Permanent hardness of water can be removed by adding ___________?

Water hardness refers to the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts in water. There are two types of hardness: temporary hardness and permanent hardness.

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  Temporary hardness is caused by bicarbonates of calcium and magnesium. It can be removed by simple boiling, which decomposes bicarbonates into insoluble carbonates that precipitate out.

  
  


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  Permanent hardness, on the other hand, is caused by chlorides and sulfates of calcium and magnesium. This type of hardness cannot be removed by boiling.

  
  

To soften water with permanent hardness, sodium carbonate (Na₂CO₃) is commonly used. Sodium carbonate reacts with calcium and magnesium ions in water to form insoluble carbonates, which precipitate and can be filtered out. The reaction is as follows:

CaSO₄ + Na₂CO₃ → CaCO₃ ↓ + Na₂SO₄
MgCl₂ + Na₂CO₃ → MgCO₃ ↓ + 2NaCl

Through this process, the calcium and magnesium ions responsible for hardness are effectively removed, leaving softer water that is more suitable for washing, cooking, and industrial use.

Looking at the options:

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  Sodium carbonate (Correct Answer): Removes permanent hardness by precipitating Ca²⁺ and Mg²⁺.

  
  


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  Alum: Used as a coagulant in water treatment but not for hardness removal.

  
  


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  Potassium permanganate: Works as an oxidizing agent for disinfection, not hardness removal.

  
  


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  Lime (CaO): Useful for removing temporary hardness but not effective alone for permanent hardness.

  
  

Permanent hardness is an important concept in everyday science because hard water causes problems like scaling in boilers, reduced soap efficiency, and damage to pipes. Industrially, methods such as ion-exchange resins and the Calgon method are also used to soften water.

Therefore, the correct answer is sodium carbonate, which effectively removes permanent hardness and makes water more suitable for domestic and industrial purposes.