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Negative X is the name given to a moisture-sensitive mixture, used in pyrotechnic demonstrations. The name itself comes from a XYZ diagram made of the 3 chemicals used in the mixture. The performance of the values of different chemical combinations in the formula were recorded and logged in the chart, and it started working when one of the chemical values was negative on the 'X' scale - hence "Negative X".
Negative X consists of powdered ammonium nitrate, ammonium chloride and fine zinc powder. The standard composition of this mixture is 14 g NH4NO3 (28% by weight), 1.5 g NH4Cl (3% by weight) and 34.5 g (69% by weight) of Zn dust.
Negative X is a grayish powder, extremely sensitive to water. Addition of a drop of water on a small pile of this material will cause it to heat up, generate lots of smoke and rapidly ignite, producing a blue-green flame. The reaction is not immediate, and there is a delay of a few seconds before the reaction starts.
The irony of of this mixture is that the addition of both ammonium nitrate and/or ammonium chloride in water is very endothermic process (they're used in instant cold packs), while the Negative X mixture will heat up and ignite if water is added.
The reactions involved are the following:
The chloride ions (Cl-) from the ammonium chloride act as a catalyst on the decomposition of ammonium nitrate:
- NH4NO3 → N2O + 2 H2O
The water in the reaction causes the decomposition of more NH4NO3, which is an autocatalytic effect. The reaction melts the NH4NO3 and allows the oxidation of the zinc. The overall reaction is:
- Zn + NH4NO3 → N2 + ZnO + 2 H2O
While the prepared mixture is not sold by any supplier due to its obvious dangers, the three individual components can be bought online or even from the same supplier.
UnitedNuclear explains more about this mixture.
Negative X can be easily prepared by finely grinding ammonium nitrate and ammonium chloride, then mix the two with zinc dust.
A simple mixture can be made by grinding 6 g of ammonium nitrate with 1 g of ammonium chloride. 14 g of very finely powdered zinc powder is added. The mixture is shaken and must be used immediately, as it is very sensitive to moisture.
Handling and safety
Negative X is supersensitive to any kind of water or moisture. Sicne there is a delay in the ignition reaction, it can be very dangerous to tell when the mixture will ignite, meaning that every second you waste letting the mixture stand, you are putting yourself in unnecessary danger. If the environment where you're preparing this mixture has high humidity, the mixture may ignite during the preparation or if you let it sit in that environment. Here's one such example of spontaneous combustion from a mixture that sit in humid air for a few minutes outside.
Ammonium nitrate from commercial sources tends to have humidity since it's hygroscopic, which may prematurely ignite the mixture.
The ignition reaction generates lots of zinc fumes, which are harmful. Inhalation of said fumes may lead to metal fume fever.
DO NOT STORE THIS MIXTURE!
If you plan on using it, keep its components in separate containers and only mix them right before you want to initiate the reaction.
Split the powder in small batches, then carefully and slowly hydrolyze them.
DO NOT POUR IT DOWN THE DRAIN!