Question

What chemical or physical reaction occurs in a flame?

Answer 1

Some (chemical) combustion reactions.

Explanation:

Fuel particles and oxygen molecules undergo exothermic reactions to produce heat. Thermal energies released would lead to the emission of photons through means of black body radiation and electron transitions, producing flames that are iconic to this type of reactions. [1]

Taking the combustion of methane `"CH"_4`- a.k.a. "natural gas"- as an example:
`"CH"_4 (g)+2"O"_2(g) to "CO"_2(g)+2"H"_2"O"(g)`
`Delta"H"=-882.0color(white)(l)"kJ"*"mol"^(-1)` [2]

Each mole of methane reacts with two moles of oxygen to produce one mole of carbon dioxide and two moles of water.

This reaction is spontaneous when carried out under standard conditions and give rise to significant amounts of heat; combustion of each mole of methane will release `882.0` kilojoules of thermal energy. [2]

References
[1] Wikipedia contributors. "Flame." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Flame 9 Apr. 2018. Web. 20 Apr. 2018.

[2] Wikipedia contributors. "Methane (data page)." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Methane_(data_page) 26 Jan. 2018. Web. 20 Apr. 2018.