W

Baking soda does not contain water of crystallization. The chemical formula of baking soda is NaHCO₃. Its formula does not show any association with water molecules.

Other three contain water of crystallization blue vitriol (CuSO₄ .5H₂O), washing soda (Na₂CO_{3 .}10H₂O) and gypsum (CaSO_{4 .}2H₂O).

Note:

The number of water molecules that are attached to a molecule of salt is called its water of crystallization. It is also known as water of hydration. This is necessary to maintain the crystalline structure of a salt. The salts that contain water of crystallization are called hydrated salts.