Which one among the following salts causes hardness to the water?

Water hardness is primarily caused by the presence of dissolved multivalent metallic cations, specifically calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are introduced into water through the dissolution of minerals like bicarbonates, chlorides, and sulfates [4]. Hardness is categorized into two types: temporary and permanent. Temporary hardness is specifically caused by dissolved calcium bicarbonate and magnesium bicarbonate. When water containing these bicarbonates is boiled, they decompose into insoluble carbonates (like calcium carbonate), which precipitate out of the solution [1]. Permanent hardness is caused by sulfates and chlorides of calcium and magnesium [4]. While calcium carbonate is the standard unit for expressing hardness, it is relatively insoluble in water and is the precipitate formed during softening, rather than the dissolved salt causing the initial hardness [1]. Sodium and potassium salts do not contribute to water hardness [3].

Sources

1. [3] https://www.griet.ac.in/nodes/EC_UNIT_1.pdf
2. [4] https://uptti.ac.in/classroom-content/data/Unit-1_Water%20and%20its%20treatment.pdf
3. [1] https://www.sciencedirect.com/topics/engineering/carbonate-hardness
4. [2] https://www.mtnregionalwaterutah.gov/files/882bc66a4/MRW-Water-Hardness.pdf