Question

Assertion: The temporary hardness of water is due to presence of chloride of magnesium
Reason: Temporary hardness is removed by Clark’s method.
A) Both assertion and reason are correct and reason is the correct explanation for assertion
B) Both assertion and reason are correct but reason is not the correct explanation of assertion
C) Assertion is correct but reason is incorrect
D) Assertion is incorrect but reason is correct

Answer 1

Hint:Hard water is water that does not form lather with soap instead they form scum with soap. Hard water hence uses more soap than with soft water. Hard water contains high mineral content. Clark’s method is a type of water treatment used for water softening, to remove the hardness of it.

Complete answer:
Hardness of water is due to the presence of some salts of calcium and magnesium. Hardness of water can be temporary and it can be permanent. Temporary hardness can be removed by heating the water, and that precipitates the salts. Temporary hardness is due to presence of calcium and magnesium carbonates and bicarbonates. Boiling promotes the formation of carbonate from bicarbonate and precipitates calcium carbonate out of the solution, leaving water that is softer upon cooling.

Permanent hardness cannot be eliminated by boiling. Permanent hardness is due to calcium and magnesium nitrates, sulphates, and chlorides. These salts do not precipitate out from the water on boiling. They can be removed using a water softener or ion exchange column.

Water softening is the removal of calcium and magnesium cations from the hard water. One of the methods for removing hardness of water is lime softening also called Clark's method. In this method we add lime water calcium hydroxide to remove hardness by precipitation. As we add lime water to the water, the pH is raised and equilibrium of carbonates species in the water is shifted. Dissolved carbon dioxide is changed into bicarbonate and then carbonate, and this causes calcium carbonate to precipitate due to exceeding the solubility product of it.

So the correct option is D. Temporary harness is because of carbonates and bicarbonates of calcium and magnesium not chlorides. Reason is that Clark’s method is used in removal of temporary hardness from water.

Hence the correct answer is option ‘D’.

Note:The calcium and magnesium ions when present in water react with the soap and remove its surfactant properties and form a solid precipitate (scum). For example sodium stearate which is the main component in soap, when reacted with calcium ion produces calcium stearate. But detergent does not have these groups hence they can be used against hard water.