thermodynamics - What exactly is fire?

What is fire? Is it a wave or is it matter?

Where does fire come from?

Does everything burn with fire? (for example: water and some metals don't burn).

Answer

I'll let the far greater minds than me at Minute Physics give the bulk of the answer. Turns out that the way you perceive fire is the result of numerous chemical and physical processes. I'll discuss and expand on just two major ones that were mentioned in the video I linked:

1. Atomic Transitions- The distinct colors that you see in a flame are the result of electrons changing energy levels within at atom. When an excited electron drops to a lower energy level, a photon is released, in order to conserve energy. The frequency of the photon released depends on the exact transition the electron makes. In fact, the energy levels of a particular element are somewhat unique, and so the exact colors emitted by electrons changing energy levels are somewhat unique to the element. In chemistry class you may have done a flame ion lab at some point to see that different metals produce flames of different colors, as a result of the different transitions taking place, unique to each element. In the second video I just linked, you can see a variety of colors such as green, blue, red, or orange depending on the element.


2. Thermal Radiation- However, the red/orange colors you typically see from most combustion reactions in real life are actually the result of a different process. As mentioned in the Minute Physics video, impure flames usually burn reddish orange because the soot and ash particles around the flame are so hot that they emit visible light. This is a different process than the atomic transitions I mentioned. All objects, when heated, emit radiation in the form of electromagnetic waves and the temperature of the object determines the exact wavelengths emitted. You are actually emitting "light" as we speak, but you are so cool that the radiation you emit is in the infrared range, and therefore not visible.