why does the linear spectrum emitted ONLY in seprate atoms or gaseous state under low pressure?

Question

why does the linear spectrum emitted ONLY in seprate atoms or gaseous state under low pressure?

Answer 1

I think by linear you mean containing discrete emission lines. These lines are produced by orbital electrons changing orbit in a given atom, and the energy change is quantized, resulting in emission of photons with a specific energies and therefore light of a specific color. This is called an emission spectrum.
When the atoms/molecules are close together, collisions due to thermal motion occur, and the energy of the collisions is variable and has a wide distribution, resulting in a continuous spectrum.
With low molecular density such as in a low-pressure gas, the emission spectrum predominates. As molecular density increases (as in a solid or high-pressure gas), the continuous spectrum becomes more and more dominant. See the references.