If you did not use a watch glass to cover your stock iodine solution, and the concentration of iodine decreased over the course of your trials, how would this affect the quality of your data

:

Lower rate of reaction

Explanation:

Lower concentration of reactant in an experiment is indicative of fewer ions or atoms present, which means a slower rate of reaction. In a titration reaction for instance, the end point would take longer to be substantiated, thereby increasing volume of titres. This is further backed up by collision theory which states that more particles in a system improves combinations of molecules.

This however may not be the case if the iodine in question is  catalyst, in which case the change in concentration has no effect whatsoever on the reaction rte.

I hope this explanation fits the brief.