Question

why a crude NaCl absorbs moisture from atmosphere​

Answer 1

Answer:

Not all salts are hygroscopic in nature. Pure Sodium chloride does not attract water from just moist air.

If the relatively humidity is low, the mineral Halite (NaCl) will not be changed in decades.

Calcium chloride, however, is do hygroscopic that it will not only pick up water from moist air, it will remove water vapor from almost dry air to the point of forming a solution.

The chemical nature of hygroscopy is not well understood as to predicting which salts will adsorb or absorb water from the atmosphere and which will not. Emperical results are usually the best method for determining how much water any given salt will attract.