Permanent hardness of water is due to the presence of

Water hardness is classified into temporary and permanent types based on the dissolved mineral salts. Temporary hardness is caused by the presence of dissolved calcium and magnesium bicarbonates (hydrogen carbonates), which can be removed by boiling [1]. In contrast, permanent hardness, also known as non-carbonate hardness, is caused by the presence of sulfates, chlorides, and nitrates of calcium and magnesium. These salts do not precipitate upon boiling and require chemical treatments like the addition of washing soda (sodium carbonate) for removal. Specifically, calcium sulfate and magnesium sulfate are primary contributors to this permanent condition. While sodium and potassium salts may be present in water, they do not contribute to hardness because they do not react with soap to form scum or cause scaling in the same manner as multivalent cations like Ca2+ and Mg2+ [1].

Sources

1. [1] https://www.sciencedirect.com/topics/engineering/carbonate-hardness