Hardness of Water

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Hardness of Water - Temporary and Permanent Hardness in Water

What Causes Temporary and Permanent Hardness of Water?

It is often seen that no lather forms while we wash our hands with soap when the water is hard. So, what makes the water hard? 

Well, Calcium and Magnesium ions present in water make it hard. 

Soaps contain sodium salt of fatty acids that forms lather. So, when we dissolve soap in the water, the Ca2+ and Mg2+ ions present in the water react with these fatty acids to give a salt of calcium and magnesium formed with the fatty acids.

The Compounds that may Form During the Reaction are: 





CaSO4, or


These appear as a precipitate on the surface of the water, which is scum. 

Temporary Hardness of Water

When Ca2+ and Mg2+ ions are present in the form of Ca(HCO3)2, Mg(HCO3)2 in water, such type of hardness is temporary. This hardness can be removed by boiling the water. When we boil the water, the reaction takes place in the following manner:

Ca(HCO3)2                     →    CaCO3            +             H2O +     CO2

Calcium Bicarbonate  Calcium Carbonate   Water    Carbon Dioxide


Mg(HCO3)2                         →     Mg(OH)2                      +                 CO2

Calcium Bicarbonate  →   Magnesium Hydroxide      Carbon Dioxide

CO2 escapes the surroundings. The resulting salts CaCO3 and Mg(OH)2 do not dissolve in water and form precipitates. 

Removal of the Temporary Hardness of Water

Let us discuss the methods of removing the temporary hardness of water:

  1. Filtration

We remove the precipitates by filtration. This is how we can get rid of Ca2+ and Mg2+ ions.

  1. Clark Method

In this method, we add some amount of lime (Slaked lime) to the water. The reaction between Ca(HCO3)2  and Ca(OH)2 takes place in this way:

  Ca(HCO3)2        +      Ca(OH)2 →      CaCO3              +       H2O

 Calcium Bicarbonate     Slaked lime    Calcium Carbonate   Water

The lime reacts with Mg(HCO3)2:

Mg(HCO3)2        +      Ca(OH)2                  →   CaCO+  Mg(OH)2   +  H2O

We already know that CaCO3 and  Mg(OH)2 are precipitates, which we can remove by filtering. This is how we can get rid of these precipitates and remove the temporary hardness of the water.

Now, let us study the permanent hardness of the water.

Permanent Hardness of Water

You might have seen a hard white coating on electric kettles, showers, or any bath asset or utensils in which we boil the water. This white coating is due to the permanent hardness of the water.

So, what is the permanent hardness of the water?

The Permanent Hardness Occurs because of the Following Compounds (or Salts):

  1. CaCl2

  2. MgCl2

  3. CaSO4, and

  4. MgSO4.

Thus, the white coating we notice on the utensils is the deposit of these salts. This hardness cannot be removed by boiling. 

So, How to Remove the Permanent Hardness of Water?

We use certain chemical methods to remove the permanent hardness of water, which are:

  1. Treating the Water with Washing Soda

In this method, we add washing soda, i.e., Na2CO3 to the hard water. It combines with chloride salts of calcium and magnesium present in the water to form compounds. The reaction is:

         CaCl2       +        Na2CO3  →            CaCO3                +    2NaCl

      Calcium Chloride  Sodium Carbonate  Calcium Carbonate Sodium Chloride

It reacts with MgCl2 to form MgCO3 and  2NaCl. It also reacts with sulfate salts of calcium and magnesium to form carbonates and Sodium sulfate. 

The reaction takes place in this way:

      CaSO4  + Na2CO3  → CaCO+ Na2SO4 , and

       MgSO4+ Na2CO3  → MgCO3 + Na2SO4

Now, the precipitates of CaCO3 and MgCO3 are removed by filtration, which frees the water from Ca2+ and Mg2+ ions. After these ions are removed, the water becomes soft (just like rainwater).

  1. Calgon’s Method

In this method, when Calgon, i.e Sodium Hexametaphosphate (Na6P6O18) is added to the water, each of its molecules ionizes to give two Na+ ions and one complex anion.

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This complex anion further releases Na+ ions and captures all the Ca2+ or Mg2+ ions.  

These ions become a part of the complex anion. In this manner, water is freed from Ca2+ or Mg2+ ions. Now, it contains only Na+ ions only. Therefore, it no longer remains hard, i.e., becomes soft.                               

  1. Ion-Exchange Method

 In this method, we add a Permutit called zeolite (Sodium Aluminum Silicate) or (AlNa12SiO5) to the water, which is insoluble in water. On adding this to water, the ion-exchange process starts between it and the Ca2+ or Mg2+ ions. 

In this way, water becomes free from Ca2+ or Mg2+ ions, and no longer remains hard.

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  1. Synthetic Resins Method

In this method, we use synthetic ion-exchange resins (RNa+), which is insoluble in water. When it is added to water, an exchange between  RNa+ and  Ca2+ or Mg2+ ions occurs. This process frees water from  Ca2+ or Mg2+ ions. So, water no longer remains hard, and it becomes soft.

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FAQ (Frequently Asked Questions)

Q1: What are the Units of Water Hardness?

Ans: We measure water hardness in the following units:

  • Grains per gallon

  • Milligrams of Ca per liter

  • Parts per million (ppm)

Q2: Why doesn’t the Hard Water Get along with Soap?

Ans: When hard water is treated with soap solution, the  Ca2+ or Mg2+ ions present in the hard water react with the anions of fatty acids present in soap and form a curdy white precipitate, i.e., scum. So, hard water doesn’t get along with soap.

The reaction is:

C17H35COO- Na+ + CaCl2 → (CHCOO)2Ca + 2 NaCl

   (soap)                                    (Scum)

C17H35COO- Na+ + MgSO4 → (CHCOO)2Mg+ Na2SO4

In making the precipitate of these ions, a lot of soap is wasted. That’s why it is not suitable for washing.

Q3: Which Performs better in Hard Water, Soap, or Detergent?

Ans: Detergents can lather well in the hard water. Detergent is the sodium salt of a long chain benzene sulphonic acid (or the sodium salt of a long chain of alkyl hydrogen sulfate), which repels the Ca2+ or Mg2+ ions. 

When added to water, they don’t produce insoluble precipitates, and therefore dissolve easily in the hard water. That’s why they have a stronger cleansing action than soaps.

Q4: What are the Disadvantages of having Soft Water?

Ans: The soft water contains a lot of sodium ions. Therefore, regular drinking of soft water may lead to serious health issues like an increase in blood pressure.