Which of the following dissolved salts cause temporary hardness in natural water? 1. Mg(HCO3)2 2. Ca(HCO3)2 3. CaCl2 4. MgSO4

Introduction / Context:
This question tests your understanding of water hardness in environmental chemistry. Hardness of water is a very important practical concept because it affects soap consumption, boiler operation, and household water quality. There are two main types of hardness, temporary and permanent, and they are caused by different dissolved salts. Here you need to identify which of the listed salts produce temporary hardness in water.

Given Data / Assumptions:

• Four salts are listed: Mg(HCO3)2, Ca(HCO3)2, CaCl2 and MgSO4.
• The question asks specifically about temporary hardness, not total hardness.
• Assume standard definitions where temporary hardness is removed by boiling.
• Assume that each salt, when dissolved, provides either bicarbonate ions or non bicarbonate anions.

Concept / Approach:
Temporary hardness is mainly due to the presence of bicarbonates of calcium and magnesium in water, usually written as Ca(HCO3)2 and Mg(HCO3)2. When hard water containing these bicarbonates is boiled, the bicarbonates decompose, producing insoluble carbonates that precipitate out and so the hardness is removed. Permanent hardness, on the other hand, is due to chlorides and sulfates such as CaCl2 and MgSO4. These salts are not removed simply by boiling and require chemical softening methods. So we must identify which of the listed salts are bicarbonates.

Step-by-Step Solution:

Verification / Alternative check:
In textbook explanations of temporary hardness, you will usually see the equations describing the decomposition of bicarbonates on heating. For example, Ca(HCO3)2 when heated gives CaCO3 plus water plus carbon dioxide. The solid CaCO3 precipitates out of solution and can be removed. Similar equations apply for Mg(HCO3)2. In contrast, salts like CaCl2 and MgSO4 remain dissolved even when the water is boiled. This is why boiling removes only temporary hardness and not permanent hardness. Matching this information with the options confirms that 1 and 2 are correct.

Why Other Options Are Wrong:

• 3 and 4 only: These are chlorides and sulfates that cause permanent, not temporary, hardness.
• 2 and 3 only: This incorrectly includes CaCl2, which does not produce temporary hardness.
• 1 and 4 only: This incorrectly includes MgSO4, which is a permanent hardness causing salt.
• 1, 2, 3 and 4 all cause only temporary hardness: This is wrong because only the bicarbonates cause temporary hardness, while chlorides and sulfates cause permanent hardness.

Common Pitfalls:
A common mistake is to assume that any calcium or magnesium salt must contribute to temporary hardness, but the type of anion is crucial. Bicarbonates are associated with temporary hardness, while chlorides and sulfates are associated with permanent hardness. Another pitfall is to confuse the terms temporary and permanent with the idea of how important the problem is, instead of how easy it is to remove by boiling. Always connect temporary hardness with bicarbonates and boilable removal to answer such questions correctly.

Final Answer:
The salts that cause temporary hardness in water are 1 and 2 only, that is Mg(HCO3)2 and Ca(HCO3)2.