Carborundum is used as an abrasive, because it
- A.has high thermal conductivity
- B.has low coefficient of expansion
- C.has high chemical stability
- D.is extremely hard
Correct answer
Explanation
The correct answer is D, is extremely hard. Carborundum, or silicon carbide (SiC), is one of the hardest substances known, so it can grind, cut and polish other materials, which is what an abrasive must do.
Silicon carbide has a hardness of about 9 to 9.5 on the Mohs scale, close to diamond at 10. Its hardness comes from a strong network in which every silicon atom is bonded to four carbon atoms, much like the structure of diamond. It was first made by Edward Acheson in 1891 by heating sand and coke in an electric furnace, and it is used in grinding wheels, sandpaper and cutting tools.
A is wrong because good heat conduction helps in some uses but does not make a material scratch or wear away another. B is wrong because a low expansion coefficient keeps a material stable when heated, which is not the property an abrasive relies on. C is wrong because chemical stability is useful but many stable substances are soft and useless as abrasives.
Exam tip: carborundum = silicon carbide, Mohs hardness about 9.5, made by the Acheson process.
Practice Questions
View allWhich one of the following is the purest form of Carbon ?
- A.Charcoal
- B.Coke
- C.Fullerene
- D.Carbon black
Show answer
Fullerenes are made of carbon atoms alone, with no surface to trap other substances, so they are counted as the purest form of carbon.
Read the full explanation →A planet has a mass M₁ and radius R₁. The value of acceleration due to gravity on its surface is g₁. There is another planet 2, whose mass and radius both are two times that of the first planet. Which one of the following is the acceleration due to gravity on the surface of planet 2?
- A.g₁
- B.2g₁
- C.g₁/2
- D.g₁/4
Show answer
Surface gravity is g = GM/R², so doubling the mass doubles g while doubling the radius divides it by four, leaving g₁ × 2/4 = g₁/2.
Read the full explanation →Which one of the following is the major constituent of biogas ?
- A.Carbon dioxide
- B.Nitrous oxide
- C.Methane
- D.Oxygen
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Methane makes up the largest share of biogas, usually about 55 to 75 per cent, and it is the gas that burns.
Read the full explanation →A lemon kept in water in a glass tumbler appears to be larger than its actual size. It is because of
- A.reflection of light
- B.scattering of light
- C.refraction of light
- D.polarization of light
Show answer
The lemon looks bigger because light from it bends as it passes from water through the curved glass into air.
Read the full explanation →Which one of the following conclusions could not be derived from Rutherford's α-particle scattering experiment?
- A.Most of the space in the atom is empty.
- B.The radius of the atom is about 10⁵ times the radius of the nucleus.
- C.Electrons move in a circular path of fixed energy called orbits.
- D.Nearly all the mass of the atom resides in the nucleus.
Show answer
. Orbits of fixed energy were proposed by Niels Bohr in 1913; Rutherford's experiment said nothing about the energy of electrons.
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