Which raises the pH of pool water?

Raising the pH of pool water is an essential aspect of maintaining a safe and pleasant swimming environment. The correct option, sodium carbonate (soda ash), is a compound specifically used to increase the pH level of water. When added to pool water, sodium carbonate dissociates in solution, increasing the concentration of carbonate ions, which reacts with water to elevate the pH.

Proper pH levels in swimming pools are crucial because they help ensure effective sanitation, minimize eye irritation for swimmers, and prevent corrosion or scaling of pool equipment.

While chlorine and alum serve important roles in water treatment—chlorine as a disinfectant and alum as a flocculating agent to help remove impurities—they do not increase the pH of pool water. Chlorine can indeed lower pH levels when used in certain forms, and alum may have a neutral effect on pH. Therefore, they are not suitable choices for raising the pH of pool water, highlighting why sodium carbonate is the appropriate answer in this context.