Which of the following substances cause temporary hardness in water? 1. Mg(HCO3)2 2. Ca(HCO3)2 3. CaCl2 4. MgSO4 Select the correct answer using the code given below.
- A.3 and 4
- B.2 and 3
- C.1 and 4
- D.1 and 2
Correct answer
Explanation
The correct answer is D, 1 and 2. Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out. This is also why kettles and boilers collect a white scale. Temporary hardness can be removed too by Clark's method, in which a calculated amount of lime is added. Permanent hardness comes from the chlorides and sulphates of calcium and magnesium, which do not break down on boiling and need washing soda, ion-exchange resins or the Calgon process. Option A is wrong because CaCl2 and MgSO4 both cause permanent hardness. Option B is wrong because it pairs a bicarbonate with CaCl2, a cause of permanent hardness. Option C is wrong because it pairs a bicarbonate with MgSO4, also a cause of permanent hardness. Exam tip: bicarbonates make temporary hardness, removed by boiling or Clark's method; chlorides and sulphates make permanent hardness.
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