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Potassium nitrate is very popular compound in pyrotechnics, used as an oxidiser. It can be mixed with sugar to make a "rocket candy", with coal and sulphur to form black powder. You can stick it on rope to create functional fuse.

The saltpeter releases oxygen in all these reactions. But what are the results of reaction? I learned that $\ce{KNO2}$ is also oxidiser. So does the nitrate turn tu nitrite and then reacts again? Or does it form something else?

I have noticed that some strange bubbles come from my homemade fuse (photo will follow). Since potassium nitrate has melting point at 334°C (according to Google) it's likely the saltpeter melting.

And what about fumes? Does potassium nitrate or nitrite evaporate to turn the air poisonous or irritating?

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If we assume potassium carbonate ($\ce{K2CO3}$) as the only solid reaction product, the overall reaction can be described as: $$\ce{2 KNO3 + C12H22O11 + 11 O2 -> N2 + 11 CO2 + 11 H2O + K2CO3}$$

when using sucrose for candy. With sorbitol, an equation like

$$\ce{2 KNO3 + C6H14O6 + O2 -> N2 + 5 CO2 + 7 H2O + K2CO3}$$

seems fitting. In both cases, it is assumed that:

  • the nitrate is completely reduced to nitrogen
  • additional oxygen is needed

The latter is plausible, the rocket candy is supposed to burn layer-wise at the surface in contact with air. This allows for propulsion rather than one instant explosion.

Whether the nitrite is formed as an intermediate (or which organic intermediates are formed) is beyond my knowledge. However, once this partial reduction (nitrate to nitrite) has happened, I rather doubt that the reaction goes back again (to regenerate the nitrate).

As far as the bubbles are concerned, is it conceivable that the fuse wasn't completely dried and thus water was driven out?

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  • $\begingroup$ The bubbles later turn into solid white/black inhomogenous substance. I believe it's potassium nitrate that has been melted by the burning. $\endgroup$ – Tomáš Zato Sep 22 '14 at 19:44
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    $\begingroup$ I'm not so sure additional oxygen is what we want. The fumes from the rocket will blow all the oxygen away. $\endgroup$ – Tomáš Zato Sep 25 '14 at 8:17

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