How can spectroscopy be used as a qualitative analysis technique?
See explanation below.
Spectroscopy could be used as a qualitative analysis technique by monitoring a particular characteristics of products or reactants in a chemical reaction.
For example, infra-red ( IR ) spectroscopy, nuclear magnetic resonance ( NMR ) or ultraviolet-visible spectrophotometer ( UV-Vis ) could be used to monitor the appearance of a product or disappearance of a reactant.
In IR and NMR, we look for the appearance of a new peaks for products or reducing the intensity or disappearance of a particular peaks for a reactant.
Using UV-Vis spectrophotometer, we can look for new absorption wavelengths that are particular to a product, and so on.
I hope this helps.
Here is a video where we have used a spectrophotometer to monitor the progress of a reaction: