A) Name the compound which is obtained from baking soda and is used to remove permanent hardness of water.
B) Write the chemical formula
C) What happens when it recrystallizes from its aqueous solution?

Hint: Baking soda is sodium bicarbonate. When baking soda goes through thermal decomposition it yields us a product used to remove the hardness of water.

Complete step by step answer:
Baking soda $\left(\mathrm{NaHCO}_{3}\right)$ when undergoing thermal decomposition gives us Sodium carbonate, water and carbon dioxide.
$$
2 \mathrm{NaHCO}_{3} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}
$$

When the soluble salts of magnesium and calcium are present in the form of chlorides and sulfides in water, we call it permanent hardness because this hardness cannot be removed by boiling.
We can remove this hardness by treating the water with washing soda. Insoluble carbonates are formed when washing soda reacts with the sulfide and chloride salts of magnesium and calcium
and thus hard water is converted to soft water.
Washing soda is nothing but Sodium Carbonate $\left(\mathrm{Na}_{2} \mathrm{CO}_{3}\right)$
When it is recrystallized with water, it changes to washing soda, $\mathrm{Na}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$.

The answers are:
A)Sodium Carbonate
$\mathrm{B}) \mathrm{Na}_{2} \mathrm{CO}_{3}$
$\mathrm{C}) \mathrm{Na}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$

Note: Washing soda can increase the alkalinity of your water – this can cause skin to become dry, irritated and sensitive.Washing soda is a precipitating water softener i.e. it can clog your washing machine or leave residue in your bathtub.