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When kept in open air, the crystals of washing soda lose 9 molecules of water to form a monohydrate.
\[\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{.10}{{\text{H}}_{\text{2}}}\text{O}\xrightarrow[\text{to air}]{\text{exposed}}\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{.}{{\text{H}}_{\text{2}}}\text{O + 9}{{\text{H}}_{\text{2}}}\text{O}\]

This process is called:
(A) efflorescence
(B) deliquescence
(C) dehydration
(D) hydration

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Hint: Recollect what term is used for the process of removal of water from any substance. This term is generally used when water of crystallization is being removed.

Complete answer:
When kept in open air, the crystals of washing soda lose 9 molecules of water to form a monohydrate.

\[\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{.10}{{\text{H}}_{\text{2}}}\text{O}\xrightarrow[\text{to air}]{\text{exposed}}\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\text{.}{{\text{H}}_{\text{2}}}\text{O + 9}{{\text{H}}_{\text{2}}}\text{O}\]

In the above reaction, we can see that washing soda is losing 9 molecules of water of crystallization. The process in which water of crystallization is removed is called Efflorescence. It can also be defined as the process of losing water or hydration from a hydrate. Like in this case, Sodium carbonate decahydrate loses nine molecules of water to form Sodium carbonate monohydrate.
Hydration is the process of addition of water molecules in a substance in order to dissolve it. It is also called Solvation. For example, interaction of NaCl with water molecules leads to dissolution of NaCl in water.
Dehydration is the process of removal of water from any substance. An example could be removal of water from acetic acid molecules to form acetic anhydride.
Deliquescence is the tendency to absorb moisture and become liquid. It is also called as Hygroscopic property. Deliquescence occurs when the partial pressure of water vapour in the air is more than partial pressure of liquid that is formed. An example of a deliquescent compound is Sodium hydroxide pellets.
So, the correct answer is “Option A”.

Note: Don’t get confused with dehydration and efflorescence. Make a note of these very similar terms. Condensation reaction is the process of combination of two molecules usually in the presence of a catalyst, with removal of water or simpler molecules.