In the Gossage process for the preparation of caustic soda the chemical added to washing soda is

Gossage process (or) soda lime processDilute Na2CO3 (washing soda) on reaction with hot.milk of lime [Ca(OH)2] gives caustic soda (NaOH) is called Gossage processNa2CO3+Ca(OH)2→CaCO3ppt.+2NaOH(caustic soda)