We know that sodium carbonate is a salt that is basic in nature, that is it will react with acids to form salts (neutralisation reaction) and when dissolved in water, its solution will have a pH more than 7. So a substance that can liberate carbon dioxide from sodium carbonate will be acidic in nature and hence its pH will be less than 7. Following is the equation that is expected to take place:
Na2CO3 + H+ → 2Na+ + H2O + CO2(↑)
For example, hydrochloric acid (HCl), which is acidic in nature reacts with Na2CO3, which is basic in nature as per the following equation:
Na2CO3 + HCl → 2NaCl + H2O + CO2(↑)