hard water

Definition

Low-lathering water that forms scum and 'boiler scale,' and interferes with industrial processes. Hard water is alkaline due to the presence of very small quantities (measured in parts per million) of inorganic compounds (such as calcium and/or magnesium salts) picked up from rocks and soils. Water hardness is of two types (1) temporary hardness is caused by dissolved calcium bicarbonate (Ca(HCO3)2) and can be cured by boiling or by adding calcium hydroxide or lime (Ca(OH)2) to water; (2) permanent hardness is caused by dissolved calcium sulphate (CaSO4) and cannot be cured by boiling, but by certain processes such as 'ion exchange' using polyphosphates (such as 'calgon') or zeolites (such as permutit'). Both types of hardness can also be cured by adding calcium carbonate or washing soda (CaCO3). In the US, water hardness is measured in Clark degrees (CD). One CD equals 1 part calcium carbonate dissolved in 70,000 parts of water, 14.3 parts per million (PPM), 1 grain (64.8 milligrams) per gallon (GPG), or 17.1 milligram per liter.