What are the reaction products of sulfur and potassium nitrate?
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&amp;\rm Melting~point\\\hline
\rm Sulfur &amp;  \rm 115~^\circ C     \\
\rm Potassium~nitrate &amp; \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:

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

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