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Q41 (NDA-I/2013) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry Answer Verified

Which of the following statements regarding the property of hard water is/are correct? 1. Temporary hardness of water is due to the presence of soluble magnesium and calcium hydrogen carbonate. 2. Temporary hardness of water can be removed by boiling. 3. Calgon's method is applied to remove temporary hardness of water. 4. Permanent hardness of water is removed by Clark's method. Select the correct answer vising the code given below,

Result
Your answer: —  Â·  Correct: A
Explanation

Temporary hardness of water is caused by the presence of dissolved bicarbonates (hydrogen carbonates) of calcium and magnesium [1]. This type of hardness is easily removed by boiling, which causes the soluble bicarbonates to decompose into insoluble carbonates or hydroxides that precipitate out [1]. Calgon's method (using sodium hexametaphosphate) and the ion-exchange method are specifically used to remove permanent hardness, not temporary hardness. Conversely, Clark's method, which involves adding a calculated amount of lime, is a standard technique for removing temporary hardness, whereas permanent hardness (caused by chlorides and sulfates) requires chemical treatment like washing soda or ion-exchange resins [1]. Therefore, statements 1 and 2 are correct, while statements 3 and 4 are incorrect as they swap the methods for temporary and permanent hardness removal.

Sources

  1. [1] https://uptti.ac.in/classroom-content/data/Unit-1_Water%20and%20its%20treatment.pdf
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