Question

When a strontium compound is heated in a flame, red light is produced. When a barium compound is heated in a flame, yellow-green light is produced. Why does this happen?

Answer 1

See below.

Explanation:

The color of the flame is due to the light emitted by certain salts. The atoms of element contained in the salt are excited to higher energy levels due to heat of the flame. Thereafter, the excited atoms return to the ground state emitting electromagnetic radiation of characteristic frequency. This radiation is seen by the viewer as emitted color.
The energy `DeltaE` of the emitted photon is the difference of energies of the the two levels and its characteristic frequency `nu` are linked by the equation
`DeltaE=h cdot nu`
where `h` is Plancks Constant.

For example, Sodium `"Na"` produces yellow color,
Copper `"Cu"` gives blue.
Barium `"Ba"` emits green and
Strontium salts and lithium salts produce:
Lithium carbonate, `"Li"_2"CO"_3` emits red
Strontium carbonate, `"SrCO"_3` emits bright red.

Picture above shows emission spectrum of a metal halide lamp