Classification of Acids according to its strength (degree of ionization), Its source and Basicity

Acids can be classified according to their strength (degree of ionization), their source, and Basicity. Acids are classified according to their degree of ionization in aqueous solution into Strong acids and weak acids.

What Are Acids?

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They have a sour taste and a pH less than 7.

Common Examples:

• Hydrochloric acid (HCl) is found in stomach acid.
• Sulfuric acid (H₂SO₄) is used in batteries.
• Acetic acid (CH₃COOH) is found in vinegar.
• Citric acid is found in citrus fruits.

Properties of Acids

• Acids have a sour taste (don’t go taste-testing though).
• Acids are corrosive in strong concentrations.
• Acids turn blue litmus paper red.
• Acids react with metals to produce hydrogen gas.
• Acids react with bases to form salt + water (neutralization).

Classification of acids according to their strength (degree of ionization)

Strong acids

The acids that are completely ionized in the water, Their aqueous solution conducts the electric current to a large degree. They are considered as strong electrolytes.

Examples: Hydroiodic acid HI, Hydrochloric acid HCl, Hydrobromic acid HBr, Sulphuric acid H₂SO₄, Nitric acid HNO₃ and Perchloric acid HClO₄.

Weak acids

The acids that are incompletely ionized in the water, Their aqueous solution conducts the electric current to a small degree. They are considered as weak electrolytes.

Examples: Carbonic acid H₂CO₃, Phosphoric acid H₃PO₄, Acetic acid (vinegar) CH₃COOH, Formic acid, Citric acid, Oxalic acid, and Lactic acid.

There is no relation between the strength of the acid and the number of hydrogen atoms in its molecular structure. Phosphoric acid (H₃PO₄) is weaker than nitric acid (HNO₃), although it contains a greater number of hydrogen atoms.

CH₃COOH ( aq ) + H₂ O ( l ) ↔ CH₃COO⁻ ( aq ) + H₃ O⁺ (aq)

Hydrochloric acid, HCl, is stronger than acetic acid because HCl is completely ionized in the water, but acetic acid is partially ionized in the water.

HCl (aq) + H₂ O (l) → Cl⁻ (aq) + H₃ O⁺ (aq)

Classification of acids according to their sources

Acids are classified according to their sources (origin) into Organic acids and Mineral acids

Organic acids

Acids that have an organic origin (plant or animal), They are extracted from the organs of living organisms. All of them are weak acids.

Examples: Lactic acid (milk products), Acetic acid (vinegar), Citric acid (from lemon), Oxalic acid, and Formic acid.

Mineral acids

Acids that have no organic origin, They usually have non-metallic elements in their structure, like chlorine, sulphur, nitrogen, and phosphorus. Some of them are strong acids, and others are weak.

Examples: Carbonic acid H₂CO₃, Hydrochloric acid HCl, Phosphoric acid H₃PO₄, Perchloric acid HClO₄, Nitric acid HNO₃, and Sulphuric acid H₂SO₄.

Classification of acids according to basicity

The basicity of an acid 

It is the number of hydrogen ions (H⁺), that is produced by one molecule of the acid when it dissolves in the water.

Acids are classified according to their basicity into Monobasic acids (Monoprotic), Dibasic acids (Diprotic), and Tribasic acids (Triprotic).

Monobasic acids (Monoprotic)

Acids where each molecule give one proton H⁺ when it dissolves in water.

Examples: Organic monobasic acids (Formic acid HCOOH, Acetic acid CH₃COOH), Mineral monobasic acids (Hydrochloric acid HCl, Nitric acid HNO₃).

Dibasic acids (Diprotic)

Acids where each molecule give one or two protons H⁺, when it dissolves in the water.

Examples: Organic diabasic acids (Oxalic acid), Mineral dibasic acids (Sulphuric acid H₂SO₄, Carbonic acid H₂CO₃).

Tribasic acids (Triprotic)

Acids where each molecule give one, two, or three protons H⁺, when it dissolves in water.

Examples: Organic tribasic acids (Citric acid), Mineral tribasic acids (Phosphoric acid, H₃PO₄).

Both citric acid and phosphoric acid have the same degree of basicity, while they differ in their source (origin), because both of them are tribasic acids, but citric acid is an organic acid, while phosphoric acid is a mineral acid.

Classifications of bases according to strength ( degree of ionization ) and molecular structure

Properties of Acids and Bases & Theories defining acids and bases

Types of compounds, Properties of Acids, Bases (alkalis), Oxides and Salts