Which one of the following statements is NOT correct ?
- A.Diamond is hard due to extended covalent bonding in it
- B.Fullerene is composed of five and six membered rings
- C.Diamond is a good conductor of electricity
- D.Graphite is used as a lubricant
Correct answer
Explanation
The correct answer is C, Diamond is a good conductor of electricity. This statement is false: diamond is an electrical insulator because it has no free electrons.
In diamond, every carbon atom uses all four of its valence electrons to form strong covalent bonds with four other carbon atoms in a rigid three-dimensional network. With no electron left free to move, diamond does not conduct electricity. Graphite is different: each carbon bonds to only three others, leaving one free electron per atom, so graphite conducts electricity.
A is correct because the extended three-dimensional covalent network makes diamond the hardest natural substance. B is correct because fullerene (C60, buckminsterfullerene) is a football-shaped cage of 12 five-membered and 20 six-membered rings. D is correct because graphite's layers are held by weak forces and slide over one another, so it works as a dry lubricant.
Exam tip: diamond is hard and an insulator; graphite is soft, slippery and a conductor.
Practice Questions
View allWhich one of the following features is an indication for modification of stem of a plant?
- A.Presence of 'eye' on potato
- B.'Scale' found in onion
- C.'Tendril' found in pea
- D.'Hair' present in carrot
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The eyes of a potato are nodes with buds, and only a stem bears nodes and buds, so they prove that the potato is a modified stem.
Read the full explanation →Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy ?
- A.Tungsten
- B.Copper
- C.Rubidium
- D.Aluminium
Show answer
Rubidium, an alkali metal like caesium, gives off electrons easily when light falls on it, so it is used in photoelectric cells.
Read the full explanation →Tooth enamel is made up of which one of the following calcium compounds ?
- A.Calcium carbonate
- B.Calcium sulphate
- C.Calcium hydroxide
- D.Calcium phosphate
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Tooth enamel is made mainly of calcium phosphate in the form of a hard crystal called hydroxyapatite. Enamel is the hardest substance in the human body, harder even than bone.
Read the full explanation →Which one of the following viruses used to be responsible for highly infectious disease Smallpox ?
- A.Adenovirus
- B.Variola virus
- C.Aichi virus
- D.Coxsackie virus
Show answer
Smallpox was caused by the variola virus, a large DNA virus of the pox family that spread from person to person through the air.
Read the full explanation →After fertilization, the fruit and the seed are produced by
- A.ovule and ovary, respectively
- B.ovary and ovule, respectively
- C.ovary, no ovule required
- D.ovule, no ovary required
Show answer
After fertilisation the ovary of the flower grows into the fruit, and each ovule inside it develops into a seed.
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