Select the correct option regarding the following two statements labelled Assertion (A) and Reason (R). Assertion (A): Unsaturated hydrocarbons give a yellow flame with black smoke. Reason (R): They burn completely when oxygen is insufficient.
A is true, but R is false.
Unsaturated hydrocarbons (alkenes and alkynes) have a higher percentage of carbon than the alkane of the same size, so they need more oxygen for complete combustion.
In ordinary air, oxygen supply is limited relative to their carbon content, so combustion is incomplete. Unburnt carbon particles (soot) glow yellow-orange in the flame and escape as black smoke, giving the characteristic yellow sooty flame. So Assertion (A) is true.
The Reason states they burn completely when oxygen is insufficient. This is the opposite of the truth: complete combustion requires sufficient oxygen; with insufficient oxygen the burning is incomplete. So Reason (R) is false.
Hence, A is true but R is false.
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