When baking powder is heated, carbon dioxide is produced. This can be shown by following equation. $$2 mathrm{NaHCO}_{3}+mathrm{Heat} \rightarrow mathrm{Na}_{2} mathrm{CO}_{3}+mathrm{CO}_{2}+mathrm{H}_{2} mathrm{O}$$ Evolution of carbon dioxide can be confirmed by passing the gas into lime water. if limewater turns milky, it indicates evolution of carbon dioxide gas. This reaction does not happen when washing soda is heated.