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What is Hard Water?



What is Hard Water?

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 encrust.

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

Physical Conditioning

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.

The Electrochemistry

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.

Dissolved Solids

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).

The most commonly occurring cations are: 
Calcium Ca2+
Magnesium Mg
Sodium Na+

The most commonly occurring anions are: 

Chloride                    Cl­SO4

Sulphate Bicarbonate HCO

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 here.)

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 strongly alkaline.

More on Hard Water courtesy of USGS.


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