Explain why the water obtained after passing hard water through cation exchange resins is acidic.

Explanation

When hard water is passed through organic ion exchange resin, the water obtained is acidic because all the metal ions present in water are exchanged with `H^(+)` ions of the resin. As a result , the resulting water is free of cations and has a high concentration of `H^(+)` ions. So, the water turns blue litmus paper red.
`underset(("from hard water"))(M^(2) )" "+ " "underset("cation exchange resin ")(2RSO_(3)H)" "rarr" " underset("exhausted resin" )(M("RS"O_(3))_(2)) " "+ " "underset((M = Ca or Mg ))(2H^(+))`