Hard Water – the Cause of the Problem
Rainwater is slightly
acidic, i.e., soft. In heavily industrialized regions, emissions make the
rainwater more acidic. The hardness in water comes from the calcium and
magnesium salts, that are dissolved into the water from soluble rocks
through which the rainwater flows. Hard water contains both temporary and
permanent hardness. Temporary hardness in most cases is associated with
calcium and magnesium carbonates and bicarbonates. These crystal forming
salts are held in solution and will remain so, unless there is a change in
pressure or temperature which will cause the water to become supersaturated,
resulting in the precipitation of encrusting scale on hot or rough surfaces,
such as pipes and heat exchangers. Permanent hardness is due mainly to
calcium and magnesium sulphate and is not affected by heat or pressure
change. However, if the water is evaporated, it will remain and will
The water hardness problem
is sometimes exacerbated by water’s being stored in reservoirs constructed
from different materials and is also further exacerbated seasonally as the
water table rises and falls resulting in concentrations.
Fig. 1 The Water Cycle
Chemists are concerned with the chemical reaction of elements
and compounds that have formed as a result of the reactions. However, to
gain an understanding of Physical Conditioners, one has to take into
account the physical effects that occur before the reactions
take place, hence the term physical conditioning.
Stable chemical compounds
are normally electrically neutral. When they dissolve in water to form
solution, they may separate into oppositely charged particles called ions.
This process is known as dissociation and can be partial or complete.
Although ions are independent particles, the connection to their opposite is
maintained and is re-established following crystallization. This process of
dissociation in water is used widely in industry to separate metals from
their compounds, for electroplating and separating the elements of water
itself, oxygen and hydrogen gases.
The mineral salts found in water can be determined in type
and quantity by simple evaporation and weighing the residue. In addition to
hardness salts, sodium chloride, sodium sulphate and silica can be found by
other techniques. These substances do not exist in solution as definite
compounds, but as “ions” – charged soluble particles of metal (known as
cations) or as acid radicals (know as anions).
commonly occurring cations are:
The most commonly occurring anions are:
Sulphate Bicarbonate HCO3
The negative and positive signs indicate polarity of electron
charge. The negative sign indicates electron gain, the positive sign
electron loss. Contaminants can be grouped according to polarity and
magnitude of charge.
(More info on cations and anions
Neutrality of Water pH Value
Pure water in its liquid state is also slightly dissociated
into its constituent ions.
This equation suggests that
water contains hydrogen ions moving freely within the liquid. However, a
hydrogen atom with one electron removed is simply a proton. It is now
recognized that the protons attach themselves to water molecules to form a
hydronium ion H2O+. For simplicity, H+ ions
will be referred to below, although the physical reality is that such
species do not have an independent existence in water.
Both hydrogen and hydroxyl
ions are present in exactly the same quantity, so that pure water is
“neutral.” In a unit weight of pure water there will be 0.0000001 unit
weights of hydrogen ion and of hydroxyl iron, or 10-7 parts
of each. The pH value – the index of acidity, alkalinity or purity – uses
the figure 7 as a neutral or purity point of the scale. Natural pure water
is said to have a pH value of 7. pH = - log10 (H+), where H+ is the hydrogen ion concentration.
As hydrogen ion concentration is increased, the pH value
decreases. As hydroxyl concentration increases, the pH value increases.
Acidity is due to hydrogen ions, so the more acid the water
becomes, the lower its pH value, Alkalinity is due to hydroxyl ions, so the
more alkaline the water becomes, the higher its pH value. This is because
acids give hydrogen ions in solution, while alkalines give hydroxyl ions:
The pH scale covers the range of 0-14, from strongly acid to
More on Hard
Water courtesy of USGS.