Carbon dioxide gas (CO$_2$) will be evolved with baking soda (sodium bicarbonate) when it reacts with an acid. Baking soda is a base and when it reacts with an acid, it undergoes a neutralization reaction to form carbon dioxide gas, water, and a salt. The chemical equation for this reaction is:

NaHCO$_3$ (sodium bicarbonate) + HCl (hydrochloric acid) → NaCl (sodium chloride) + H$_2$O (water) + CO$_2$ (carbon dioxide gas)