Which major ion is MOST directly responsible for causing temporary hardness in water?

Calcium ions (Ca²⁺) are most directly responsible for causing temporary hardness in water. Water hardness refers to the concentration of dissolved minerals, primarily calcium and magnesium ions. Temporary hardness, also known as carbonate hardness, is specifically caused by the presence of dissolved bicarbonates of calcium (Ca(HCO₃)₂) and magnesium (Mg(HCO₃)₂). These bicarbonates form when rainwater, slightly acidic due to dissolved carbon dioxide (CO₂), reacts with minerals like limestone (calcium carbonate, CaCO₃) and dolomite (calcium magnesium carbonate, CaMg(CO₃)₂). For example, rainwater containing dissolved CO₂ reacts with limestone: CaCO₃(s) + H₂O(l) + CO₂(aq) ⇌ Ca²⁺(aq) + 2HCO₃⁻(aq). The resulting calcium ions (Ca²⁺) and bicarbonate ions (HCO₃⁻) contribute to the water's hardness. The 'temporary' nature of this hardness stems from the fact that it can be easily removed by boiling. When water containing calcium bicarbonate is heated, it decomposes, releasing CO₂ and forming insoluble calcium carbonate (limescale) which precipitates out of the solution: Ca²⁺(aq) + 2HCO₃⁻(aq) → CaCO₃(s) + H₂O(l) + CO₂(g). This precipitation removes the calcium ions, thus softening the water. Magnesium ions (Mg²⁺) contribute similarly to temporary hardness through their bicarbonate salts. Permanent hardness, in contrast, is caused by dissolved calcium and magnesium sulfates and chlorides, which do not precipitate upon boiling and require different treatment methods for removal.