Which of the following statements describes carbonate hardness?

The statement that describes carbonate hardness is that it includes both calcium and magnesium ions. Carbonate hardness specifically refers to the portion of hardness that arises from the presence of carbonate (CO3^2-) and bicarbonate (HCO3^-) ions in water, primarily in the form of calcium carbonate (CaCO3) and magnesium carbonate (MgCO3). These ions contribute to the overall hardness of water, which is a measure of the concentration of divalent metallic ions, mainly calcium and magnesium, in the water.

This is essential because carbonate hardness significantly impacts the water's buffering capacity as well as its suitability for various industrial and ecological applications. The presence of these ions can also affect the scaling potential in pipes and equipment.

The other statements do not accurately characterize carbonate hardness. For instance, measuring total acidity reflects a different property related to the presence of acidic compounds, rather than hardness. The term "general hardness" typically encompasses all sources of hardness, not limited exclusively to carbonate hardness, and negatively charged ions do not contribute to hardness measurements, as hardness is fundamentally based on the presence of positively charged metal ions.