Acids, Bases, and Salts: A Comprehensive Guide

1. Introduction to Acids and Bases

Acids and bases are fundamental substances in chemistry. According to the Arrhenius theory, an acid is a substance that produces hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in water. A base is a substance that produces hydroxide ions (OH^-) in water.

The Brønsted-Lowry theory provides a broader definition: an acid is a proton donor, and a base is a proton acceptor.

2. General Characteristics

• Acids: Sour taste, turn blue litmus paper red, have a pH less than 7, and react with metals to liberate hydrogen gas.
• Bases: Bitter taste, feel slippery/soapy, turn red litmus paper blue, and have a pH greater than 7.

3. Basicity of Acids

Basicity refers to the number of replaceable hydrogen atoms in one molecule of an acid. For example:

• Monobasic: HCl, HNO₃, CH₃COOH (1 replaceable H).
• Dibasic: H₂SO₄, H₂CO₃ (2 replaceable H).
• Tribasic: H₃PO₄ (3 replaceable H).

4. Salts

A salt is formed when the replaceable hydrogen ion of an acid is replaced by a metal or ammonium ion.

Types of Salts

• Normal Salts: Formed by complete replacement of H+ ions (e.g., NaCl, K₂SO₄).
• Acid Salts: Formed by partial replacement of H+ ions (e.g., NaHSO₄, NaHCO₃).
• Basic Salts: Contain hydroxide ions (e.g., Mg(OH)Cl).
• Double Salts: Two simple salts crystallized together (e.g., Alums like KAl(SO₄)₂·12H₂O).

5. pH and pOH Calculations

The pH is the negative logarithm of the hydrogen ion concentration: pH = -log[H⁺]. Similarly, pOH = -log[OH^-]. In any aqueous solution at 25°C, pH + pOH = 14.

6. Acid-Base Titration

This is a volumetric analysis technique used to determine the concentration of an unknown solution. The equivalence point is when the acid and base have reacted in stoichiometric proportions. An indicator is used to signal the end point of the titration through a color change.

7. Hydrolysis of Salts

Salt hydrolysis is the reaction of a salt with water to produce an acidic or basic solution.

• NH₄Cl: Hydrolyzes to form an acidic solution (Strong acid + Weak base).
• Na₂CO₃: Hydrolyzes to form a basic solution (Weak acid + Strong base).
• NaCl: Does not hydrolyze; solution remains neutral (Strong acid + Strong base).