What is carbonate hardness caused by?

Carbonate hardness is primarily caused by the presence of calcium and magnesium ions in water, specifically in the form of their carbonates (CaCO3) and bicarbonates (Ca(HCO3)2). These compounds contribute to the temporary hardness of water, which can be removed by boiling. When water interacts with limestone or other carbonate-containing minerals, the calcium and magnesium dissolve and increase the hardness level.

The relevance of bicarbonates lies in their behavior; they not only contribute to hardness but also affect the pH and buffering capacity of the water. When bicarbonate ions decompose upon heating, they can precipitate as carbonate, which further illustrates the relationship between carbonate compounds and hardness.

The other options relate to different aspects of water chemistry but do not lead to carbonate hardness. For instance, the dissolution of sulfates involves calcium and magnesium but results in non-carbonate hardness, often referred to as "sulfate hardness." Organic material can affect water quality and biological activity, while phosphates are associated with eutrophication rather than hardness. Understanding these distinctions helps clarify why the correct answer specifically pertains to carbonate-based compounds.