Question

When you buy washing soda from the market, you will get it in powder form because:
(A) the crystals of washing soda quickly lose water molecules
(B) the shopkeeper prefers to powder the crystals
(C) washing soda does not form crystals
(D) washing soda is a drying agent

Answer 1

Hint: The chemical formula for washing soda is $Na_2CO_3.10H_2O$. It is also called sodium carbonate decahydrate because of the presence of ten molecules of water, also called water of crystallization. The physical state of washing powder depends upon its nature on being exposed to the atmosphere. Washing powder is efflorescent.

Complete step by step answer:
An efflorescent substance is a substance that loses its moisture on exposure to the atmosphere. In compounds, the moisture present in the form of water molecules is called water of crystallization. It is the presence of these water molecules that imparts a compound a crystalline shape.
Compounds containing water of crystallization form crystals, whereas, compounds that do not contain water of crystallization exist in amorphous or powder form.
Washing soda, due to its efflorescent nature, readily loses the water molecules that impart it a crystalline shape and gets converted into a powder form. This is why washing powder is available in powder form in the market. The conversion of washing soda crystals into powder form can be represented by the following simple equation:

$Na_2CO_3.10H_2O\rightarrow Na_2CO_3 + 10H_2O$

Hence, the correct answer to why we get washing soda in powder form in the market is because the crystals of washing soda quickly lose water molecules.

So, the correct answer is Option A .

Note: In crystalline form, washing powder is blue due to the presence of water molecules, which changes to white powder on being to the atmosphere due to the loss of water molecules.

The opposite of efflorescent is hygroscopic. A hygroscopic substance is a substance that absorbs moisture from the atmosphere without itself getting dissolved in it.

Washing powder (dry) is hygroscopic. When washing soda powder is left in the open air, it immediately absorbs water molecules from the surroundings and recrystallizes into sodium carbonate decahydrate. This is why it is suggested to store washing powder in airtight containers to prevent contact from the air.