My class teacher asked this question, when a fire-cracker ( which has gunpowder in it) burns, it explodes and make a sound while when a candle is burnt it doesn't, and I also want to know why does gunpowder explodes?
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1$\begingroup$ Would Chemistry be a better home for this question? $\endgroup$– Qmechanic ♦Commented Jan 3, 2022 at 16:07
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$\begingroup$ At the same power output, I bet a candle and a bit of gunpowder sound about the same. Firecrackers are made to be noisy, candles aren’t… $\endgroup$– Jon CusterCommented Jan 3, 2022 at 16:21
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$\begingroup$ Might have something to do the rate of combustion being slower or faster than the speed of sound in the material. Detonation vs deflagaration $\endgroup$– DKNguyenCommented Jan 3, 2022 at 20:32
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1 Answer
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Begin by thinking about what sound is - a variation in the pressure in the air, which might be a uniform near-sinusoidal variation like a flute note, or an abrupt peak in pressure like your exploding fire cracker.
In order to produce this change in pressure, we can invoke the usual mechanisms - notably, bursting the wall of your fire cracker under the pressure of the hot contained gases of combustion will make a loud bang! But if you just have a flame without any containing casing as is the case for your candle, the flame will only cause fluctuations in surrounding air pressure if the amount of energy released by the flame, changes rapidly. So steady flames tend to be near-silent, but fluctuating ones can produce audible sound. A guttering candle flame is an example, which can produce a fairly quiet "popping" noise. A powerful gas flame makes a roaring noise because the flame is turbulent and the combustion rate varies in a rather random manner. Finally, gunpowder - even in a small heap - burns so quickly that it makes some sound when ignited.
Incidentally, there is a well-explored field of musical flames, in which a flame usually drives a resonant mode in an organ-pipe or similar tube. Search "pyrophone" for many examples.
