Question

put an unlit incence stick in a corner of your class how close do you have to go near it so as to get its smell

Answer 1

it is because of the action if fusion. The air particles fuses with the smoke of the incense stick and because of heat the rate of fusion is very high. And that is why we are able to smell the fregnance of the incense stick.

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Answer 2

When an incence stick burns the smell or smoke spreads throug diffusion throughout the class room but when it does not burns the smoke or smell does not spreads through diffusion so we have to go near it to smell it.
Hope this answer helps you !!!