why flames change color as the get hotter?
In: Chemistry
Flames emit black body radiation from soot and ash in the flame. Some fuels, like coal, have lots of carbon and ash particles, so they emit lots of black body radiation. Fuels like natural gas produce less soot and ash, so they tend to emit less black body radiation. Fuels like hydrogen produce no soot or ash, and as such, they emit even less black body radiation.
Burning fuels also emit radiation at specific frequencies related to the excited states of chemical species in the flame. In industry, this is called “luminosity”. This generally is the source for the blue color seen in natural gas and h2 flames.
For fuels like coal, the luminosity is a small part of the total radiation. For fuels like natural gas, the luminosity can be close to 50% of the radiation.
[Blackbody radiation.](https://en.wikipedia.org/wiki/Black-body_radiation) The name comes from a thought experiment where you imagine an object that *only* radiates energy, it does not at all reflect any energy at all.
All things, all the time, give off photons. Any time a charged particle does anything, it gives off a photon to “communicate” that action. Atoms are full of charged particles, electrons and protons, and because those things aren’t at absolute zero, they are wiggling with Brownian motion.
The more energy the particle has, the more energy it needs to “communicate” via photons. So, something moving quickly because it has more energy will give off more energy as photons. The frequency of a photon determines its energy: higher frequency means more energy. Frequency is also what we perceive as color. Thus: something with more thermal energy gives off photons with a higher frequency, which we perceive as a different color.
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