In water chemistry, permanent hardness of water is mainly due to the presence of which dissolved salt among the following?

Introduction / Context:
Hardness of water is an important concept in environmental chemistry and everyday life, influencing soap consumption, scale formation in boilers and the effectiveness of detergents. Hardness is classified into temporary and permanent hardness depending on the nature of the dissolved salts that cause it. This question asks you to identify the salt responsible for permanent hardness of water from a list of common calcium and magnesium compounds.

Given Data / Assumptions:
- The question refers specifically to permanent hardness, not temporary hardness.
- Options include calcium sulphate, calcium bicarbonate, sodium bicarbonate and magnesium bicarbonate.
- We assume standard definitions: temporary hardness is typically due to bicarbonates of calcium and magnesium; permanent hardness is due to sulphates and chlorides of calcium and magnesium.

Concept / Approach:
Temporary hardness arises mainly from the presence of calcium bicarbonate and magnesium bicarbonate in water. It can be removed by boiling, because bicarbonates decompose to form insoluble carbonates that precipitate. Permanent hardness is caused by salts such as calcium sulphate, magnesium sulphate and the chlorides of these metals. These salts do not decompose upon boiling and require chemical treatment, such as using washing soda or ion exchange, to be removed. Therefore, among the given options, calcium sulphate is the salt that contributes to permanent hardness.

Step-by-Step Solution:
Step 1: Recall that bicarbonates of calcium and magnesium are mainly responsible for temporary hardness and can be removed by boiling. Step 2: Identify which options are bicarbonates: calcium bicarbonate, sodium bicarbonate and magnesium bicarbonate all have the HCO3^- ion. Step 3: Calcium sulphate (CaSO4) is a sulphate salt of calcium, not a bicarbonate, and is known to cause permanent hardness. Step 4: Since the question asks specifically about permanent hardness, eliminate bicarbonate options and select calcium sulphate.

Verification / Alternative check:
To verify, consider the effect of boiling water containing each of these salts. Water with calcium bicarbonate will lose hardness after boiling because calcium carbonate precipitates, reducing hardness. This is why temporary hardness can be treated simply by boiling. In contrast, boiling water containing calcium sulphate does not significantly remove the dissolved CaSO4, and hardness remains. Industrial water treatment plants often remove permanent hardness using chemical softening or ion exchange methods, supporting the classification of calcium sulphate as a permanent hardness causing salt.

Why Other Options Are Wrong:
Calcium bicarbonate: This contributes mainly to temporary hardness, which can be removed by boiling.
Sodium bicarbonate: While it affects alkalinity, sodium salts typically do not contribute significantly to hardness because hardness is associated with calcium and magnesium ions.
Magnesium bicarbonate: Like calcium bicarbonate, this causes temporary hardness that can be removed by boiling, not permanent hardness.

Common Pitfalls:
Learners sometimes memorise lists of salts without distinguishing between bicarbonates versus sulphates and chlorides. Another pitfall is to think that any calcium salt must cause permanent hardness. In reality, the anion plays a crucial role: bicarbonates are associated with temporary hardness, while sulphates and chlorides are associated with permanent hardness. A clear mental table of these categories will help you answer many water chemistry questions correctly.

Final Answer:
Permanent hardness of water is mainly due to the presence of calcium sulphate (CaSO4).