What two ions primarily compose water hardness?

Water hardness is primarily determined by the presence of specific dissolved minerals, which are typically cations that bond with water molecules. The two main ions responsible for water hardness are calcium and magnesium. These ions come from geological formations found in the earth, often leaching into groundwater as rainwater seeps through soils and rocks.

Calcium is usually found in the form of calcium carbonate (CaCO3) and magnesium may appear as magnesium sulfate (MgSO4) or magnesium carbonate (MgCO3). When water contains high concentrations of these ions, it is classified as hard water. This type of water can cause scaling in pipes and appliances, affecting their efficiency and lifespan. Additionally, hard water can interfere with soap's effectiveness, leading to lower sudsing capacity.

The other options include ions that are not primarily linked to water hardness. Potassium, while a cation, does not significantly contribute to hardness like calcium and magnesium do. Sodium and chloride are more related to saline conditions rather than hardness, and iron and manganese, although they can be present in water, are not the primary contributors to hardness. Therefore, the clear relationship of calcium and magnesium with water hardness solidifies why this answer is the correct choice.