Which of the following would increase the pH of water?

Lime, which is primarily composed of calcium hydroxide (Ca(OH)₂), is used to raise the pH of water. When lime is added to water, it reacts with the water to produce hydroxide ions (OH⁻). This increase in hydroxide ions leads to a rise in the pH level, making the water less acidic. Higher pH levels indicate that the water is more alkaline.

In the context of water treatment, managing pH is crucial, as it affects chemical reactions and the solubility of various compounds in water. A higher pH can help in precipitating certain contaminants, enhancing disinfection processes, and improving the overall quality of the water.

Other substances listed, such as chlorine gas and carbon dioxide, would lead to a decrease in pH, making the water more acidic, while hypochlorous acid is a weak acid that also lowers pH. Therefore, lime is the only substance mentioned that effectively increases the pH of water, making it the correct choice.