Hardness of water

Hardness is the most important property i.e. impurity present in water.  So the water that does not give lather with soap is called hard water. Also, hard water has dissolved impurities which destroy the soap and will form scum over the soap. “Hence hardness is the power of not forming scum” Hard water is not fit for drinking, in boilers and industrial uses. Also, soap is salt of sodium stearate C17H35COONa

2C17H35COONa Ca/Mg salt → (C17H35COO)Ca/Mg (insoluble or scum of ppts +  2Na

What is the hardness of water?

There are two types of hardness

Temporary hardness

Temporary hardness is due to the presence of Ca and Mg carbonates and bicarbonates. Basically, their hardness is alkaline hardness. So,  it can easily be removed by simply heating

Hardness of water
Hard and soft water

i.e Ca/Mg(CO3, HCO3) temporary hardness or alkaline hardness

Ca(HCO3)HeatCaCO3↓ + H2O + CO2

Mg(HCO3)HeatMgCO3↓ + H2O + CO2

MgCO3+ H2O Mg(OH)2↓+ CO2

Also, the temporary hardness is measured by treating it with a standard acid solution. So, It is observed by scale in tea kettles.

Permanent hardness

This hardness is due to the presence of chloride sulphates of Ca and Mg. It is non-alkaline hardness. Basically, it is determined by treating it with a standard solution of soap or EDTA (Ethylene diamines tetra acetate).


This hardness can be removed by chemical methods.

Disadvantages of hard water

Hard water is very harmful to the boiler due to the deposition of salts as a result reduces boiler efficiency. Also, hard water is not suitable for laundry and other processing in the dying and finishing process. Hard water is also not suitable for drinking for a long time creating many problems and strains on the body’s skin. Also, the spot appears on fabrics

  • Firstly soap wastage
  • Secondly energy wastage
  • Boiler body scale so wastage of metal

Water treatment or water softening

The water contains a lot of impurities due to soil and atmospheric conditions. Hence it is not suitable for boiler and must be well treated to maintain the water and steam purity in order to prevent the deposition of suspended solids and scale on the boiler surface and also silica deposition and corrosion in boiler tubes. Also, the extent of pre-treatment depends on the source of raw water used. Well water usually requires simple filtration while water from rivers and lakes requires more treatment.

Removal of the temporary  hardness of water


Surprisingly soluble bicarbonate into carbonate which is insoluble simply by boiling.

Ca(HCO3)2CaCO3 ↓ +CO2+ H2O

Mg(HCO3)2MgCO3 ↓ + H2O + CO2

By Clark method

In general, in this method, bicarbonate is converted into carbonate to remove the hardness by using sodium hydro-oxides as an agent.

2Ca(OH)2  + Ca(HCO3)22CaCO3 ↓ + 2H2O

Removal of the permanent hardness of water

Colgan method

In this method Na(PO3)6 (sodium -hexa-meta-phosphate) is used as Colgan as an agent so, the salts will be converted into ions Ca++, Mg++etc.

Zeolite method

In general, In this method, zeolite(sodium-aluminium-ortho-silicate) is used to remove permanent hardness. Also, zeolite is a Na2Al2O8.KH2O.

Resine method (ion exchange)

So, In this method, the resin is used as RNH2OH salt ion as Ca++, Mg++etc. Hence it will react with this resin

RNH2OH + Mg++(RCOO)2Mg  + 2H+


Significantly, it is a process in which water is chlorinated to prevent odour.


So the suspended particles are made to coagulate by special chemicals. The coagulated material then settles down by gravity at the vessel bottom and is removed.


The chlorinated water depending upon the hardness and alkalinity undergoes softening. At first, hardness is the chief source of scale in heat exchangers, boilers, and pipelines caused by the presence of calcium and magnesium salts containing Ca and Mg ions. Alkalinity is mostly by HCO-13 but is also caused by CO-23. Also, softening is usually done in a cold process by using lime soda ash Ca(OH)2, which precipitates the bicarbonates and sulphates i.e

2Ca(OH)2  + Mg(HCO3)22CaCO3 ↓ +Mg(OH)2+ 2H2O

MgCl2  + Ca(OH)2 Mg(OH)2↓ + CaCl2

MgSO4  +  Ca(OH)2 Mg(OH)2↓ + CaSO4

CaSO4  + Na2CO3 Na2SO4 +CaCO3

Also, the products of CaCO3 and Mg(OH) 2  are insoluble in water and settle down to the bottom of the vessel. So the sludge is removed from the container.

what is precipitation?

A chemical method/chemical reaction of separating salt from water by adding different chemicals water as Lime soda ash Ca(OH)2. The solution is heated and cooled, and salt precipitates out of the solution, and settles down due to gravity.


At last, filtration is done which further removes the residual solid suspension.

Measurement of hardness

Since CaCO3 is taken as a common factor for the measurement of hardness.

