To raise the pH in a pool, which substance should be added?

To effectively raise the pH in a pool, sodium carbonate is the appropriate substance. This compound, also known as soda ash, is specifically designed to increase pH levels when added to pool water. It acts as a buffer that can elevate the hydrogen ion concentration in the water, thereby increasing the pH.

In aquatic environments, maintaining an optimal pH is critical for swimmer comfort, the effectiveness of chlorine disinfection, and the overall stability of the pool water chemistry. Sodium carbonate not only raises the pH but is also relatively easy to handle and dissolve in water.

The other options present different functions. Sodium bisulfate is primarily used to lower pH, making it unsuitable for this purpose. Calcium chloride is typically used to increase calcium hardness, which can indirectly affect pH but not effectively raise it. Calcium carbonate can raise pH as well, but it is primarily used to increase alkalinity and does not increase pH as efficiently as sodium carbonate in most situations. Thus, when the goal is to directly raise pH levels, sodium carbonate is the most effective choice.