$(a)$ Write the chemical name and chemical formula of washing soda.
$(b)$ How is it obtained from sodium chloride? Give equations of the reactions.
$(c)$ Why is it called a basic salt? Give any one of its uses.

(N/A) Chemical name: Sodium carbonate decahydrate.
Formula: $Na_{2}CO_{3} \cdot 10H_{2}O$
$(b)$ It is obtained via the Solvay process:
$NaCl + H_{2}O + CO_{2} + NH_{3} \rightarrow NH_{4}Cl + NaHCO_{3}$
$2NaHCO_{3} \xrightarrow{\Delta} Na_{2}CO_{3} + CO_{2} \uparrow + H_{2}O$
$Na_{2}CO_{3} + 10H_{2}O \rightarrow Na_{2}CO_{3} \cdot 10H_{2}O$
$(c)$ It is called a basic salt because it is a salt of a strong base $(NaOH)$ and a weak acid $(H_{2}CO_{3})$,which makes its aqueous solution alkaline.
Use: It is used for removing permanent hardness of water.