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

Introduction / Context:
Hardness of water is an important topic in environmental chemistry and everyday life because it affects soap consumption, scale formation and industrial processes. Hardness can be classified as temporary or permanent based on whether it can be removed by simple boiling. Temporary hardness is usually associated with bicarbonate salts of calcium and magnesium, while permanent hardness involves chlorides and sulphates. This question focuses on identifying the specific salt responsible for temporary hardness.

Given Data / Assumptions:

• The question asks specifically about temporary hardness.
• The options include calcium carbonate, calcium bicarbonate, magnesium sulphate and calcium sulphate.
• We assume typical natural water containing dissolved salts from rocks and soil.
• We use the standard definitions: temporary hardness is removable by boiling, permanent hardness is not.

Concept / Approach:
Temporary hardness is mainly caused by the presence of soluble bicarbonates of calcium and magnesium, such as Ca(HCO3)2 and Mg(HCO3)2. When such water is boiled, these bicarbonates decompose to form insoluble carbonates, carbon dioxide and water, thereby removing hardness. In contrast, sulphates and chlorides of calcium and magnesium cause permanent hardness because they do not decompose upon boiling. Calcium carbonate itself is largely insoluble and does not generally remain dissolved in water to cause hardness; instead, it often precipitates as scale.

Step-by-Step Solution:
Step 1: Recall that temporary hardness is removed by boiling water, which suggests the presence of bicarbonate salts.Step 2: Calcium bicarbonate, Ca(HCO3)2, is soluble in water and is a classic cause of temporary hardness.Step 3: On boiling, calcium bicarbonate breaks down to form calcium carbonate, carbon dioxide and water, and the insoluble calcium carbonate can be removed.Step 4: Calcium carbonate, CaCO3, is not very soluble in water and, when it forms, usually precipitates rather than remaining in solution to cause hardness.Step 5: Magnesium sulphate and calcium sulphate contribute to permanent hardness, not temporary hardness, because they do not decompose on boiling.Step 6: Therefore, the correct answer for temporary hardness is calcium bicarbonate.

Verification / Alternative check:
You can verify by recalling the standard chemical equation for removal of temporary hardness: Ca(HCO3)2 (aq) on boiling gives CaCO3 (s) + CO2 (g) + H2O (l). This reaction is widely shown in textbooks to explain why boiling can soften water containing bicarbonate hardness. There is no similar simple decomposition for sulphates or chlorides at normal boiling temperatures. Thus, calcium bicarbonate fits perfectly with the concept of temporary hardness, whereas the other salts do not.

Why Other Options Are Wrong:
Option a, calcium carbonate, is mostly insoluble and is more commonly associated with scale deposits rather than dissolved hardness. Option c, magnesium sulphate, and option d, calcium sulphate, both cause permanent hardness and cannot be removed just by boiling. Since the question specifically refers to temporary hardness, any salt associated with permanent hardness must be ruled out. Only calcium bicarbonate matches the definition and behaviour of temporary hardness in water.

Common Pitfalls:
Students sometimes confuse the terms carbonate and bicarbonate, or they assume that any calcium salt must cause the same type of hardness. Another mistake is to think that calcium carbonate itself is responsible for temporary hardness because it appears in the boiling reaction. In fact, calcium carbonate is the precipitated product of boiling, not the original cause of hardness. To avoid confusion, always link temporary hardness with bicarbonates and permanent hardness with sulphates and chlorides of calcium and magnesium.

Final Answer:
Temporary hardness of water is mainly due to the presence of calcium bicarbonate in solution.