Explain based on the HSAB concept ("hard and soft acids and bases") why NaCl is very soluble in water, where as AgCl is not.

Na⁺ tends to interact with the hardest base, which is water. Ag⁺ tends to interact with the softest (hardless) base, which is Cl⁻.

Explanation:

The HSAB concept says that hard acids are small ions with low electronegativity, while hard bases are electron donating groups with high electronegativity and low polarizability. The HSAB concept also says that hard acids will tend to react with hard bases. The opposite is valid for soft acids and soft bases.

Na⁺ is a hard acid

Ag ⁺ is a soft acid

Cl⁻ is a hard base

H₂O is a harder base than Cl⁻

Therefore, when in water, the Na⁺ tends to react with water, because it is a harder base than Cl⁻. However, as Ag⁺ is a soft acid, it will tend to stay with the less hard base, which is Cl⁻.