Question

You want to test for hardness of water but hard water is not available in the laboratory. Which of the following compounds may be dissolved in pure water to make it hard?

(i) Hydrogen Carbonate of Sodium
(ii) Sulphate of Magnesium
(iii) Chloride of Calcium
(iv) Carbonate of Sodium

(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv) ​

Answer 1

To make water hard in a laboratory, one can dissolve Hydrogen Carbonate of Sodium and Sulphate of Magnesium in pure water, which contributes to temporary and permanent hardness respectively. The correct answer to the student's question is (a) (i) and (ii).

Step-by-step explanation:

To test for the hardness of water in a laboratory setting, you would need to add compounds that contain calcium or magnesium ions because these contribute to water hardness. Choices (i) Hydrogen Carbonate of Sodium (NaHCO3) and (ii) Sulphate of Magnesium (MgSO4) will result in hard water when dissolved in pure water. The Hydrogen Carbonate of Sodium will increase the carbonate hardness (temporary hardness), while Sulphate of Magnesium contributes to the permanent hardness due to the sulfate ion. Therefore, the answer would be (a) (i) and (ii).

Calcium (Ca2+) and magnesium (Mg2+) are the primary minerals that make water hard. While Sodium Carbonate (Na2CO3) does contain carbonate, which can be associated with hardness, it does not contribute to hardness in the same way as calcium or magnesium compounds because sodium does not precipitate with soap, a common test for hardness. The Chloride of Calcium (CaCl2), on the other hand, would contribute to water hardness, but it's not one of the options paired with the correct Sodium Hydrogen Carbonate in the provided choices.