IC6.11.5 Water hardness and limescale
Water hardness and limescale
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IC6.11.5 Water hardness and limescale
There are two types of hard water. These depend on
the type of rock the water has flowed over and through,
and the different minerals they contain.
 Permanent hardness is caused by dissolved
calcium sulfate.
 Temporary hardness is caused by dissolved
calcium hydrogencarbonate.
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IC6.11.5 Water hardness and limescale
When water with
permanent hardness is
boiled, the hardness is not
removed.
When water with temporary
hardness is boiled, the
hardness is removed
because calcium
hydrogencarbonate
decomposes when heated.
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IC6.11.5 Water hardness and limescale
calcium
calcium
carbon

+ water +
hydrogencarbonate
carbonate
dioxide
Ca(HCO3)2

CaCO3
+ H2O
+
CO2
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IC6.11.5 Water hardness and limescale
This is essentially the reverse
of the process that produced
the hard water in the first place.
It can lead to deposits of
calcium carbonate as limescale
in kettles, irons, and
heating systems.
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IC6.11.5 Water hardness and limescale
This copper hot water pipe is almost completely blocked by a thick
layer of limescale.
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IC6.11.5 Water hardness and limescale
 Limescale causes damage to equipment that
involves hot water and increases energy costs.
 A layer just 1 mm thick can increase energy costs by
around 8%.
 Hard water costs British industry an estimated £1
billion per year.
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IC6.11.5 Water hardness and limescale
Limescale can be removed with weak acids, like citric
acid. These do not react with the metal of the
equipment, but will react with the limescale.
For example:
calcium
carbon
calcium
citric

+ water +
+
carbonate
dioxide
citrate
acid
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IC6.11.5 Water hardness and limescale
Why do you think some people use boiled water
in their steam irons?
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