Flexi Says: Baking soda is sodium bicarbonate that reacts when dissolved in water and forms CO2 as gas. When the solution is heated the decomposition reaction takes place:
NaHCO3 + H2O → NaOH + CO2 +H2CO3
The carbonic acid is further decomposed on heating:
H2CO3 → H2O + CO2
Heating the solution until the evaporation of water is complete leaves the decomposed sodium hydroxide only.