Slow Burn

[DrP]

I spilt some Potassium Permanganate solution onto a tissue. I left it there and it dried a purple/brown colour. 2 or 3 weeks on this stain is now black... First I though it was the KMnO7 going to manganese oxide or something, which is sorta black, but after poking it with a gloved finger, the tissue crumbled to ash. It appears that (maybe unsurprisingly) that the KMnO7 has oxidised the paper fully to ash and it looks the same as if you had hit it with a blow torch. Presumable the paper is oxidised and thus the chemistry is the same as if it had been burnt. Therefore the 'slow burn'.

 

 

[OldChemE]

"Unsurprisingly" is quite correct. The speed of oxidation with Potassium Permanganate, IIRC, depends on the presence of a medium for the reaction to take place in. Very slow in this case presumably because the tissue was dry. I used to demonstrate this to my Chemistry students by mixing a few grams of Potassium Permanganate with a few grams of skin moisturizer in a small plastic cup. Nothing happens for the first minute or so, and then it burns very hot and very quickly (like a highway flare), consuming the moisturizer and the plastic cup.