Dissolve baking soda in water, and boil for 15 minutes. This will expell excess carbon dioxide and convert the sodium bicarbonate (baking soda) NAHCO3 to sodium carbonate (washing soda) NA2CO3.
Add calcium hydroxide (Lime) CA(OH)2 to the solution, and continue boiling for about 30 minutes.
Remove from heat, and allow to settle.
The solid residue at he bottom is Calcium carbonate CACO3, while the clear solution at the top is sodium hydroxide NAOH.
Boiling baking soda is lost of time, moreover 100 C is a temperature to low to decompose NaHCO3 in 15 min.
But this procedure will work because:
2NaHCO3 + Ca(OH)2 = Ca
2+ + 2HCO
3- + 2Na
+ + 2OH
- and during warming, even to temperature lower then 100C
Ca
2+ + 2HCO
3- =CaCO3(s) + H2O
and, in fact, you will obtain solution of NaOH
When you will use an excess of Ca(OH)2, the solid residue will conain also insoluble Ca(OH)2 - NaOH diminshes solubility of Ca(OH)2