Question

Explain why you usually can't use just a single spectrum line to confirm the identity of an element

Answer 1

Usually, you can't only use a single spectrum line to prove an element's identity, since there are times where the emission line is not well defined. Compared to the surrounding noise, when the emission line is very weak, the more data points you have to build confidence in the nature of a single emission range, the better.

Answer 2

You can't usually just use a  single spectrum line to confirm the identity of an element because there are cases that the emission line id not clearly defined.  When the emission line is very weak compared to surrounding noise, in which case the more datapoints you have to build up confidence for the existence of a particular emission spectra, the better.