Hard Water

Step 1: Understand what causes hardness in water. Hardness in water is primarily caused by the presence of multivalent metallic cations. Common cations that contribute to water hardness include calcium (Ca ) and magnesium (Mg ). Step 2: Evaluate the ions listed. - and are well-known for causing hardness in water. - (Strontium) can also contribute to hardness, although it is less common. Step 3: Identify the outlier. - (Carbonate) does not cause hardness. Instead, it can react with calcium and magnesium to form precipitates, thus potentially reducing hardness by removing these ions from solution.