Which one of the following statements is not correct?
- A.Most carbon compounds are good conductors of electricity.
- B.Bonding in organic compounds is covalent.
- C.Graphite is used as a lubricant.
- D.Diamond is an allotrope of carbon.
Correct answer
Explanation
The correct answer is A. Most carbon compounds are poor conductors of electricity, because their atoms are joined by covalent bonds and they do not form ions or free electrons that could carry a current.
Carbon has four valence electrons and completes its octet by sharing them, so its compounds are covalent. Such compounds usually have low melting and boiling points and do not conduct electricity in the solid, liquid or dissolved state. Graphite is a special case: each carbon atom bonds with only three others, leaving one free electron per atom, so graphite conducts electricity and is used for electrodes.
B is correct as a statement because organic compounds are held together by shared electron pairs.
C is correct as a statement because graphite's layers slide over one another easily, making it a good dry lubricant for machine parts.
D is correct as a statement because diamond is a crystalline form of pure carbon.
Exam tip: carbon compounds are covalent and non-conducting; graphite is the one form of carbon that conducts.
Practice Questions
View allThe potential difference between the terminals of an electrical heating device is 40 V, when it draws a current of 2 A from the source. Which one of the following is the correct value of current drawn by the heating device if the potential is increased to 140 V?
- A.7 A
- B.5 A
- C.4 A
- D.2 A
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By Ohm's law the heater's resistance is 40 V divided by 2 A, which is 20 ohms, so at 140 V it draws 140 divided by 20, or 7 A. Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the potential difference across…
Read the full explanation →Consider the following statements in the light of octane number and cetane number of fuels: 1. Cetane number is used for gasoline and octane number is used for diesel. 2. Octane number of n-hexadecane is 100 and cetane number of isooctane is 100. Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Show answer
Both statements swap the two scales: octane number rates petrol, cetane number rates diesel, and their reference fuels are the other way round.
Read the full explanation →Which one among the following statements related to human eye is correct?
- A.For a person suffering from short-sight defect, the image of a distant object is focused behind the retina.
- B.Short-sight defect can be corrected by a converging spectacle lens.
- C.For a person suffering from long-sight defect, the image of a near object is focused behind the retina.
- D.Long-sight defect can be corrected by a diverging spectacle lens.
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In long-sightedness (hypermetropia) the eye cannot see nearby objects clearly because their image is formed behind the retina.
Read the full explanation →When sunlight (white light) falls on a triangular glass prism, a band of colours called a spectrum is obtained. Which one among the following represents this effect appropriately?
- A.Reflection
- B.Diffraction
- C.Refraction
- D.Dispersion
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The splitting of white light into its seven component colours by a prism is called dispersion. White light is a mixture of colours, and glass bends each colour by a different amount because its refractive index depends on wavelength.
Read the full explanation →Which one of the following statements is correct?
- A.The number of electrons in alpha particle is 2.
- B.The coordination number of EDTA is 6.
- C.The charge of beta (β) particle is +2.
- D.Both carbon and silicon form ionic compounds.
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EDTA is a hexadentate ligand: one EDTA ion binds a metal ion through six donor atoms, so it forms complexes with coordination number 6.
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