Which of the following salts does not contain water of crystallization?
(A) Blue vitriol
(B) Baking soda
(C) Washing soda
(D) Gypsum

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Hint: The water that is stoichiometrically bound into a crystal when crystallized out of water is known as water(s) of crystallization or water(s) of hydration.

Complete step by step solution:
-In simpler words, the number of water molecules present in one formula unit of salt is known as the water of crystallization. Water being the common solvent to be found in crystals because of its polarity and smaller size hence may sometimes incorporate within the crystal lattice.
-Water of crystallization refers to the water that is found in the crystalline framework of a metal complex or a salt and is not bonded directly to the metal cation.
-The crystal salts which have water(s) of crystallization are known as hydrates.
-This crystal of water does not include contaminants and is easily affected by the heat.
-The chemical formula of blue vitriol which is copper sulphate pentahydrate is $CuS{{O}_{4}}.5{{H}_{2}}O$. It has 5 moles of water.
-The chemical formula of baking soda which is sodium bicarbonate is $NaHC{{O}_{3}}$. It does not have any water molecule incorporated in its crystal lattice.
-The chemical formula of Washing soda which is sodium carbonate monohydrate is $N{{a}_{2}}C{{O}_{3}}.{{H}_{2}}O$. It has one mole of water.
-The chemical formula of Gypsum which is sulphate dihydrate is $CaS{{O}_{4}}.2{{H}_{2}}O$. It has 2 moles of water.

So, the correct answer is option (B).

Note: The water molecules present in hydrated molecules are held by hydrogen bonding. Hydrogen bonding is a type of weak force that results in the formation of dipole-dipole interaction between the hydrogen atom and an electronegative atom that is strongly bonded to another electronegative atom. Since the water of crystallization in a crystal is much less intimately combined with the salt than the water of constitution, and therefore on heating, these water of crystallization are more easily expelled, often losing the crystalline properties.

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