Short Answer

Some salts remain insoluble in water because the forces holding their ions together (lattice energy) are stronger than the forces of attraction between ions and water molecules. As a result, the salt does not dissolve appreciably.

For example, barium sulfate (BaSO₄) and silver chloride (AgCl) are largely insoluble. Insoluble salts are important in precipitation reactions, water purification, and industrial applications.

Detailed Explanation :

Definition of Insoluble Salts

An insoluble salt is a compound that does not dissolve significantly in water, forming very few ions in solution. The extent of solubility is extremely low, often measured in grams per liter or molarity.

• These salts are sparingly soluble or practically insoluble.
• Solubility depends on lattice energy, hydration energy, and interactions with solvent molecules.

1. Factors Responsible for Insolubility

1. Lattice Energy
   • The energy that holds the ions together in a solid salt.
   • Higher lattice energy → stronger ionic bonds → lower solubility.
   • Example: BaSO₄ has strong ionic bonds; water cannot break them effectively.
2. Hydration Energy
   • Energy released when ions interact with water molecules.
   • If hydration energy < lattice energy, the salt remains insoluble.
   • Example: AgCl: lattice energy is too high for water molecules to dissolve it significantly.
3. Charge of Ions
   • Salts with highly charged ions (like Mg²⁺, Al³⁺, SO₄²⁻) have stronger attraction → lower solubility.
4. Size of Ions
   • Smaller ions with high charge density form stronger ionic bonds → low solubility.

2. Examples of Insoluble Salts

1. Barium Sulfate (BaSO₄)
   • Used in medical X-rays as a contrast medium because it does not dissolve in the body.
2. Silver Chloride (AgCl)
   • Used in photography and precipitation reactions.
3. Calcium Sulfate (CaSO₄)
   • Used in construction materials; remains mostly insoluble in water.
4. Lead(II) Chloride (PbCl₂)
   • Low solubility; used in laboratory experiments.

3. Role of Insoluble Salts in Chemistry

1. Precipitation Reactions
   • Insoluble salts form precipitates in solutions.
   • Example: AgNO₃ + NaCl → AgCl(s) + NaNO₃
2. Water Purification
   • Insoluble salts remove unwanted ions through precipitation.
   • Example: CaCO₃ precipitates hardness ions.
3. Industrial Applications
   • Insoluble salts are used in ceramics, paints, and construction.
4. Biological Systems
   • Some salts like CaCO₃ remain insoluble in bones and shells, providing structural support.

4. Solubility Product (Ksp) and Insoluble Salts

• Insoluble salts have very low Ksp values.
• Ksp indicates the maximum concentration of ions in a saturated solution.
• Example: For AgCl, Ksp ≈ 1.8 × 10⁻¹⁰ → extremely low solubility.
• Even when some dissolution occurs, equilibrium limits the concentration of ions.

5. Summary

• Insoluble salts remain undissolved because lattice energy > hydration energy.
• Factors affecting insolubility: ion charge, ion size, lattice energy, hydration energy.
• Examples: BaSO₄, AgCl, CaSO₄, PbCl₂.
• Importance: precipitation, water purification, industrial and biological applications.

Conclusion

Some salts remain insoluble due to strong ionic bonds and low interaction with water, making them sparingly soluble or practically insoluble. Understanding this helps predict precipitation reactions, solubility behavior, and industrial applications. Insoluble salts like BaSO₄ and AgCl are essential in chemistry labs, medicine, and environmental applications, demonstrating the importance of solubility principles in practical and theoretical chemistry.