‘‘Sodium hydrogen carbonate is a basic salt.’’ Justify this statement. How is it converted into washing soda?

Sodium hydrogen carbonate is a salt produced by the neutralization reaction between a strong base (NaOH) and a weak acid (H₂CO₃), hence it is a basic salt. 

$NaOH + H_2CO_3 → H_2O + NaHCO_3$

When Sodium hydrogen carbonate is heated strongly it produces sodium carbonate which recrystallizes to produce washing soda.

The equation for the reaction is given below:

$2NaHCO_3 \xrightarrow{heat} Na_2CO_3 + H_2O + CO_2$

$Na_2CO_3 + 10H_2O → Na_2CO_3.10H_2O$