CDS (I) 2022 General Knowledge · Question 11 of 120

Correct — B, made of unsaturated hydrocarbons. A sooty yellow flame is the signature of incomplete combustion, and unsaturated compounds are the ones that burn incompletely in an ordinary air supply. The reason lies in their composition: a double or triple bond means two fewer hydrogens for the same number of carbons, so the molecule is proportionally richer in carbon and needs more oxygen per unit mass to burn cleanly. When the air supply cannot keep up, some carbon never reaches carbon dioxide. It is released as fine unburnt particles which glow as they are heated — that glow is the yellow of the flame — and then escape as the black smoke. Saturated hydrocarbons, with their fuller complement of hydrogen, ordinarily burn with a clean flame under the same conditions.

• (a)made of saturated hydrocarbons. — The opposite of what is observed. Saturated hydrocarbons burn with a clean flame in a good air supply; they will smoke only if the air supply itself is choked off, which is not what the option claims.
• (c)burning completely. — Complete combustion is the clean case, in which every carbon leaves as carbon dioxide and nothing is left over to glow or to escape as soot. Black smoke is unburnt carbon, so it is direct evidence that the combustion was not complete.
• (d)wet. — A wet fuel spits and sputters and is hard to light, because energy is spent boiling the water off. That does not produce a carbon-rich sooty flame — soot comes from carbon that failed to find oxygen, not from water.

Hydrocarbons are saturated when every carbon–carbon bond is a single bond, as in the alkanes, and unsaturated when at least one double or triple bond is present, as in the alkenes and alkynes. The distinction shows up in three standard tests. Unsaturated compounds add hydrogen across the multiple bond over a nickel catalyst; they decolourise bromine water, while saturated ones do not; and they burn with a yellow, sooty flame where saturated ones burn cleanly. All three come back to the same structural fact — a multiple bond means less hydrogen and a higher proportion of carbon.

The reasoning is a chain of two links, and both are worth holding. Sooty flame means unburnt carbon, and unburnt carbon means the fuel had more carbon than the available oxygen could convert. Then ask which molecules carry proportionally more carbon, and the answer is the unsaturated ones. The same physics explains something on any kitchen stove: if the air inlet of a gas burner is blocked, even the saturated hydrocarbons in cooking gas will burn with a yellow flame and deposit soot on the vessel, which is a warning sign because incomplete combustion also produces carbon monoxide.

• Saturated hydrocarbons contain only single carbon–carbon bonds; unsaturated ones contain at least one double or triple bond.
• Unsaturated hydrocarbons burn with a yellow flame and heavy black smoke; saturated ones generally give a clean flame.
• The black smoke is unburnt carbon, and its incandescence is what makes the flame look yellow.
• Unsaturated compounds decolourise bromine water, which is the standard laboratory test for a multiple bond.
• A yellow, sooty flame on a domestic gas burner usually means the air inlet is blocked, and incomplete combustion also releases carbon monoxide.

• Reading a sooty flame as a sign of complete combustion because the flame is bright.
• Assuming a smoky flame must mean a damp or dirty fuel rather than a carbon-rich one.
• Forgetting that a saturated fuel will also smoke if its air supply is restricted, so the test assumes an ordinary air supply.

As an inference from the appearance of the flame, as here, or as a straight comparison of how saturated and unsaturated compounds burn.

• practice — not a real PYQ

  A hydrocarbon decolourises bromine water on shaking. The hydrocarbon is

  • (a)saturated, and contains only single bonds
  • (b)unsaturated, and contains at least one double or triple bond
  • (c)necessarily a member of the alkane family
  • (d)necessarily a solid at room temperature

  Answer(b) unsaturated, and contains at least one double or triple bond — bromine adds across the multiple bond, which is why the colour disappears.

• practice — not a real PYQ

  The flame of a domestic gas burner turns yellow and blackens the bottom of the vessel. The most likely reason is that

  • (a)the gas supply pressure has risen
  • (b)the air inlet holes of the burner are blocked
  • (c)the vessel is made of steel rather than aluminium
  • (d)the gas has been stored for too long

  Answer(b) the air inlet holes of the burner are blocked — with too little oxygen the combustion is incomplete, unburnt carbon glows yellow and deposits as soot.