Naphthalene burns with a yellow sooty flame. This is because
- (a)carbon to hydrogen ratio is low
- (b)there is incomplete combustion
- (c)there is excess supply of air
- (d)of presence of impurities of nitrogen and sulfur
Correct — B, there is incomplete combustion. Naphthalene is C₁₀H₈, a pair of benzene rings fused along one edge, and that is a great deal of carbon for very little hydrogen. Burning it in ordinary air, the oxygen reaching the flame cannot oxidise all of that carbon to carbon dioxide. The carbon that misses out is left as fine solid particles inside the flame; heated white-hot they glow, which is where the yellow comes from, and the ones that escape unburnt leave as soot. NCERT's Class 10 chemistry sets this out and uses naphthalene itself as the demonstration — Activity 4.3 asks students to burn naphthalene, camphor and alcohol and hold a metal plate over each, and the text explains that unsaturated compounds 'will give a yellow flame with lots of black smoke', that limiting the supply of air 'results in incomplete combustion... giving a sooty flame', and that 'incomplete combustion gives soot which is carbon'.
- (a)carbon to hydrogen ratio is low — The right idea printed backwards. Naphthalene's ratio is high, not low — ten carbons to eight hydrogens, against one carbon to four hydrogens in methane, which burns with a clean blue flame. Had the paper written 'high' this would have been a serious rival to the keyed answer, since the high ratio is what makes the combustion incomplete in the first place.
- (c)there is excess supply of air — The exact opposite of what produces soot. Give a flame more air and it burns cleaner and bluer, which is why a kitchen stove has air inlets; starve it of air and it turns yellow and smoky.
- (d)of presence of impurities of nitrogen and sulfur — Naphthalene contains no nitrogen or sulfur — it is a pure hydrocarbon. The soot in its flame is carbon, and it comes from the fuel's own structure rather than from any contaminant.
A hydrocarbon flame is yellow and sooty when the fuel supplies more carbon than the available oxygen can convert. Complete combustion sends every carbon atom to carbon dioxide and every hydrogen to water, and the flame is blue and almost invisible. Incomplete combustion leaves carbon behind as glowing particles, and it is those incandescent particles, not any gas, that make the flame luminous. Two things push a flame that way: a fuel rich in carbon relative to hydrogen, and a restricted air supply.
The trap here is a well-made one. Option (a) names the correct physical quantity and then states it the wrong way round, so a candidate who reasons 'yes, carbon to hydrogen ratio, that is the thing' and stops reading is caught. Check the formula and the option collapses: C₁₀H₈ has more carbon atoms than hydrogen atoms, which almost no simple fuel does. That leaves the keyed answer, which is the mechanism rather than the cause — the high carbon content is why the combustion is incomplete, and the incomplete combustion is why the flame is sooty. One printed oddity in the paper: option (d) begins with 'of', so that it reads as 'This is because of presence of impurities...', while the other three complete the stem without it. The mismatch is only in the grammar and does not change what is being asked.
- Naphthalene is C₁₀H₈, a fused pair of benzene rings, and is the traditional ingredient of mothballs.
- NCERT Class 10 Science, Activity 4.3, uses naphthalene itself as a compound to burn while watching for smoke and a sooty deposit.
- The same chapter: limiting the supply of air 'results in incomplete combustion of even saturated hydrocarbons giving a sooty flame'.
- The yellow colour comes from solid carbon particles glowing in the flame; NCERT notes that 'incomplete combustion gives soot which is carbon'.
- Naphthalene is used in daytime pyrotechnics precisely because it produces very clear black smoke trails.
Either a carbon-rich fuel or a starved air supply will do it; naphthalene supplies the first.
- Choosing the option that names the carbon-to-hydrogen ratio without checking which way it points. Naphthalene's ratio is high.
- Thinking more air makes a flame smokier. More air makes it cleaner.
- Blaming impurities. Naphthalene is a pure hydrocarbon and the soot is its own carbon.
As a reason for a sooty or luminous flame, as a comparison of saturated with unsaturated hydrocarbons, or as a question on why a gas stove flame turns yellow.
No directly related past PYQ was found.
- practice — not a real PYQ
A gas stove flame turns yellow and leaves a black deposit on vessels. The most likely reason is that
- (a)the air inlets are blocked
- (b)the gas pressure is too high
- (c)the burner is too clean
- (d)the vessel is made of aluminium
Answer(a) the air inlets are blocked — a restricted air supply makes the combustion incomplete, and the unburnt carbon glows yellow and settles as soot.
- practice — not a real PYQ
The yellow luminosity of a candle flame is due to
- (a)sodium vapour
- (b)glowing particles of unburnt carbon
- (c)carbon dioxide
- (d)water vapour
Answer(b) glowing particles of unburnt carbon — incomplete combustion leaves solid carbon in the flame, and heated to white heat it radiates the yellow light.