Why does flame look blue sometimes?

Blue light has highest energy compared other colours,when any substance burns it emits energy in the form of light.Often some substances while burning emits energy corresponding to the energy of blue light thats why in that case flame looks blue

The short answer: 

light of different colours is given off at different energies. Blue flame is higher energy light than red or yellow flame. Light of different energies is radiated by different materials when they burn, so one sort of material might prefer to give off higher energy light than another.

The long answer:

Think about the structure of the atom. Atoms have a nucleus surrounded by shells called electron orbitals. Electrons can occupy a range of different orbitals, and orbitals represent different levels of energy held by the electron. An electron in a high orbital, visualised as being far out from the nucleus, has a higher energy than the electrons that occupy positions closer in.

When you send energy into an atom by heating it, electrons will gain energy and jump (make a "quantum leap") instantly into a higher energy orbital. But if there are no electrons filling up the orbitals below it, this will be an unstable situation. The electron wants to get closer in, but it has too much energy. So it needs to loser energy.

The energy it loses is fired off in the form of a photon - that is, light. Light comes in different colours depending on the energy making it up. Ultraviolet light has more energy than blue light, blue light has more energy than red light, red light has more energy than infra-red light, and so on and so forth.

The colour of the light that is sent out by an electron depends on the ''gap'' that it needs to drop to get into a lower electron orbital, closer to the nucleus of the atom. Depending on the structure of an atom - a carbon atom has a different structure to a helium atom, for example - the electron might need to drop different ''distances'' between one orbital and the next, giving off light of different energies and therefore different colours.

But it doesn''t just depend on the atoms, but also on the molecules in a substance. Molecule are groups of atoms that are bound together by electron orbitals. These orbitals can have a different shape and different energy-differences between each other than the orbitals of a simple, single atom. So more variation is introduced between materials.

Therefore, burning different materials - pushing energy into different kinds of atoms and molecules - produces different colours of light.

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