Question

Hardness in water is NOT caused by ______

• A. $Ca^{2+}$
• B. $Si^{2+}$
• C. $Mg^{2+}$
• D. $CO^{2-}$

Answer 1

The Correct Option is D

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\(^{2+}\)) and magnesium (Mg\(^{2+}\)). Step 2: Evaluate the ions listed. - \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \) are well-known for causing hardness in water. - \( \text{Sr}^{2+} \) (Strontium) can also contribute to hardness, although it is less common. Step 3: Identify the outlier. - \( \text{CO}_3^{2-} \) (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.