Which method is used to increase the pH level in pool water?

To increase the pH level in pool water, the appropriate method is to add a base. Bases, such as sodium carbonate (also known as soda ash), are used specifically for this purpose. When a base is added to water, it reacts by increasing the concentration of hydroxide ions, which in turn raises the pH level. Maintaining proper pH levels in pool water is crucial because it not only affects the effectiveness of chlorine as a sanitizer but also helps prevent corrosion of pool equipment and ensures the comfort of swimmers.

The other methods mentioned involve actions that do not increase pH. Adding an acid would lower the pH level, making the water more acidic. Chlorine is primarily used for sanitizing and does not directly affect pH significantly. Adding water can dilute the concentration of chemicals in the pool but does not specifically target pH increase. Therefore, using a base is the correct and effective method to elevate the pH in pool water.