permanent hardness

noun

chem hardness of water that cannot be removed by boiling as it results mainly from the presence of calcium and magnesium chlorides and sulphates

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Permanent hardness is caused by calcium sulphate which is soluble in 400 parts of water and cannot be removed by boiling.

What do the terms "temporary" and "permanent" hardness of water mean?

Calcium sulphate, or gypsum, on the other hand, imparts permanent hardness.

Boiling the water does not affect these salts; hence such waters are said to have permanent hardness.

Permanent hardness in water, due to the presence of gypsum or sulphate of lime in solution, may be remedied by addition of caustic soda.

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