This reaction works best with finely divided KMnO4 and glycerine, which has a little water in it. Very dry glycerine makes it less reactive. The
amount of water in the glycerine must be a few percent, not more. Where I live, standard glycerine has a density of 1.23 g/ml, while pure glycerine is
1.26 g/ml. The standard glycerine works quite well with KMnO4, while the sg 1.26 material works not as well. Probably a little amount of water makes
the KMnO4 more soluble and allows the reaction to start easier.
The phenomenon that the presence of a little amount of water is beneficial is quite common though. Some other examples:
- Mix powdered AgNO3 and Mg. On addition of a small drop of water, the mix inflames with a bright white flame.
- Mix powdered Mg and crushed I2. On addition of a small drop of water you get a puff of iodine vapor.
- Mix powdered Zn and NH4NO3. On addition of a small drop of water the mix ignites with a greenish/grey flame.
- Mix powdered NaClO2, acetone and sulfuric acid. On addition of a single drop of water the mix explodes with a bright orange flash.
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