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Q7 (CDS-I/2015) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Applied everyday chemistry Answer Verified

When hard water is evaporated completely, the white solid remains in the container. It may be due to the presence of 1. Carbonates of Ca and Mg 2. Sulphates of Ca and Mg 3. Chlorides of Ca and Mg Select the correct answer using the code given below :

Result
Your answer: —  Â·  Correct: B
Explanation

Hardness in water is primarily caused by the presence of dissolved bicarbonates, chlorides, and sulphates of calcium and magnesium [2]. When hard water is evaporated completely, these dissolved minerals remain as a white solid residue. Temporary hardness is caused by bicarbonates, which upon heating or evaporation, decompose into insoluble carbonates (CaCO3) or hydroxides (Mg(OH)2), forming a solid scale [1]. Permanent hardness is attributed to the presence of chlorides (CaCl2, MgCl2) and sulphates (CaSO4, MgSO4) of calcium and magnesium [2]. Unlike bicarbonates, these salts do not decompose upon boiling but remain in the container as solid mineral matter once the solvent (water) is entirely removed through evaporation [2]. Therefore, the white solid residue consists of carbonates, sulphates, and chlorides of calcium and magnesium.

Sources

  1. [1] https://annamalaiuniversity.ac.in/studport/download/engg/chemistry/resources/Chemistry_19_ETBS202_2020_II_semester.pdf
  2. [2] https://www.uptti.ac.in/classroom-content/data/Unit-1_Water%20and%20its%20treatment.pdf
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SIMILAR QUESTIONS

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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,

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NDA-I · 2011 · Q35 Relevance score: -2.74

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NDA-I · 2017 · Q7 Relevance score: -3.35

Temporary hardness in water is due to which one of the following of Calcium and Magnesium ?