Why does the tip of a wick (made of thin cotton) of a lamp emit smoke with a certain smell while extinguished when oil in the lamp is exhausted, though the rest of the wick is still soaked in oil?

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Let us see how a lamp emits light. The wick is soaked in oil. It has interstitial voids in its thread, in which the oil rises by capillary action. At the tip of the wick, due to the heat of the burning flame the oil vaporises and burns easily. One thing to be noted is that, a flame is produced only when a gas burns. A burning liquid does not produce a definite and steady flame. It produces an unsteady and irregular flame unsuitable for illumination. So the oil slowly burns by a cycle of gradual capillary suction, vaporisation and inflammation. When the oil is exhausted or if the flame is put off, a smoke with certain smell is produced due to unburnt oil particles, soot and ash of the wick. These particles are carried away with the combustion products, such as carbon dioxide and carbon monoxide and cause a characteristic smell. The rest of the wick in contact with the oil does not get burnt out due to the simple reason that it has not attained its ignition temperature when it is still soaked in oil.

Source: thehindu.com