Question 4: Why do solids give rise to continuous spectrum while hot gases to line spectrum?
In solids atoms are not free but they are packed so tightly that their orbits overlap with each other. The energy changes in a particular atom are impacted by the other atoms in the surrounding. The impact is the radiations of all wavelengths are emitted. Therefore, solids give rise to continuous spectra.
On the other hand, atoms of the hot gases are isolated and far apart from one another. No atom of the gas influence the other atoms, and hence, radiations of specific wavelengths and frequencies are emitted. Therefore, hot gases give rise to line spectra.