Permanent hardness of water can be removed by adding:

1. Potassium permanganate
2. Chlorine
3. Bleaching powder
4. Washing soda

Permanent hardness in water, caused by calcium and magnesium sulfates or chlorides, resists boiling and requires specific treatment. Washing soda (sodium carbonate) effectively removes it by precipitating these minerals as insoluble carbonates. Unlike chlorine or bleaching powder, which disinfect rather than soften, washing soda targets hardness directly. Historically, this method dates back to 19th-century industrial water treatment.