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I saw a post on a online forum that potassium nitrate and petroleum jelly could be mixed to create a rocket propellant. What is the reaction that would take place?

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Yes, a bit crude as a rocket propellant, but it works as igniting the petroleum jelly in air leads to the associated thermal decomposition of the potassium nitrate liberating oxygen:

$\ce{2KNO3 (s) → 2KNO2(s) + O2 (g)}$

where the generated oxygen accelerates the burning.

The petroleum jelly itself is not particularly volatile, so the dangers of an explosion are low.

In general, however, I would recommend exercising considerable care when working with a fuel and oxidizers mix.

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    $\begingroup$ @ AJKOER. 1) Volatility and risk of explosion are not necessarily related. 2) your equation corresponds to what happens when heating pure $\ce{KNO3}$. When mixed with carbon containing substances $\ce{KNO3}$ looses all its oxygen atoms. For example, the famous black powder is a mixture $\ce{KNO3 + C + S}$ that reacts according to $$\ce{2KNO3 + 3 C + S -> K2S + 3 CO2 + N2}$$ $\endgroup$
    – Maurice
    Sep 28, 2022 at 11:06
  • $\begingroup$ Good points Maurice, hence my cautionary note on handling such compositions, especially in larger amounts. $\endgroup$
    – AJKOER
    Sep 28, 2022 at 14:01

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