Zeolite can be used to soften hard water when:

\(Na^+\) ions in zeolite are replaced by \(Ca^{2+}\) or \(Mg^{2+}\) ions in hard water

The correct answer is option 3. \(Na^+\) ions in zeolite are replaced by \(Ca^{2+}\) or \(Mg^{2+}\) ions in hard water. 

Hard water contains high concentrations of calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions. These ions can form insoluble compounds with soap, leading to scale formation in pipes, appliances, and boilers.

Zeolites are naturally occurring or synthetic aluminosilicate minerals with a crystalline structure that contains a network of pores. They can trap and exchange ions within these pores. Common examples include sodium zeolite (also known as sodium aluminosilicate). The structure of zeolites includes a framework of silica and alumina tetrahedra, which create a three-dimensional lattice with interconnected cavities. This structure allows the zeolite to act as a molecular sieve.

Ion Exchange Process

The ion exchange process involves the substitution of one type of ion for another. In the context of water softening, zeolites exchange their sodium (\(Na^+\)) ions for the calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions in hard water.

Working:

Zeolite Composition: Zeolites typically have sodium ions (\(Na^+\)) associated with them. These sodium ions are loosely bound and can be exchanged with other cations.

Water Softening:

Hard Water: Contains calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions.

Zeolite Bed: When hard water passes through a bed of zeolite, the \(Ca^{2+}\) and \(Mg^{2+}\) ions in the water are attracted to the negatively charged sites in the zeolite structure.

Exchange Reaction: The sodium ions (\(Na^+\)) in the zeolite are released into the water and replace the \(Ca^{2+}\) and \(Mg^{2+}\) ions. The overall reaction can be represented as:

\(\text{Na}_2\text{Zeolite} + \text{Ca}^{2+} + \text{Mg}^{2+} \rightarrow \text{CaZeolite} + \text{MgZeolite} + 2 \text{Na}^+\)

Detailed Explanation of Each Option

1. \(Ca^{2+}\) or \(Mg^{2+}\) ions in zeolite are replaced by \(Na^+\) ions in hard water

This is incorrect. This is not the correct description of the ion exchange process. In reality, the \(Ca^{2+}\) and \(Mg^{2+}\) ions from the hard water are exchanged for \(Na^+\) ions from the zeolite, not the other way around.

2. \(Ca^{2+}\) ions in zeolite are replaced by \(Mg^{2+}\) ions in hard water

This is incorrect. This option is incorrect because zeolites do not initially contain \(Ca^{2+}\) ions. Instead, they contain sodium ions (\(Na^+\)). The process involves replacing \(Ca^{2+}\) and \(Mg^{2+}\) ions in the hard water with \(Na^+\) ions from the zeolite.

3. \(Na^+\) ions in zeolite are replaced by \(Ca^{2+}\) or \(Mg^{2+}\) ions in hard water

This is correct. This correctly describes the ion exchange process. In water softening, the sodium ions (\(Na^+\)) from the zeolite are exchanged for calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions in the hard water.

4. None is correct

This is incorrect. This option is incorrect because Option 3 accurately describes the process used in water softening with zeolites.

Conclusion

The correct option is: 3. \(Na^+\) ions in zeolite are replaced by \(Ca^{2+}\) or \(Mg^{2+}\) ions in hard water.

This reflects the actual mechanism by which zeolites soften hard water by exchanging sodium ions for the hardness-causing calcium and magnesium ions.