Which among the following statements is not correct with respect to allotropes of Carbon?
- (a)Graphite is a good conductor of electricity.
- (b)Diamond is the hardest substance known.
- (c)Fullerene is one of the allotropes of carbon.
- (d)Next to diamond, graphite is the second hardest known substance.
Correct — D, 'Next to diamond, graphite is the second hardest known substance.' This is FALSE — graphite is in fact one of the softest solids, soft and slippery enough to be used as a lubricant and in pencil leads. The other three statements about carbon allotropes are all correct, so D is the incorrect one.
- (a)Graphite is a good conductor of electricity. — This is true — each carbon in graphite has a delocalised (free) electron, so graphite conducts electricity; it is a correct statement, not the answer.
- (b)Diamond is the hardest substance known. — This is true — diamond's rigid three-dimensional tetrahedral network makes it the hardest naturally occurring substance.
- (c)Fullerene is one of the allotropes of carbon. — This is true — fullerenes such as C60 ('buckyballs') are a genuine molecular allotrope of carbon.
Allotropes are different structural forms of the same element. Carbon has several — diamond, graphite and the fullerenes — all made only of carbon atoms but arranged differently, which is why their properties differ so sharply. Diamond is very hard and non-conducting, while graphite is soft and conducts electricity.
The question asks for the false statement, so the trick is to spot the property that contradicts what you know. Diamond being hardest, graphite conducting, and fullerene being an allotrope are all correct; calling graphite the second hardest substance clashes with the fact that graphite is soft.
- In diamond each carbon is bonded to four others in a tetrahedral network, making it very hard and a non-conductor.
- Graphite has layered hexagonal sheets with free electrons, so it is soft, slippery and conducts electricity.
- Fullerenes such as C60 and carbon nanotubes are molecular allotropes of carbon.
- All these forms contain only carbon atoms but differ in structure, hence in properties.
- Assuming every form of carbon is hard.
- Thinking graphite does not conduct electricity.
NDA/UPSC ask which statement about carbon allotropes is correct or incorrect — remember diamond is hard and non-conducting, graphite is soft and conducting.
No directly related past PYQ was found.
- practice — not a real PYQ
Graphite is used as a lubricant and in pencils because it is
- (a)extremely hard
- (b)soft and slippery
- (c)a poor conductor
- (d)a gas at room temperature
Answer(b) soft and slippery — its layers slide over one another, making graphite a good lubricant and writing material.
- practice — not a real PYQ
Which allotrope of carbon is the hardest natural substance?
- (a)Graphite
- (b)Fullerene
- (c)Diamond
- (d)Charcoal
Answer(c) Diamond — its rigid tetrahedral carbon network makes it the hardest naturally occurring substance.