NDA (I) 2018 General Ability Test · Question 85 of 150

Correct — C, By burning hydrocarbons in a limited supply of air. Carbon black is soot made deliberately. When a hydrocarbon burns with plenty of air, every carbon atom ends up as carbon dioxide and nothing solid is left. Starve the same flame of air and combustion cannot go to completion, so a fraction of the carbon never reaches the gas phase and settles out as an extremely fine black powder. Industrially the feedstock is a heavy oil or natural gas, burnt or thermally cracked with only part of the oxygen it would need, and the powder is collected from the smoke. The same chemistry on a domestic scale is the reason a kerosene lamp or a badly adjusted burner blackens the vessel above it.

• (a)By heating wood at high temperature in absence of air — That is the recipe for charcoal, not carbon black. Heating wood out of contact with air is destructive distillation — the volatile matter is driven off and a porous black residue of carbon remains.
• (b)By heating coal at high temperature in absence of air — This produces coke, along with coal gas and coal tar. Coke is the hard grey-black fuel and reducing agent charged into a blast furnace; it is not the fine powdery carbon black.
• (d)By heating coal at high temperature in presence of air — Heating coal with air available simply burns it. The carbon leaves as carbon dioxide and what remains is ash, so no useful carbon product is collected at all.

The impure forms of carbon are told apart by how each is made. Charcoal comes from heating wood without air; coke comes from heating coal without air; lampblack and carbon black come from burning a hydrocarbon with too little air; animal charcoal or bone black comes from heating bones without air; and gas carbon is the deposit left inside the retorts used to make coal gas. Carbon black matters commercially far more than its humble origin suggests. The great bulk of the world's output goes into rubber, where it is mixed into tyre compounds as a reinforcing filler that makes the rubber tougher and far more resistant to wear; the rest goes into printing inks, paints, plastics and toners.

The four options here are not random — three of them are the correct answers to three neighbouring questions, so the item really tests whether a candidate can keep the preparations apart rather than whether they know carbon black. The distinguishing variables are only two, the starting material and whether air is present, so build a small grid on those axes: wood without air gives charcoal, coal without air gives coke, hydrocarbon with limited air gives carbon black. Notice as well that the phrasing of the correct option is precise in a way worth copying — 'limited supply of air', not 'absence of air'. With no air at all a hydrocarbon does not burn; carbon black needs a real flame, just a starved one.

• Carbon black is made by burning hydrocarbons in a restricted supply of air, so combustion stays incomplete.
• Charcoal is made by heating wood in the absence of air; coke is made by heating coal in the absence of air.
• Most of the world's carbon black goes into rubber, chiefly as a reinforcing filler in tyres.
• It is also the pigment in printing inks, black paints and toners.
• All these are impure or amorphous forms of carbon, as distinct from the crystalline allotropes diamond and graphite.

• Reading 'limited supply of air' as 'absence of air' — the difference separates carbon black from charcoal and coke.
• Assuming carbon black and coke are the same thing because both are black forms of carbon from fuels.
• Forgetting that heating coal WITH air simply burns it and yields nothing worth collecting.

NDA sets these preparations either as a direct 'how is X obtained' item or as a match-list pairing each form of carbon with its source.

• practice — not a real PYQ

  Coke is obtained by

  • (a)burning coal in excess air
  • (b)heating coal strongly in the absence of air
  • (c)burning natural gas in limited air
  • (d)heating wood in the absence of air

  Answer(b) heating coal strongly in the absence of air — destructive distillation, which also yields coal gas and coal tar.

• practice — not a real PYQ

  The largest single industrial use of carbon black is in

  • (a)printing inks
  • (b)rubber and tyre manufacture
  • (c)pencil leads
  • (d)water purification

  Answer(b) rubber and tyre manufacture — it is added as a reinforcing filler that improves strength and wear resistance.