In: Chemistry
Since you haven’t specified what is the compound for which you are performing steam distillation, I cannot be specific, but will give a qualitative explanation.
Suppose that we have a compound containing a polar function like –OH or –C=O or –COOH or –COOR. Toward the end of the distillation technique, addition of NaCl helps to increase the polarity of the compound. NaCl is highly polar and dissociates completely into constituent ions, Na+ and Cl-. Na+ acts as a Lewis acid and can accept the lone electron pair from O-atom of these groups. This leads to generation of partial charge on the O-atom which increases the polarity of the molecule. In case the compound is non-polar like a hydrocarbon, then also the technique can be useful to isolate impurities with polar functional groups.
I hope this information gives you some idea; knowing the exact distillation experiment would have helped to give you a more detailed answer.