# How is it possible some substances burn with an invisible flame

I know some substances such as Methanol fuel burn invisibly, but according to the following picture, every black-body emitter still emits light at the blue part of the spectrum, even when it's incredibly hot.

And besides, according to this article, both ethanol and methanol burn at less than 2000 degrees Celsius, so that would leave them having yellowish orange flames.

So my question is: how come the flames of some substances can be invisible? And if the answer is that those reaction products don't have soot in them, then why are hydrogen flames not invisible even though hydrogen most definitely does not have soot in it?

The reason flames such as from burning hydrogen produce any light is that the temperature rise causes an increase in the average velocity of the gas molecules. The velocity distribution is related to the gas temperature by the Maxwell-Boltzmann equation, and while the vast majority of gas molecules have energies around $kT$ a small proportion of them have enough energy for their collisions to cause electronic transitions. The excited atoms/molecules then decay by emission of a photon, and it's the light produced from these decays that we see as the colour of the flame.