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When we light a solid lump of explosive it burns slowly layer by layer, but when we light up powder form of explosive it just burns with a huge explosion and sound why does it happen. And one more question, what fraction of the energy in a blast is lost just in the form of sound?

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I think you should read the wikipedia article on explosive material en.wikipedia.org/wiki/Explosive_material to realize the complexity of explosions and materials. –  anna v Mar 6 '13 at 5:55
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1 Answer 1

Gunpowder is a low speed explosive (by strict modern standards it isn't really explosive)

A burning explosive isn't the same as detonating. The combustion (burning) is spread from one part of the explosive to the next by the flame on the surface. In a large piece of explosive the burning can only reach the center slowly as the explosive burns away. In a finely powdered explosive there is a lot of surface area for the flame to reach quickly.

In a detonating explosive the detonation is set off by a supersonic shockwave so the entire explosive reacts 'almost' instantly

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sorry but the term explosive was not coming in my mind I actually meant explosive's i have edited the question –  Akash Mar 6 '13 at 4:09
    
The answer is for a low-explosive like gunpowder. For a high explosive burning is different from exploding. I fixed up the English in the question –  Martin Beckett Mar 6 '13 at 4:12
    
I actually asked for explosive –  Akash Mar 6 '13 at 4:16
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