Which two hardness-forming cations are most commonly present in significant concentrations?

Which two hardness-forming cations are most commonly present in significant concentrations?

• A. Magnesium and Aluminum
• B. Sodium and Potassium
• C. Calcium and Magnesium
• D. Calcium and Iron

Answer: (C)

Hardness in water comes mainly from multivalent cations, especially calcium and magnesium. These two ions are the most abundant hardness-forming cations in natural waters because they originate from common minerals in rocks, such as calcite and dolomite, which readily dissolve to release Ca2+ and Mg2+ into water. Sodium and potassium are also cations but are monovalent and contribute far less to hardness. Aluminum and iron can be present, but they do not typically drive hardness to the same extent as calcium and magnesium in most waters. Therefore, calcium and magnesium are the two most significant hardness-forming cations.

Hardness in water comes mainly from multivalent cations, especially calcium and magnesium. These two ions are the most abundant hardness-forming cations in natural waters because they originate from common minerals in rocks, such as calcite and dolomite, which readily dissolve to release Ca2+ and Mg2+ into water. Sodium and potassium are also cations but are monovalent and contribute far less to hardness. Aluminum and iron can be present, but they do not typically drive hardness to the same extent as calcium and magnesium in most waters. Therefore, calcium and magnesium are the two most significant hardness-forming cations.