Also, 1 gram of CaCOin one gallon of water is called 10 hardness.

In other words, 1 gallon= 70,000 grains of H2O (nowadays international unit)

Also,1 ppm of hardness = 1 milligram of CaCO3 in one Kg of water (1000,000 milligrams).


So, one part of CaCO3 in 1000,000 parts of  H2O. Still, many countries express hardness in their own ways.

America 1 G.P.G= 1 grain of CaCOper US gallon of water.

British 1 degree = 1 grain of CaCOper British gallon of water.

French 1 part of CaCO3 per 100,000 parts of H2O.

German 1 part of CaO per 1000,000 parts of H2O

Russian 1 part of Ca per 1,0000,000 parts of H2O

The above scales can convert international units as follows

American x 17.1=international

British x 14.3=international

French x 10 =international

German 17.9=international

Russian x 2.5=international

What is total hardness?

In other words, the hardness measured is total hardness = temporary hardness + permanent hardness

Up to 150 ppm = soft water

Also, 150~350 ppm hard depending upon use.

Salts/minerals Type of hardness Solubility ppm Hardness in ppm (in CaCO3) Hardness in British degree
Ca(HCO)3 Temporary 1180 730 51
Mg(HCO)3 Temporary 31,000 36750 2570
Fe(HCO)3 Temporary 4000 3450 240
CaCO3 Permanent 68 68 4.8
MgCO3 Permanent 700 830 58
Ca(SO4) Permanent 1980 1450 102
Ca(OH)2 Permanent 1700 2300 161
Mg(OH)2 Permanent 9 15.4 1.1

Side effects of hard water

  • Firstly cloth appearance looks dull
  • Secondly, salts are deposited in water pipes and in boilers, etc.
  • Also, soap efficiency will reduce
  • Stain spots on metal parts while touching with hard water continuously
  • Due to salt deposition, more energy is required as a layer created on it

How hardness of water affects industries

The hardness of water is the most serious problem and affects the processing in a variety of manners. Some of the problems are discussed as follows

I)-Process involving the use of soap

The non-alkali metals containing e.g. Ca, Mg, Fe, and maybe lead etc in water will react with soap to convert it into a useless product, rather a troublesome product.

C17H35COONa  +  Fe+3 /Ca+2 /Mg+2 (C17H35COO)2Ca( insolveble soap grease)  +  2Na+

The textile industry uses a lot of soap for soaping dying ( Vat, azoic, sulphur, etc). Similarly, printed goods require thorough soaping. Soap is also used for degumming silk, scouring raw wool(wax) and wool fabrics, creping silk and as softener finishes to textiles. It is estimated that I kg soap will be precipitated with a hardness of 100 p.p.m in 100 litres of water.

Besides this wastage Ca, Mg soaps restrict to fibre giving harsh and light colour spots. These soaps decompose during the storage of material giving a foul smell and occasionally causing mildew.

ii)-Processing requiring alkalies at elevated temperature

Whenever hard water comes in contact with caustic soda ashes and it is also heated or boiled in salts of Mg, Ca and it precipitates the fabrics. Also, this solution is often met in the processing of the cotton textile e.g. (Kier boiling), dying various cloths, both in hot and cold water rinsing out alkalies, for package dying. These insoluble compounds often precipitate and stick to the fibre and although their quality is small yet they reduce quality appreciably.

On dying the material further, these white ppts on the amount of their presence, reduce the depth of shade, purity and brightness of colour will be marked ( finished)


In the package dying of yarn ppts hinder the free flow of liquor through the package and causes unevenness dying.


Certain dyes are very sensitive to  Ca and Mg Compounds. In the presence of a dye Ca, Mg compounds will form and the less soluble Ca and Mg complexes or salts are formed, The ppts dye cause non-uniform dying besides weakening the depth of the shade.

Dissolved gases

Dissolved gases like O2 with the dying of certain interfere with the dying of certain cloth of dyes especially the vat dyes while CO2 in the water reacts with iron precipitates carrying water to form ferrous bicarbonate.

Fe  + 2CO2 + 1/2O2 +  H2O →  Fe(HCO3)2

Fe(HCO3)2 can produce colour spots on the processed goods and also catalyse the bleaching action of H2O2. The excessive reactivity of peroxide weakens the fibre and may even produce holes in the material.

Total dissolved solids (T.D.S)

If the concentration of dissolved solids especially those of sodium salt is high the finished textile material will absorb the moisture from the atmosphere and give the cloth an undesirable damp and limp feeling.

Generally, the stiffness of textile fabric dried after rinsing with hard water felt. The solid content of rinsed water in greater amounts also gives a stiff feeling.

Water quality requirements in the textile industry

Turbidity 3 ppm
Colour 5 Hazen units
Hardness 25 ppm
Iron 0.01 ppm
Mn 0.02 ppm
Total alkalinity 64 ppm
T.D.S 150 ppm




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