Short Answer

Some salts form acidic solutions because they are made from a strong acid and a weak base. In water, the cation of the weak base reacts with water to release H⁺ ions, increasing acidity.

For example, ammonium chloride (NH₄Cl) forms an acidic solution because NH₄⁺ (from weak base NH₃) hydrolyzes, releasing H⁺ ions. The stronger the acid and weaker the base, the more acidic the solution becomes.

Detailed Explanation :

Acidic Salts

Salts are formed by the neutralization of acids and bases. While many salts produce neutral solutions, some produce acidic solutions due to the hydrolysis of ions in water. An acidic salt solution has a pH less than 7.

Acidic solutions form when a cation derived from a weak base interacts with water, generating hydrogen ions (H⁺). The strength of the parent acid and base determines whether the salt will behave acidic, basic, or neutral.

How Acidic Solutions Form

When salts dissolve in water, their ions dissociate:

Salt → Cation⁺ + Anion⁻

• If the cation is from a weak base (like NH₄⁺ from NH₃), it reacts with water:

NH₄⁺ + H₂O → NH₄OH + H⁺

• The H⁺ ions increase the acidity, lowering the pH.
• The anion is from a strong acid (like Cl⁻ from HCl) and does not hydrolyze because strong acids fully dissociate and their anions are neutral.

This combination produces a solution with excess H⁺, making it acidic.

Examples of Acidic Salts

1. Ammonium Chloride (NH₄Cl)

• Strong acid: HCl
• Weak base: NH₃
• pH < 7 due to NH₄⁺ hydrolysis

1. Ammonium Sulfate ((NH₄)₂SO₄)

• Acid: H₂SO₄ (strong)
• Base: NH₃ (weak)
• Hydrolysis of NH₄⁺ makes solution acidic

1. Aluminium Chloride (AlCl₃)

• Forms acidic solution due to hydrolysis of Al³⁺ ions

Factors Affecting Acidity of Salt Solutions

1. Strength of the Parent Acid

• Stronger acids (HCl, H₂SO₄, HNO₃) lead to more pronounced acidic solutions.

2. Weakness of the Parent Base

• Weaker bases produce cations that hydrolyze more readily, increasing H⁺ concentration.

3. Concentration of the Salt

• Higher concentration of hydrolyzing cations increases acidity.

4. Temperature

• Temperature changes can slightly influence hydrolysis and pH.

Explanation Using Hydrolysis

Acidic salt formation is closely linked to salt hydrolysis:

• Salt hydrolysis occurs when ions from a salt react with water.
• Cations of weak bases react with water to produce H⁺ ions.
• Anions of strong acids do not react because they are stable.

This selective hydrolysis explains why salts from strong acid + weak base always give acidic solutions.

Applications and Importance

1. Laboratory Chemistry

• Acidic salts are used in experiments to study pH, hydrolysis, and buffer solutions.

2. Industrial Processes

• Ammonium salts are used in fertilizers and cleaning products; understanding acidity helps in handling.

3. Environmental Science

• Hydrolysis of acidic salts affects soil and water pH.
• For example, ammonium salts in fertilizers can slightly acidify soil.

4. Biological Systems

• Certain acidic salts help regulate pH in biological fluids and solutions.

Summary of Why Some Salts Are Acidic

• Salt is formed from a strong acid and weak base.
• The cation from the weak base hydrolyzes in water.
• This produces H⁺ ions, lowering pH.
• The anion from the strong acid does not hydrolyze.
• The net result: an acidic solution.

Conclusion

Some salts form acidic solutions because they are composed of a cation from a weak base and an anion from a strong acid. In water, the cation undergoes hydrolysis, producing H⁺ ions that increase the solution’s acidity. Examples include ammonium chloride and ammonium sulfate. The pH of these solutions depends on the relative strengths of the parent acid and base, salt concentration, and temperature. Acidic salts are important in laboratories, industry, environmental science, and biological processes for understanding and controlling pH.