What happens when you heat sodium hydrogen carbonate? 

$2{\mathrm{NaHCO}}_3 +\mathrm{heat} \to {\mathrm{Na}}_2{\mathrm{CO}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

When sodium hydrogen carbonate or its solution is heated, it decomposes to give sodium carbonate with the evolution of carbon dioxide gas. The above reaction takes place during the cooking of food.