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YouTube shows videos of dropping a piece of elemental sulfur into molten potassium nitrate and applying additional heat to accelerate the process. A white precipitate falls out to the bottom. There is never any mention as to what this material is. An internet search yields not a clue. Sulfur dioxide is obviously evolved, but KNO doesn't seem to exist if one relies on an internet search.

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There are at least two videos (first and second) showing the reaction between sulfur and potassium nitrate. For the chemicals:

\begin{array}{cc} \rm Species&\rm Melting~point\\\hline \rm Sulfur & \rm 115~^\circ C \\ \rm Potassium~nitrate & \rm 334~^\circ C \end{array}

It isn't clear if nitrogen, some nitrogen oxides, or both would be produced. My guess is that it would depend on particle size, mixing and so forth, thus that a mixture of nitrogen oxides would be very likely. Also the test tube starts with a tiny amount of oxygen gas from the air. One possible balanced reaction would be:

$$\ce{4KNO3 + 5S -> 2K2O + 5SO2_{(g)} + 2N2_{(g)}}$$

Looking at the products of black powder burning $\ce{K2SO4}$ and $\ce{K2S}$ can also be formed.

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    $\begingroup$ I suspect $\ce{K2SO4}$ is also possible. $\endgroup$ – Ivan Neretin Dec 27 '15 at 10:26
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    $\begingroup$ Excuse me, I have been away from chemistry a sinfully long time, but how, in an environment "hot" enough to allow reactions, could $\ce{K_2O}$ and $\ce{SO_2}$ coexist whithout reacting? $\endgroup$ – Gyro Gearloose Dec 27 '15 at 16:50
  • $\begingroup$ @GyroGearloose - the notion is that the gases produced blow bits of sulfur or partially reaction products onto the walls of the closed tube so that there is an incomplete reaction. This isn't suggesting that most of the products are from incomplete combustion, just some. $\endgroup$ – MaxW Jan 3 '16 at 5:14
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When potassium nitrate is heated to its melting point, oxygen gas is evolved and potassium nitrite is formed. Any sulphur present would be rapidly oxidised to sulphur dioxide and possibly some sulphur trioxide. The residual nitrite could further react with excess sulphur to produce potassium derivatives such as potassium sulphide and further sulphur dioxide.

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