73Noncarbonate hardness is a measure of calcium and magnesium salts other than carbonate and
bicarbonate salts. These salts are calcium sulfate, calcium chloride, magnesium sulfate (MgSO 4 ),
and magnesium chloride (MgCl 2 ). Calcium and magnesium combined with nitrate may also
contribute to noncarbonate hardness, although it is a very rare condition. For carbonate and
noncarbonate hardness,
Total hardness = carbonate hardness + noncarbonate hardnessWhen hard water is boiled, carbon dioxide (CO 2 ) is driven off, and Bicarbonate salts of calcium and
magnesium then settle out of the water to form calcium and magnesium carbonate precipitates.
These precipitates form the familiar chalky deposits on teapots. Because it can be removed by
heating, carbonate hardness is sometimes called “Temporary hardness.” Because
noncarbonated hardness cannot be removed or precipitated by prolonged boiling, it is sometimes
called “permanent hardness.”
Objections to Hard Water
Scale Formation
Hard water forms scale, usually calcium carbonate, which causes a variety of problems. Left to
dry on the surface of glassware and plumbing fixtures, including showers doors, faucets, and sink
tops; hard water leaves unsightly white scale known as water spots. Scale that forms on the
inside of water pipes will eventually reduce the flow capacity or possibly block it entirely. Scale
that forms within appliances and water meters
causes wear on moving parts.
When hard water is heated, scale forms much
faster. In particular, when the magnesium hardness
is more than about 40 mg/l (as CaCO 3 ),
magnesium hydroxide scale will deposit in hot
water heaters that are operated at normal
temperatures of 140-150oF (60-66oC).
A coating of only 0.04 in. (1 mm) of scale on the
heating surfaces of a hot water heater creates an
insulation effect that will increase heating costs by
about 10 percent.
Effect on Soap
The historical objection to hardness has been its effect on soap. Hardness ions form precipitates
with soap, causing unsightly “curd,” such as the familiar bathtub ring, as well as reduced efficiency
in washing and laundering. To counteract these problems, synthetic detergents have been
developed and are now used almost exclusively for washing clothes and dishes.
These detergents have additives known as sequestering agents that “tie up” the hardness ions so
that they cannot form the troublesome precipitates. Although modern detergents counteract many
of the problems of hard water, many customers prefer softer water. These customers can install
individual softening units or use water from another source, such as a cistern, for washing.