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Rajarshi Banerjee from DELHI PUBLIC SCHOOL, asked a question
Subject: Science , asked on 31/10/13

in the chapter - combustion ,

what i do not understand is how appearance of black circular mark on the glass plate is related to the wax getting completely burnt ( or not completely burnt ). can u pls explain?????

EXPERT ANSWER

Mohit Gupta , Meritnation Expert added an answer, on 4/11/13

Dear student,

In this video, 3 different zones of the flame are shown to us:

· Innermost zone

· Middle zone or the luminous zone

· Outermost or the non-luminous zone.

Out of which outermost zone is the zone of complete combustion , rest of the two zones shows incomplete or partial combustion.

This can be shown with the help of an activity.

If you introduce a glass plate in the innermost zone then a black ring on the glass will be observed, this is due to the fact that this zone contain unburnt wax because of incomplete combustion.

Similarly, If you introduce glass plate in the middle zone then you will observed the same result. This is again due to the presence of unburnt wax vapour i.e. ( partial combustion).

However no such black ring is observed when glass is introduced in the outermost zone which indicates that this is the zone of complete combustion.

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Rakshit Pathade From Swarnleela International School, added an answer, on 31/10/13
261 helpful votes in Science

in that chapter that activity states the zones of candle and their efficiencies . when the glass plate is take under the innermost zone the black circular spot occurs because there is lack of oxygen and no combustion takes place same happens in middle zone and when brought in outermost zone there we see no black dark spots because there is oxygen and combustion takes place. hope it helps.

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