By comparing the bright-line spectrum of the unknown sample with reference metal ion spectra, one can determine whether it is a pure substance (if it matches one reference) or a mixture of salts (if lines from multiple references are found).
To determine whether the unknown sample is a pure substance or a mixture of different salts, we must analyze the bright-line spectrum displayed. If the unknown sample's spectrum shows lines that correspond to multiple reference spectra of the metal ions, this indicates the presence of more than one type of ion, and therefore the sample is a mixture. Alternatively, if it matches with just one reference spectrum, the sample is likely a pure substance.
The concept that each metal salt has a distinctive spectral signature is crucial to this analysis. This means that the emission lines in the spectrum correspond to specific electron arrangements unique to each type of metal. When a metal salt is heated, it emits photons at specific wavelengths which appear as lines in its emission spectrum. In the case of the unknown sample, overlapping lines from several reference spectra would suggest that multiple types of metal ions are present, resulting in different colors being produced during the flame test.
In practice, qualitative analysis is used to identify the composition of complex mixtures. This involves selectively precipitating and identifying ions in a stepwise manner, but in the case of a flame test, we are looking for direct visual evidence from the spectrum.
From historical research, we know that pure substances emit or absorb specific patterns, and mixtures show spectral lines corresponding to each component. Hence, by comparing the line emission spectra of an unknown with known samples, we can deduce whether it is a pure substance or a mixture.