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I'm preparing a chemistry show where I end the show with yellow powder bang.

The powder is a mixture of:

$$\ce{K2CO3 + KNO3 + S} $$ My question is, what's the reaction between these chemicals?

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    $\begingroup$ Not sure about this, but the nitrate might oxidise the sulphur to sulphur dioxide or trioxide. $\endgroup$ – Binary Geek Mar 18 '15 at 13:28
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    $\begingroup$ It seems that nobody really knows about the role of the carbonate, but there's a nice video in the Free Range Chemistry series of the RSC. $\endgroup$ – Klaus-Dieter Warzecha Mar 18 '15 at 16:09
  • $\begingroup$ It looks like a variation on a gunpowder mixture which is usually sulfur, KNO3 and charcoal $\endgroup$ – bon Mar 18 '15 at 18:34
  • $\begingroup$ Yellow powder is said to be unpredictable, so it is probably better to use a different bang. $\endgroup$ – permeakra Mar 19 '15 at 12:19
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Here's just a guess, but I do have some confidence that I'm right:
$\ce{KNO3 + S -> K2SO4 + K2SO3 + K2S + K2O + SO2 + N2}$

$\ce{K2CO3 ->[\Delta] K2O + CO2}$
The first, unbalanced equation is the reaction between potassium nitrate and sulfur, producing quite some gas as product, and a variety of potassium compounds left behind, with the composition depending on your formulation of the powder. Of course there might be some sulfur trioxide produced, but the major oxide of sulfur should be the dioxide. In the second equation is the thermal decomposition of potassium carbonate releasing carbon dioxide. This is the part that I'm not very sure of, but it may be to release more gas to give your reaction more puff. However, IIRC the decomposition requires a very high temperature. In contrast, bicarbonate has a much lower decomposition temperature and also release more gas(carbon dioxide and water), but the products would likely be absorbed by potassium oxide to form hydroxide and carbonate, so bicarbonate might not help much.

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