MPSC 2021 Prelims Paper-I · Question 67 of 100

Correct — option (1), All of the above. All four statements are true, and they are true for one connected reason: diamond and graphite are allotropes of the same element, built from the same carbon atoms joined in different ways, and every property in the list follows from the difference in the arrangement. Take diamond first. Each carbon atom in diamond is bonded to four others at the corners of a tetrahedron by strong covalent bonds, and the pattern repeats through the whole crystal, so a diamond is one giant molecule. Every one of carbon's four outer electrons is locked into a bond, and there is no electron free to move through the structure. An electric current is a flow of mobile charge, so diamond does not conduct electricity, and statement (a) is correct. The same rigid three-dimensional network of covalent bonds makes diamond the hardest naturally occurring substance, and hardness is exactly what a drilling or cutting tool needs: diamond-tipped bits are used for drilling rock in mining and oil exploration, and diamond is used industrially as an abrasive and for cutting and grinding, so statement (d) is correct as well. Now graphite. Each carbon atom in graphite is bonded to only three others, and those bonds lie in a plane, so the atoms build up flat sheets of linked hexagons — the two-dimensional sheet-like structure of statement (b), which is correct. The sheets are stacked one above another and held together only by weak forces, which is why graphite is soft and slippery and why it works as a dry lubricant and as the 'lead' of a pencil, a sheet at a time being left behind on the paper. The fourth outer electron of each carbon atom is not used in the bonding of the sheet at all; it is free to move along the layers. Those mobile electrons are what make graphite a conductor of electricity, so statement (c) is correct too, and graphite is used for electrodes in electrolysis and in furnaces for that reason. So the same element gives the hardest known natural material and one of the softest solids in common use, an insulator and a conductor, a transparent crystal and an opaque grey solid, purely by rearranging the bonds — which is what makes this pair the standard illustration of allotropy in the syllabus. Since no statement in the list fails, the option that includes all of them is the answer. Note in passing that this paper spells its escape options a dozen different ways; the exact phrase 'All of the above' is printed only twice in the English column of the whole paper, here and in the question that follows.

• (2)Only (a), (b), (c) — This accepts the three statements about structure and conduction but drops statement (d), the use of diamond for rock drilling — and that statement is true. Diamond's hardness comes from the same feature that explains its failure to conduct: every carbon atom is joined to four neighbours by strong covalent bonds in a rigid three-dimensional network, so the crystal cannot be scratched, deformed or cut by anything softer, and there are no free electrons. Diamond is in fact used far more widely in industry than in jewellery — as the cutting edge of drilling bits for rock and for oil wells, as an abrasive in grinding wheels and polishing pastes, and in glass cutters and surgical instruments — and a large share of the diamond used for such purposes is synthetic rather than mined. An option that quietly omits one true statement is the commonest way of dressing up a question whose real answer is that all of them hold.
• (3)Only (a), (b) — This keeps only the two statements about diamond's failure to conduct and graphite's sheet structure, and rejects both the conductivity of graphite and the use of diamond in drilling, each of which is true. The conductivity of graphite is the most instructive of the four statements, because it is the property that surprises: carbon is a non-metal, and non-metals as a class do not conduct, yet graphite does. The explanation is that only three of each carbon atom's four outer electrons are used in bonding within a sheet, leaving the fourth delocalised and mobile along the layers, so graphite conducts along its sheets while remaining a poor conductor across them. That is why graphite is used for the electrodes of dry cells and of electrolytic cells, where a chemically inert conductor is needed. A candidate who has learnt only that non-metals do not conduct electricity is likely to reject the statement and land here.
• (4)Only (b), (c) — This keeps the two statements about graphite and rejects both statements about diamond, although both are correct. Diamond is a non-conductor of electricity because all four of its outer electrons are held in covalent bonds and none is free to carry charge, and it is used for rock drilling because the same bonding makes it the hardest naturally occurring substance. One refinement is worth carrying, because examiners like it: diamond is a very poor conductor of electricity but an excellent conductor of heat, better than most metals, since heat travels through the rigid lattice as vibrations rather than by moving electrons. Electrical and thermal conduction are therefore not the same property, and a substance can be strong at one and useless at the other. Choosing this option usually means the candidate has assumed that whatever is true of graphite must be false of diamond, which is the wrong way to read a list of independent statements.

Allotropy is the existence of an element in two or more forms in the same physical state, differing in the way the atoms are bonded and therefore in properties. Carbon is the classic example. In diamond every carbon atom forms four single covalent bonds directed to the corners of a tetrahedron, producing a rigid three-dimensional network; the result is the hardest naturally occurring substance, transparent, with a very high melting point, chemically unreactive, an excellent conductor of heat but an insulator of electricity because no electron is free. In graphite every carbon atom forms three covalent bonds within a plane, producing flat sheets of fused hexagons; the sheets are stacked and held to one another only by weak forces, so they slide easily, making graphite soft, greasy to the touch and useful as a lubricant and in pencils, while the fourth electron of each atom is delocalised and makes graphite a conductor of electricity along its layers. Graphite is also, unusually, less dense than diamond and is the thermodynamically stable form of carbon at ordinary conditions. Beyond these two classical forms the syllabus now includes the fullerenes, of which buckminsterfullerene, a cage of sixty carbon atoms shaped like a football, is the best known; carbon nanotubes, which are rolled sheets of the graphite type; and graphene, a single such sheet one atom thick. Amorphous forms such as coke, charcoal, lampblack and carbon black have no long-range order. Allotropy is not confined to carbon: oxygen exists as dioxygen and as ozone, phosphorus as white, red and black, and sulphur as rhombic and monoclinic.

