Question
Why do soaps not work in hard water?
✓
Answer
Hard water contains calcium and magnesium ions. These ions form insoluble calcium and magnesium soaps respectively when soaps are dissolved in hard water.
$\ \ \ \ \ \ \ 2\text{C}_{17}\text{H}_{35}\text{COONa}\ \ \ \ +\ \ \ \ \ \text{Cacl}_{2}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \rightarrow\ \ \ \ \ \ \ 2\text{Nacl}\ \ + \ \ \text{(C}_{17}\text {H}_{35}\text{Coo)}_{2}\text{Ca}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{sope}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(in hard water)}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Insoluble calcium stearate (soap)}$
These insoluble soaps separate as scum in water and are useless as cleansing agent. Therefore, soaps are not work in hard water.
$\ \ \ \ \ \ \ 2\text{C}_{17}\text{H}_{35}\text{COONa}\ \ \ \ +\ \ \ \ \ \text{Cacl}_{2}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \rightarrow\ \ \ \ \ \ \ 2\text{Nacl}\ \ + \ \ \text{(C}_{17}\text {H}_{35}\text{Coo)}_{2}\text{Ca}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{sope}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(in hard water)}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Insoluble calcium stearate (soap)}$
These insoluble soaps separate as scum in water and are useless as cleansing agent. Therefore, soaps are not work in hard water.
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