Which chemical can be added to lower the pH of pool water?

Adding muriatic acid to pool water is an effective method for lowering the pH. This strong acid dissociates in water, increasing the concentration of hydrogen ions, which in turn decreases the pH level. Properly managing the pH is crucial for maintaining a balanced pool environment, as it affects sanitization efficiency, swimmer comfort, and equipment longevity.

Other substances listed do not serve the same purpose. Baking soda and sodium bicarbonate are used to raise pH and alkalinity, which is the opposite of what is needed in this scenario. Calcium carbonate also raises pH levels and is used primarily to increase calcium hardness, not to decrease acidity. Thus, muriatic acid is the correct choice for lowering the pH of pool water effectively.