Board Thread:Questions and Answers/@comment-4441317-20180306065027/@comment-25296247-20180307120415

Fire is kinda fun in physics. You see, everything around us creates light (or more correctly, electromagnetic radiation). Because everything has temperature. The higher the temperature is, the higher the frequency of that radiation. For things at room temperature, this frequency is low. Infrared, microwave... But when you heat something up enough, the radiation's frequency reaches visible part of the spectrum. For example, if you heat up metal enough, it becomes red, then orange, then it melts. Important thing is, it becomes red at around 700 Kelvin.

Now, when something rapidly oxidizes, the oxidation is highly exotermic and heats up the gas it reacts with. And depending on material, the gas's temperature can reach 700 Kelvin or more. That's what we call fire. When the gas is heated up to this point. Little less, the fire is invisible. Add more temperature (like a burner for example), the fire shifts on the spectrum to blue and then to ultraviolet (which we can't see).

Now, fire DOES produce plasma, IF it's hot enough. At 700 K, that temperature is hardly enough to create plasma, but some might occur. However, things like bunsen burner would definitely create plasma.

Lightning produces plasma because its temperature is 30,000 K. But technically, lightning has nothing to do with fire. Fire is a gas from chemical reaction, lightning is a flow of electrons from electrostatic physical reaction.

Finally, if you want to analyze that old concept of 4 elements, it kinda fits the 4 basic states of matter (even though there are more than 10 states of matter), but that's just how people tried to explain the world. We know that things are made out of atoms, they thought they were made out of different amounts of these elements. For example, they thought soul was made out of fire and wind. Some cultures used different equivalents. 5 element model of Fire-Air-Water-Earth-Aether. Chineses Fire-Water-Earth-Wood-Metal model... It's just very old science.