II) 2019 General Ability Test · Question 59 of 148

Correct — B. Diamond and graphite are allotropes of the same element, carbon, so their chemical properties are essentially the same — both are pure carbon and both burn in oxygen to give only carbon dioxide. Only their physical properties differ, because the atoms are bonded differently. So the claim that 'both physical and chemical properties are different' is not correct; the other three statements describe genuine physical differences.

• (a)Diamond has a tetrahedral structure, whereas graphite has a hexagonal planar structure. — This is true — diamond forms a three-dimensional tetrahedral network while graphite forms flat hexagonal layers, so it is a correct statement, not the answer.
• (c)Graphite is soft but diamond is hard. — True — graphite's layers slide over one another making it soft and slippery, while diamond is the hardest natural material; a real physical difference.
• (d)Graphite is a good conductor of electricity while diamond is not. — True — graphite has delocalised electrons between its layers and conducts, whereas diamond, with all four valence electrons locked in bonds, does not.

Allotropes are different structural forms of the same element. Because they are the same element, allotropes share their chemical properties but can differ sharply in physical properties owing to different bonding and arrangement. Diamond (a rigid tetrahedral network) and graphite (soft layered sheets) are the classic allotropes of carbon.

The false statement over-claims by saying the chemical properties also differ. Both solids are pure carbon and behave the same chemically — burning to carbon dioxide — so only the physical properties (hardness, structure, conductivity) set them apart.

• Diamond and graphite are allotropes of carbon — same element, same chemistry.
• Both burn in oxygen to give only carbon dioxide.
• Diamond is hard and non-conducting with a tetrahedral network; graphite is soft and conducting with a layered structure.
• Other carbon allotropes include fullerenes and carbon nanotubes.

• Assuming allotropes must differ chemically — they share chemical properties, differing only physically.
• Forgetting graphite conducts electricity while diamond does not.

Asked as 'which statement about diamond and graphite is not correct' — the false option usually claims their chemistry differs.

No directly related past PYQ was found.

• practice — not a real PYQ

  Diamond and graphite are

  • (a)isotopes of carbon
  • (b)allotropes of carbon
  • (c)isomers of carbon
  • (d)compounds of carbon

  Answer(b) allotropes of carbon — different structural forms of the same element.

• practice — not a real PYQ

  Graphite conducts electricity because

  • (a)it is very hard
  • (b)it has free (delocalised) electrons between its layers
  • (c)it contains metal atoms
  • (d)it is a compound

  Answer(b) it has free (delocalised) electrons between its layers, which carry current.