Carbon's allotropes are among the most dependable topics in MPSC's chemistry section, because a single comparison — diamond against graphite — supports questions on bonding, on hardness, on electrical conduction, on lubrication and on industrial use, and every one of those answers can be derived from the two structures rather than memorised. The Commission likes the form used here, a list of independent statements to be judged true or false, and this question is a reminder that a statement list may contain no false statement at all. Candidates lose marks on such items in a particular way: having found three statements true, they begin to distrust the pattern and look for the flaw in the fourth. The discipline is to judge each statement entirely on its own merits and then to select whichever option matches the verdicts, without any expectation about how many ought to be true. It is also worth noticing how this paper handles escape options. It spells them in many different ways, and the exact phrase 'All of the above' — the wording printed here — occurs only twice in the whole hundred questions, so a candidate should read the option text rather than reach for a remembered position.

• Diamond and graphite are allotropes of carbon: the same element in the same physical state but with different bonding arrangements, which is why their properties differ so sharply.
• In diamond each carbon atom is covalently bonded to four others in a rigid three-dimensional tetrahedral network, leaving no free electrons, so diamond is the hardest natural substance and does not conduct electricity — though it conducts heat exceptionally well.
• In graphite each carbon atom is bonded to only three others within a plane, giving two-dimensional sheets of hexagons stacked on one another and held by weak forces, which makes graphite soft and slippery and useful as a lubricant and in pencils.
• The fourth outer electron of each carbon atom in graphite is delocalised and free to move along the sheets, which is why graphite, alone among the common non-metals, conducts electricity and is used for electrodes.
• Diamond's hardness makes it an industrial material before it is a gem: it is used in rock-drilling and oil-drilling bits, in abrasives and grinding wheels, and in glass cutters, and much of the diamond so used is synthetic.

• Assuming that no non-metal can conduct electricity, when graphite conducts because one electron per carbon atom is delocalised along its sheets
• Expecting a statement list to contain at least one false statement, and hunting for a flaw that is not there
• Confusing electrical with thermal conduction, since diamond is an insulator of electricity yet an outstanding conductor of heat
• Reaching for a remembered position instead of reading the option text, in a paper that spells its escape options in many different ways

MPSC asks the carbon allotropes as property questions — which form conducts, which is hardest, which is used as a lubricant — as structure questions about the number of bonds each atom forms, and as statement lists of the type set here. The Commission also connects the topic to industry, since graphite electrodes, diamond abrasives and synthetic diamond production sit in the applied part of the syllabus, and to the newer materials, where fullerenes and graphene turn up in science-and-technology current affairs. Because every property in the classical comparison can be derived from the two structures, the efficient preparation is to be able to draw both — a tetrahedral network for diamond, stacked hexagonal sheets for graphite — and to read the properties off the drawing. That method also protects against the reverse form of the question, in which a property is stated and the candidate is asked which form it belongs to.

No directly related past PYQ was found.

• practice — not a real PYQ

  Graphite conducts electricity whereas diamond does not, because

  • (a)graphite contains metallic impurities while diamond is pure carbon
  • (b)in graphite one electron of each carbon atom is delocalised and free to move, while in diamond all four are used in covalent bonds
  • (c)graphite has ionic bonds and diamond has covalent bonds
  • (d)graphite is softer than diamond

  Answer(b) In graphite one electron of each carbon atom is delocalised and free to move, while in diamond all four are used in covalent bonds — each carbon in graphite is bonded to only three neighbours within a sheet, so the fourth outer electron is not tied to any bond and can travel along the layers, and a flow of such mobile electrons is an electric current. In diamond each carbon atom is bonded to four neighbours in a tetrahedral network, so every outer electron is committed and none is available to carry charge. Both forms are pure carbon with covalent bonding, and softness by itself has nothing to do with conduction.

• practice — not a real PYQ

  Which one of the following pairs consists of allotropes of the same element ?

  • (a)Diamond and silicon carbide
  • (b)Ozone and hydrogen peroxide
  • (c)Graphite and buckminsterfullerene
  • (d)Deuterium and tritium

  Answer(c) Graphite and buckminsterfullerene — both are forms of the single element carbon in the same physical state, differing only in how the atoms are bonded, which is exactly what allotropy means; buckminsterfullerene is the cage of sixty carbon atoms shaped like a football. Silicon carbide is a compound of two elements, and hydrogen peroxide is a compound as well, so neither pairing can be allotropic. Deuterium and tritium are isotopes of hydrogen — same element, same chemistry, different numbers of neutrons in the nucleus — which is a different idea from allotropy altogether.