Soap on reaction with hard water forms insoluble scum due to the formation of

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${(C_{17} H_{35} COO)}_{2} Ca$

${CaCl}_{2}$

$C_{17} H_{35} COONa$

$Nacl$

answer is D.

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Detailed Solution

Hard water contains calcium and magnesium ions.
These ions form insoluble calcium and magnesium soaps respectively when sodium or
potassium soaps are dissolved in hard water.
$\underset{soap}{2 C_{17} H_{35} COONa} + {CaCl}_{2} \to \underset{insoluble calcium stearate (soap)}{{(C_{17} H_{35} COO)}_{2} Ca} + 2 NaCl$
These insoluble soaps separate as scum in water and are useless as cleansing agent.
In fact, these are hinderance to good washing, because the precipitate adheres onto the fiber of the cloth as gummy mass.
Hair washed with hard water looks dull because of this sticky precipitate.

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