When testing hardness of water, the water may be titrated with standard soap solution. The usual end point in the titration is :

when blue litmus turns red

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when a lather that presists for a given time is formed

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when the alkaline soap is decomposed completely

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when a maximum amount of scum is produced

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Solution

The correct option is B when a lather that presists for a given time is formed
Soluble soaps consist of sodium (or) potassium salts of higher fatty acids. When soap solution is added to hard water, the hardness causing ions first react to form precipitate of calcium and magnesium soaps which are insoluble in water. Thus, after all the hardness causing metal ions present in the hard water sample are precipitated, the further addition of soap gives lather. The total hardness of water sample can be determined by titrating an aliquot of the sample against a standard soap solution in alcohol. The appearance of a stable lather persisting even after shaking for about 2 min marks end point.
Thus option B is correct.

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