permanent water hardness. Compare with temporary hardness and water hardness. Water hardness that remains after boiling the water, mainly due to dissolved calcium sulfate. Chlorides also contribute to permanent hardness.
Water hardness due to the presence of the chlorides and sulfates of calcium and magnesium, which will not precipitate by boiling. This term is largely replaced by "noncarbonate hardness". (See noncarbonate hardness.)
The type of hardness which cannot be removed by boiling the water; it is caused by dissolved sulphates. Sulphates are unaffected by heat.