What does constant hardness mean in practice?
Remains after removing the carbonate part and results mainly from calcium and magnesium salts with chlorides, sulfates or nitrates.
When does it matter?
Boiling does not remove it, so the water may still need to be softened for technical uses.
Which solutions are related to it?
It is calculated as the difference of the appropriate components and removed by ion exchange, nanofiltration or RO.