In water treatment and basic chemistry, permanent hardness of water can be removed by adding which chemical substance to precipitate calcium and magnesium ions?

Introduction / Context:
This question focuses on the concept of water hardness in environmental and applied chemistry. Hard water contains dissolved calcium and magnesium salts. Permanent hardness is mainly due to chlorides and sulfates of calcium and magnesium, which do not disappear upon boiling. Understanding how to remove permanent hardness is important in water softening, boiler maintenance, and household applications such as laundry and soap use.

Given Data / Assumptions:

• The type of hardness considered is permanent hardness.
• We are asked which chemical can remove this permanent hardness by addition.
• Options include alum, lime, sodium carbonate, potassium permanganate, and calcium sulfate.

Concept / Approach:
Permanent hardness due to calcium and magnesium chlorides and sulfates can be removed by adding washing soda, which is sodium carbonate. Sodium carbonate reacts with soluble calcium and magnesium salts to form insoluble carbonates that can be removed by filtration. Alum is used for coagulation, lime is more effective for temporary hardness, and potassium permanganate is an oxidizing agent. Therefore, sodium carbonate is the correct chemical for removing permanent hardness by precipitation.

Step-by-Step Solution:

Verification / Alternative check:
Standard water treatment processes include the lime soda process where sodium carbonate is specifically listed as the reagent used to remove the non carbonate (permanent) hardness. Textbooks and exam guides often state that washing soda can remove both temporary and permanent hardness by precipitation of metal carbonates. Experimental demonstrations of water softening frequently feature sodium carbonate as the main softening agent for permanently hard water. This evidence strongly supports sodium carbonate as the correct answer.

Why Other Options Are Wrong:

• Alum: Works mainly as a coagulating agent to settle suspended impurities, not to precipitate calcium and magnesium from solution.
• Lime: It helps in softening but is especially used with sodium carbonate and is more closely associated with temporary hardness removal.
• Potassium permanganate: A strong oxidizing agent used for disinfection and oxidation of organic matter, not for hardness removal.
• Calcium sulfate: It is itself a salt that contributes to hardness rather than removing it.

Common Pitfalls:
Students sometimes confuse temporary and permanent hardness and may incorrectly think that boiling is always sufficient. Others may pick alum or lime because they are familiar water treatment chemicals. Remembering that washing soda, which is sodium carbonate, is the key agent for precipitating calcium and magnesium from permanently hard water helps answer many questions about water softening accurately.

Final Answer:
Permanent hardness of water may be removed by the addition of Sodium carbonate.