Sciencemadness Discussion Board

Weird Microwave Plasma from KNO3

mauricethegangsteroflove - 3-2-2026 at 17:08

Howdy all

I recently recrystallized some KNO3 from stump remover, and while waiting for it to dry I began wondering how I could dry it faster. I ended up using the oven on low heat and it worked out fine, but a stupid thought briefly crossed my mind.

Would it be faster to just microwave it?

The sane part of my brain said hell no, and I used the oven. But just to see what would happen, I took a small crystal and microwaved it on a paper plate. I figured it would either do nothing or maybe catch the plate on fire, but instead it began to give off sparks like electricity and a second later made a lightsaber noise and shot what appeared to be a jet of plasma, like you'd get from microwaving a grape.

After the initial "oh crap" turned to "cool, plasma" I found this was repeatable. It would shoot the plasma after about 3 seconds. The KNO3 didn't burn, it just melted. What would cause this?

mauricethegangsteroflove - 3-2-2026 at 17:14

A brief bit of searching the net for info on microwaving KNO3 turned up little beyond "please do not microwave KNO3."

yobbo II - 3-2-2026 at 17:19

Quote: Originally posted by mauricethegangsteroflove  
A brief bit of searching the net for info on microwaving KNO3 turned up little beyond "please do not microwave KNO3."


Pay no attention to bots!

Yob

Mateo_swe - 4-2-2026 at 03:21

I would guess its something similar to when you microwave some carbon or other items.
There seem to be some sharp points in the material that makes sparks emerge and the microwave energy feeds this reaction sometimes creating like a ball lightning fenomena in the microwave.
You can also make this by putting 2 toothpicks crossed in some putty and lit on fire in the microwave. This will make the ball lightning thing after a few seconds.
Edit. Be aware that lots of energy is released in this ball lightning ball, be ready to turn off micro fast.
I would guess the KNO3 will melt and produce some gas that will be ignited and thats probably what makes the lightsaber noise.
Do NOT put larger amounts of KNO3 in the microwave, it could explode so only small pieces that cause no harm if it does.

[Edited on 2026-2-4 by Mateo_swe]

Sulaiman - 4-2-2026 at 20:07

Quote: Originally posted by Mateo_swe  
...Do NOT put larger amounts of KNO3 in the microwave, it could explode...
How/Why ?

Precipitates - 4-2-2026 at 22:12

Potentially violent decomposition

heist_2009 - 13-2-2026 at 05:54

Quote: Originally posted by Mateo_swe  
I would guess its something similar to when you microwave some carbon or other items.
There seem to be some sharp points in the material that makes sparks emerge and the microwave energy feeds this reaction sometimes creating like a ball lightning fenomena in the microwave.
You can also make this by putting 2 toothpicks crossed in some putty and lit on fire in the microwave. This will make the ball lightning thing after a few seconds.
Edit. Be aware that lots of energy is released in this ball lightning ball, be ready to turn off micro fast.
I would guess the KNO3 will melt and produce some gas that will be ignited and thats probably what makes the lightsaber noise.
Do NOT put larger amounts of KNO3 in the microwave, it could explode so only small pieces that cause no harm if it does.

[Edited on 2026-2-4 by Mateo_swe]





if this were the case would not common salt would also produce plasma when kept in the microwave??

teodor - 13-2-2026 at 06:55

You said your KNO3 was not dry. It can explain heating. To check wether it is the case try to repeat experiment with perfectly dry KNO3. I don't see how it can explode. The decomposition is 2KNO3 -> 2KNO2 + O2 before the point of nitric oxides, and after that point there is no any redox reaction also.
For ammonium cation it would be different of course, it can explode.

Solubility and the melting point of NaCl probably doesn't allow accumulate enough energy for melting before evaporation of water, and concentrated solution of KNO3 has very high boiling point and can be overheated to pass from solution to the melt which is highly conductive, so the conductance will grow almost exponentially under microwaves. There is no sharp difference between KNO3 solution in H2O and H2O gas solution in KNO3 near the melting point. This is how I see it.


[Edited on 13-2-2026 by teodor]