Permanent hardness of water can be removed most effectively by adding which of the following substances?

Introduction / Context:
This question belongs to the topic of water chemistry and hardness. Hard water contains dissolved salts of calcium and magnesium that interfere with soap action and can form scale in boilers and pipes. Temporary hardness can be removed by boiling, but permanent hardness requires chemical treatment. The question asks which substance is commonly used to remove permanent hardness of water.

Given Data / Assumptions:

• The type of hardness considered is permanent hardness, mainly due to calcium and magnesium sulphates and chlorides.
• The options include bleaching powder, chlorine, washing soda (sodium carbonate) and potassium permanganate.
• The chemical added should convert soluble hardness causing salts into insoluble compounds that can be removed.
• Standard school level methods for softening hard water are assumed.

Concept / Approach:
Permanent hardness is usually caused by soluble salts like calcium sulphate and magnesium chloride. One common method of removing this hardness is to add washing soda, which is sodium carbonate. Sodium carbonate reacts with the calcium and magnesium ions to form insoluble carbonates, such as calcium carbonate and magnesium carbonate, which precipitate out. This process reduces the concentration of hardness causing ions and softens the water. Bleaching powder and chlorine are used mainly for disinfection. Potassium permanganate is an oxidising agent used for removing iron or organic impurities, but not specifically for permanent hardness.

Step-by-Step Solution:
Step 1: Identify the main cause of permanent hardness. It is due to calcium and magnesium salts that do not decompose on boiling. Step 2: Recall that one standard method of softening such water is to add sodium carbonate, commonly known as washing soda. Step 3: Sodium carbonate reacts with soluble calcium and magnesium salts to form insoluble carbonates like CaCO3 and MgCO3. Step 4: These insoluble precipitates can be removed by filtration, resulting in softened water. Step 5: Bleaching powder and chlorine are used to kill bacteria and disinfect water rather than to remove hardness. Step 6: Potassium permanganate is a strong oxidising agent useful in some purification processes, but it does not directly remove permanent hardness. Step 7: Therefore, washing soda (sodium carbonate) is the correct choice.

Verification / Alternative check:
Textbooks on water treatment describe several methods of softening hard water: lime soda process, ion exchange and addition of washing soda. In the lime soda process, both calcium hydroxide (lime) and sodium carbonate are used to precipitate hardness causing ions. For small scale domestic use, washing soda is often mentioned as a convenient chemical for reducing hardness. The description that sodium carbonate converts soluble salts of calcium and magnesium into their insoluble carbonates matches this function. By contrast, bleaching powder and chlorine are always discussed under disinfection rather than softening.

Why Other Options Are Wrong:
Bleaching powder (calcium hypochlorite) is used to disinfect water and make it safe for drinking by killing microbes. Chlorine gas or chlorine solutions also disinfect water but do not precipitate hardness causing ions. Potassium permanganate is used as an oxidising agent to remove iron and manganese or to control taste and odour in water, but not to remove calcium or magnesium salts that cause permanent hardness. None of these substances directly produce insoluble carbonates of calcium and magnesium, so they are not effective softening agents for permanent hardness.

Common Pitfalls:
Students sometimes confuse water purification for health, which uses disinfectants, with water softening for industrial or domestic use, which uses chemicals that remove hardness. Seeing familiar water treatment chemicals like chlorine and bleaching powder, they may choose them without recalling the specific processes involved. To avoid this, remember that disinfection kills germs, while softening removes hardness ions, often using sodium carbonate or ion exchange resins. Associating washing soda with the term lime soda process can also help in recalling its role in softening hard water.

Final Answer:
Permanent hardness of water can be removed by adding washing soda (sodium carbonate).