Zeolite can be used to soften hard water when:

The correct answer is Option (3) → Na⁺ ions in zeolite are replaced by Ca²⁺ or Mg²⁺ ions in hard water

Zeolite (permutit) is hydrated sodium aluminium silicate, written NaZ. When hard water passes through a bed of zeolite, the \( \ce{Ca^2+} \) and \( \ce{Mg^2+} \) ions that cause hardness are taken up by the zeolite and an equivalent amount of \( \ce{Na+} \) passes into the water (ion exchange):

    \( \displaystyle \ce{2NaZ(s) + M^2+(aq) -> MZ2(s) + 2Na+(aq)}\qquad (\text{M = Ca or Mg}) \)

Sodium salts do not cause hardness, so the water is softened. The exhausted zeolite is regenerated with aqueous NaCl:

    \( \displaystyle \ce{MZ2(s) + 2NaCl(aq) -> 2NaZ(s) + MCl2(aq)} \)

• Option (1) Ca²⁺ or Mg²⁺ ions in zeolite are replaced by Na⁺ ions in hard water — Incorrect. It reverses the exchange: fresh zeolite contains \( \ce{Na+} \) and removes \( \ce{Ca^2+} \)/\( \ce{Mg^2+} \) from the water.

• Option (2) Ca²⁺ ions in zeolite are replaced by Mg²⁺ ions in hard water — Incorrect. Replacing \( \ce{Ca^2+} \) by \( \ce{Mg^2+} \) would only swap one hardness-causing ion for another; the water would stay hard.