Which of the following is most likely to decrease pH levels in water?

Carbon dioxide is most likely to decrease pH levels in water because it reacts with water to form carbonic acid, which in turn dissociates into bicarbonate and hydrogen ions. The release of hydrogen ions increases the acidity of the water, lowering the pH. This natural process occurs in various aquatic environments and is a significant contributor to acidification, particularly in water bodies exposed to increased levels of carbon dioxide, whether from natural processes or human activities.

In contrast, caustic soda and soda ash are both alkaline substances that would raise pH levels when added to water. Caustic soda (sodium hydroxide) is a strong base, while soda ash (sodium carbonate) is used to increase alkalinity. Alkaline materials, by definition, counteract acidity and thus would also serve to elevate pH rather than decrease it. Understanding the chemical reactions and interactions of these compounds helps clarify why carbon dioxide is the choice that most directly contributes to lowering pH levels in water.