What happens to air at 1000 degrees Celsius I was looking at the graphs for the density of air at different temperatures (link below) and I was wondering if air would not react with itself at high temperatures? I read that oxygen and nitrogen react at 1600 degrees Celsius, but even below that average temperature (because of the Maxwell Boltzmann distribution) I’m guessing the reaction takes place? Also would it just catch fire with contact with anything in the environment like carbon / almost anything? I’m wondering about the hypothetical use in hot air balloons with very high temperatures and low volumes.

https://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html
At $\pu{1000^oC}$, oxygen and nitrogen will not react substantially or probably even noticeably if at all. Besides that, whatever organic material you are using for a hot air balloon will most definitely ignite immediately. The average temperature that most organic materials char and eventually ignite is $\pu{270^oC}$. At almost double the kinetic energy at $\pu{1000^oC}$, most things will almost instantaneously ignite. Note that in air, temperature diffuses rather quickly so even if the fire or whatever you are using to get to $\pu{1000^oC}$ will probably be significantly colder when it reaches anything flammable. In fact, most fuel used in normal hot air balloons burn at over $\pu{1000^oC}$, but it diffuses very quickly that the average temperature of the air in the balloon is around $\pu{110^oC}$