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Which of the following ions will cause hardness in water sample?
A. `Ca^(2+)`
B. `Na^(+)`
C. `Cl^(-)`
D. `K^(+)`

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Correct Answer - A
Bicarbonate, chlorides and sulphates of Ca and Mg are responsible for the hardness of water.
Note: Hard water forms scum/precipiate with soap. Soap canatining out Ca/Mg stearate.
`(C_(17)H_(35)COONa)` reacts with hard water to precipiate out ca/Mg stearate.
` 2C_(17)H_(35)COONa(aq) + M^(2+)(aq) to (C_(17)H_(35)COO)_(2) M darr + 2Na^(+)(aq) ("where", M = Ca//Mg)`

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