Which property of water is affected by the addition of carbon dioxide?

The correct choice is related to the pH of water, which is influenced by the addition of carbon dioxide. When carbon dioxide dissolves in water, it reacts to form carbonic acid, which leads to an increase in the acidity of the water. This process decreases the pH level, making the water more acidic. Monitoring pH is crucial in water treatment processes, as the acidity can impact chemical reactions, biological processes, and the solubility of various minerals and metals present in the water.

The other properties do not directly correlate with the addition of carbon dioxide in the same way. Hardness typically refers to the concentration of calcium and magnesium ions, while turbidity pertains to the clarity of water based on suspended particles. Alkalinity, though related to the capacity of water to neutralize acids, is not directly affected by carbon dioxide in the same immediate and straightforward manner as pH.