Which substance is often added to water to control pH levels?

Lime is frequently added to water to control pH levels because it is an alkaline substance that reacts with acidity in the water, increasing the pH. This adjustment is essential for several reasons, including preventing corrosion of pipelines, optimizing the effectiveness of disinfectants, and improving the solubility of minerals that may be needed in the water. Additionally, lime can precipitate hardness from the water, aiding in softening processes.

Other substances mentioned, such as chlorine, soda ash, and sulfuric acid, serve different purposes in water treatment. Chlorine is primarily a disinfectant and is used for killing pathogens in water but does not specifically adjust pH. Soda ash (sodium carbonate) can also be used to raise pH, though lime is often the more commonly utilized option for larger scale operations due to its effectiveness and cost-efficiency. Sulfuric acid is typically used to lower pH levels, which is contrary to the function addressed in the question. Hence, the role of lime in increasing pH levels makes it the correct answer for controlling acidity in water treatment processes.