CDS (II) 2021 General Knowledge · Question 9 of 120

Correct — B, 0.025%. Carbon is everywhere in living matter and almost nowhere in the rocks — the standard textbook line is that the earth's crust has only 0.02% carbon, in the form of minerals such as carbonates, hydrogen-carbonates, coal and petroleum, while the atmosphere holds about 0.03% carbon dioxide. Geochemical surveys of crustal abundance put the figure in the same narrow band, a couple of hundred parts per million, which is a few hundredths of one per cent. Set against the four numbers offered, that lands squarely on 0.025% and rules the others out by an order of magnitude in one direction or a factor of two in the other. The comparison worth carrying away is the contrast rather than the decimal: oxygen accounts for roughly 46% of the crust by mass and silicon for about 27%, so carbon — the element on which every living structure is built — is a trace constituent of the rock beneath our feet.

• (a)0.045% — Roughly double the accepted band. No standard source puts crustal carbon above about 0.03%.
• (c)0.015% — Below the band. It would place crustal carbon at about 150 parts per million, under the textbook and survey figures alike.
• (d)0.005% — Smaller than the accepted value by roughly a factor of four or five, which would make crustal carbon rarer than it actually is.

Crustal abundance measures how much of the earth's solid outer shell, by mass, a given element makes up. Oxygen and silicon dominate, together accounting for close to three-quarters of the crust, followed by aluminium, iron, calcium, sodium, potassium and magnesium. Carbon does not make that list at all; it is present in a few tenths of a per mille, locked up in carbonate rocks such as limestone, in bicarbonates, and in the fossil fuels. Its importance to us rests on chemistry — catenation and tetravalency let it build millions of compounds — rather than on how much of it there is.

The item rewards knowing the order of magnitude rather than a precise decimal. Anyone who remembers the textbook opening — carbon at a few hundredths of one per cent in the crust and carbon dioxide at about 0.03% in the atmosphere — can eliminate 0.005% and 0.045% at once and then choose between two neighbours. One honest caveat belongs on this card: the exact figure is not a single settled number. The textbook prints 0.02%, published crustal abundance tables generally give something between 0.018% and 0.03%, and none of the four printed options reproduces the textbook value exactly. The key takes 0.025% because it is the only option inside that band, and this card is written to the key on that basis rather than certifying 0.025% as a published measurement.

• The standard school text states that the earth's crust has only 0.02% carbon, in the form of minerals such as carbonates, hydrogen-carbonates, coal and petroleum.
• The same passage puts carbon dioxide in the atmosphere at about 0.03%.
• Published crustal abundance figures for carbon fall in the range of roughly 180 to 300 parts per million, that is about 0.018% to 0.03% by mass.
• Oxygen makes up about 46% of the crust by mass and silicon about 27%; carbon is not among the major constituents.
• Carbon's significance comes from catenation and tetravalency, which let it form millions of compounds, not from its abundance.

• Confusing the crustal figure for carbon with the atmospheric figure for carbon dioxide; the two are different quantities in different reservoirs.
• Assuming an element important to life must be abundant in rock.
• Treating a textbook percentage as an exact constant — crustal abundances are estimates and vary between sources.

As a straight recall of the percentage, as an ordering of the major crustal elements, or as a statements item pairing crustal carbon with atmospheric carbon dioxide.

• practice — not a real PYQ

  The approximate percentage of carbon dioxide in the earth's atmosphere is

  • (a)0.3%
  • (b)0.03%
  • (c)3%
  • (d)0.003%

  Answer(b) 0.03% — the standard textbook figure, against about 0.02% carbon in the earth's crust.

• practice — not a real PYQ

  In the earth's crust, most of the carbon occurs in the form of

  • (a)free graphite
  • (b)carbonate and hydrogen-carbonate minerals, coal and petroleum
  • (c)diamond
  • (d)carbon monoxide trapped in rock

  Answer(b) carbonate and hydrogen-carbonate minerals, coal and petroleum — free carbon is a very small part of the crust's carbon budget.