noun
permanent hard water
Technical term in chemistry and water treatment; refers to water whose hardness cannot be removed by boiling, typically due to dissolved calcium and magnesium sulfates or chlorides.
永久硬水は煮沸しても軟化しない。
Permanent hard water does not soften even when boiled.
Temporary hard water, whose hardness can be removed by boiling.
Compound of 永久 (permanent) and 硬水 (hard water).