Question

Which of the following is the chemical formula for washing soda?
(a). CaO
(b). $\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$
(c). $\text{N}{{\text{H}}_{2}}\text{Cl}$
(d). $\text{MgC}{{\text{l}}_{2}}$

Answer 1

Hint: Let us discuss the physical appearance of Washing Soda. It is a transparent crystalline solid and one of the few metal carbonates which are soluble and alkaline in water which turns red litmus to blue.

Complete step by step solution:
Washing Soda is sodium carbonate containing 10 molecules of water of crystallisation. So, we can say that washing soda is sodium carbonate decahydrate ($\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}.10{{\text{H}}_{2}}\text{O}$).
The sodium carbonate which does not contain any water of crystallisation is called anhydrous sodium carbonate or soda ash ($\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$).
In the following three steps it is discussed how washing soda is prepared from sodium chloride or common salt.
Firstly, a cold and concentrated sodium chloride solution is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate.
The sodium hydrogen carbonate is slightly soluble in water, so it precipitates out as solid.

$\text{NaCl+N}{{\text{H}}_{3}}+\text{C}{{\text{O}}_{2}}+{{\text{H}}_{2}}\text{O}\to \text{NaHC}{{\text{O}}_{3}}\text{+N}{{\text{H}}_{4}}\text{Cl}$

Secondly, now sodium hydrogen carbonate is separated by filtration, dried and heated.
$\text{2NaHC}{{\text{O}}_{3}}\to \text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}+{{\text{H}}_{2}}\text{O+C}{{\text{O}}_{2}}$

On heating sodium hydrogen carbonate decomposes to form sodium carbonate. This anhydrous sodium carbonate is known as soda ash.

The preparation of Sodium Carbonate is also known as Solvay process.

Therefore, we can conclude that the correct option would be (b) .

Note: It is hygroscopic in nature that is, it absorbs moisture from the air. It is highly soluble in water and forms a strongly alkaline water solution.