Hardness of Water

There are two types of water: Soft water is water which form lather readily with soap. Soft water is useful in scientific research works, 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.

HARDNESS OF WATER

Hard water is water which does not form lather with soap readily. The inability of hard water to form lather readily with soap is due to the presence of certain compounds such as calcium and magnesium salts.

TYPES OF HARDNESS OF WATER

Hardness in water are of two types which are
  1. Temporary hardness of water
  2. Permanent hardness of water.

Temporary hardness of water

Temporary hard water is the one that contains dissolved salts of calcium hydrogencarbonate or magnesium hydrogencarbonate. These are formed when rain water containing dissolved carbon dioxide passes through underground rocks made of limestone and chalk. The water dissolves the rocks to form soluble hydrogencarbonate.
H2O + CO2 + CaCO3 →Ca(HCO3)2
The other compounds that cause hardness when dissolved by rain, are dolomite (CaCO3.MgCO3) and gypsum (CaSO4.2H2O) from rocks and soil.

Permanent hardness of water.

Permanent hardness of water is caused by the presence of soluble calcium sulphate or chloride and magnesium sulphate or chloride.
Generally hard water does not form lather with soap and forms scum (white precipitates formed by the reaction of the dissolved salt in water with soap).
 Calcium sulphate + sodium stearate (soap)→Calcium stearate (scum)+ sodium sulphate.

CaSO4 + C17H35COONa  →Ca (C17H35COO)2 + Na2SO4.
Soapless detergents (detergents) do not produce a scum because they do not react with the substances present in hard water. Most washing-up liquids, washing powders and shampoos are soapless detergents.

Ways of softening water

  1. Boiling
    2.  This method removes temporary hard water. When heated the calcium or magnesium hydrogencarbonates decompose producing insoluble calcium or magnesium carbonate. The precipitates can be removed by filtration. Ca(HCO3)2 →CaCO3 + H2O + CO2
  2. Addition of correct amount of slaked lime (Ca(OH)2)
    3.  This method removes hard water as follows.
    Ca(OH)2 + Ca(HCO3)2 →CaCO3 + H2O
  3. By addition of ammonia solution
    4.  Ammonia solution reacts with calcium hydrogencarbonate or magnesium hydrogencarbonate to form insoluble precipitates of magnesium or calcium carbonates and are filtered out.
    CaHCO3 + 2NH4OH → (NH4)2CO3 + CaCO3 + H2O
  4. Addition of washing soda (Na2CO3.10H2O).
    5.  This method removes both temporary and permanent hardness of water. A solution of washing soda is added to the hard water and it precipitates both calcium and magnesium carbonates.
    For temporary
    Ca(HCO3)2 + Na2CO3 →CaCO3 + NaHCO3
    Mg(HCO3)2 + Na2CO3 →MgCO3 + NaHCO3
    For permanent
    MgSO4 + Na2CO3 →MgCO3 + Na2SO4
    CaSO4 + Na2CO3 →CaCO3 + Na2SO4
  5. Distillation
    6.  Under this method, the hard water is boiled to vapour and then condensed back to water. Distillation removes both temporary and permanent hardness of water. It is the most expensive method.
  6. Ion exchange column(resin)
    7.  The hard water is passes through a container filled with a suitable resin containing sodium ions.

    The calcium or magnesium ions causing the hardness in water are exchanged for the sodium ions in the resin.

    Ca2+ + Na-R(s) →Ca2+-R + Na
    R designed to stand for resin

    When all the sodium ions have been removed from the resin, it can be regenerated by pouring a solution of a suitable sodium chloride through it.

Advantages & Disadvantages of Hard water

Advantages Disadvantages
Provides calcium required by body for strong bones and teeth. Wastes soap because some of the soap is used to produce scum, therefore more soap is required for washing .
Has a better taste than soft water Scum leaves marks on clothes, sinks and baths.
Coats lead pipes with thin layer of lead (II) sulphate or lead(II) carbonate, this prevents the possibility of lead poisoning. Causes kettles and boilers to fur. They usually called boiler scale
It provides calcium for the formation of shells and eggs of many living organisms Scums formed stains clothes leaving them with changed color.
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