Question

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Why do substances burn with or without a flame ?

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Answer 1

Hi there✋✋✋✋

Substance burn with flame are vapourise during heating.
Example-When we heat kerosene, a flame released.
While some substances do not burn with flame because they do not vapourise during heating.
Example-When we heat coal, it doesn't burn with a flame.

So the main reason for the burning of any substance with or without flame is the vapourisation during heating.

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Answer 2

ANSWER--

The substances which vaporise during burning give flames. For e.g. kerosene oil. Flame may be bright yellow due to incomplete combustion. But on complete combustion, flame is colourless.

Those substance which do not vaporise during burning do not form flames. For example, charcoal, salt, sugar etc. which do not vaporise.