why flame is yellow?

Answer 1

Well that's not correct, not all flames are yellow. The colour of the flame depends on the flame temprature and the substance burning with the oxygen. For lower flame tempratures, meaning bad oxygen-fuel mixture, the flame is coloured yellow. But for good mixtures and high tempratures the flame turns blue. Moreover as I said before there is dependence on what you're burning, so different substances after burnt create different flame colours such as green, blue,red etc (an example of that is fireworks with all these different colours). The reason of all these different colours has entirely to do with the energy released by the reaction (high energies give colours towards violet, lower energies red) and the molecular and atomic structure of the products of the reaction. Different atoms => different bonds and different number of electrons rotate at different orbits=>
different energy released upon excitation or ionization.

Answer 2

the flame is yellow due to the inssufient oxygen and cause the
combusion can't take place completely then the left out substance like carbon cause yellow flame produce..a normal flame is not yellow colour!!

Answer 3

Flame is yellow when some unburnt carbon particles are present which emit light. If all the carbon is burnt and a little excess oxygen is present, the flame will be almost invisible with a faint bluish tinge.