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Soft and Hard Water

There are two kinds of water, soft and hard water. soft water is water which contains little or no dissolved solid impurities. Soft water is useful in scientific research work, pharmaceuticals, food processing and other processes that require high degree of purity of materials. However, soft water may corrode lead pipes through which it is transported, resulting in the water being contaminated of lead.

Hard water can be defined as water, which will not readily form lather with soap. This may be due to any salt of calcium, magnesium, or iron(II), all of which are likely to be present in under ground water, due to the dissolution of certain rocks, example, Limestone (CaCO₃); Gypsum (CaSO₄.2H₂O); CaF₂; Ca₃(PO₄)₂; Dolomite (MgCO₃.CaCO₃); Canalite (KCl.MgCl₂.6H₂O); MgSO₄.7H₂O; Haematite (Fe₂O₃); and Magnetite (Fe₃O₄).

Note: the inability of hard water to form lather with soap is due to the precipitation of the soap, in the presence of any salt of Ca, Mg or Fe(II), as a stearate of calcium, magnesium or iron(II), which is insoluble.

Example, 2Nast(aq) + CaSO₄(aq) → Na₂SO₄(aq) + Cast₂(s)

Note: Nast is Sodium octadecanoate (Sodium stearate or soap, soluble); Cast₂ is Calcium octadecanoate (Calcium stearate, the insoluble scum)

Hardness in water are: temporary and permanent hardness.

Temporary Hardness

Temporary hardness is hardness due to the presence of soluble Ca(HCO₃)₂, Mg(HCO₃)₂, or Fe(HCO₃)₂ in water. It is called temporary hardness because it can be removed by heating the water - the trioxocarbonate(IV) is precipitated, while CO₂ is released.

Example, Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

CaCO₃ is insoluble and does not cause hardness. The presence of the dissolved hydrogen trioxocarbonate(IV) of the metals in the water, e.g. Ca(HCO₃)₂ is due to dissolution of the trioxocarbonate(IV), example, CaCO₃ in water in which carbon(IV) oxide had dissolved - this makes the trioxocarbonate(IV) soluble in water.

Example, CaCO₃(s) + H₂O (l) + CO₂(g) → Ca(HCO₃)₂(aq)

MgCO₃(s) + H₂O(l) + CO₂(g) → Mg(HCO₃)₂(aq)

Effects of Temporary Hardness:

(i). The furring of kettles or boilers - this is as a result of the decomposition of Ca(HCO₃)₂ into CaCO₃, and this coats the inside of a kettle or boiler used in heating water containing dissolved Ca(HCO₃)₂.

Also, pipes used in transporting hot water may become blocked with CaCO₃, formed from the decomposition of Ca(HCO₃)₂ in the temporarily hard water.

(ii). Stalagmites and stalactites: these are CaCO₃ structures in hot caves that grow upward from the floor of the cave to its roof (stalagmites); and downward, from the roof to the floor of the cave (stalactites).

This phenomenon is due to temporary hard water flowing over the roof of a hot cave and dripping through cracks in the roof. Due to the heat in the cave, the Ca(HCO₃) in the water is decomposed to deposit CaCO₃.

Permanent Hardness

Permanent hardness is due to the presence of soluble CaSO₄, MgSO₄, and FeSO₄ in water. Permanent hardness is not removed by heating the water.

Related Tutorials

Water - Composition and Reaction
Removal of Hardness from Water
Purification of Town Water Supplies