Why does hard water not from lather with soap ?

Explanation

Hard water contains `Ca^(2+) and Mg^(2+)` ions which of the soap to from corresponding calcium and magnesium salts that are insoluble and get precipitated. Therefore, soap is simply wasted without producing any lather.
`underset("Soap")(2RCOONa)+underset("(Hard water"))(Ca^(2+))tounderset("(ppt)")(RCOO)_(2)Ca+2Na^(+)`
`underset("Soap")(2RCOONa)+underset("(Hard water)"(Mg^(2))tounderset("(ppt)")((RCOO)_(2))Mg+2Na^(+)`