Acids, Bases, and Salts: Principles & Theories

Acids

Acids are substances that release hydrogen ions (H⁺) when dissolved in water.

Properties of Acids

1. Sour taste
2. Turn blue litmus paper red
3. pH value less than 7
4. Conduct electricity in aqueous solution

Examples: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), Acetic acid (CH₃COOH).

Types of Acids

1. Strong Acids: Ionize almost completely in water (e.g., HCl, H₂SO₄, HNO₃).
2. Weak Acids: Ionize partially in water (e.g., CH₃COOH, H₂CO₃).

Chemical Properties of Acids

Reaction with Metals:
Acids react with most metals above hydrogen in the reactivity series to produce a salt and hydrogen gas.

Example: 2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

Reaction with Metal Carbonates and Bicarbonates:
Acids react with metal carbonates and bicarbonates to produce a salt, water, and carbon dioxide gas.

Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Reaction with Metal Oxides (Basic Oxides):
Acids react with basic oxides to form salt and water. This is a neutralization reaction.

Example: 2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l)

Reaction with Bases:
Acids react with bases to form salt and water. This is a neutralization reaction.

Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Reaction with Water:
Acids reacts with water to ionise as given below.

HCl + H₂O → H₃O⁺ + Cl⁻

Bases

Bases are substances that release hydroxide ions (OH⁻) when dissolved in water.

Properties of Bases

1. Bitter taste
2. Slippery feel
3. Turn red litmus paper blue
4. pH value greater than 7

Alkalis are bases that are soluble in water (e.g., NaOH, KOH).

Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Ammonium hydroxide (NH₄OH).

Types of Bases

1. Strong Bases: Ionize almost completely in water (e.g., NaOH, KOH).
2. Weak Bases: Ionize partially in water (e.g., NH₄OH, Ca(OH)₂).

Chemical Properties of Bases

Reaction with Non-metal Oxides (Acidic Oxides):
Bases react with acidic oxides to form salt and water.

Example: 2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l)

Reaction with Acids:
Bases react with acids to form salt and water (neutralization).

Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Reaction with Certain Metal Salts:
Some bases can precipitate insoluble metal hydroxides from salt solutions.

Example: 2NaOH(aq) + FeCl₃(aq) → Fe(OH)₃(s) + 3NaCl(aq)

Reaction with Water:
Bases reacts with water to ionise as given below.

NaOH → Na⁺ + OH⁻

Salts

Salts are ionic compounds formed by the neutralisation reaction between an acid and a base.

Types of Salts

1. Neutral Salts: Formed from strong acid and strong base (e.g., NaCl, KNO₃). pH is approximately 7.
2. Acidic Salts: Formed from strong acid and weak base (e.g., NH₄Cl). pH is less than 7.
3. Basic Salts: Formed from weak acid and strong base (e.g., Na₂CO₃, CH₃COONa). pH is greater than 7.

General Reaction

Acid + Base → Salt + Water

Salts are generally neutral in nature (pH ≈ 7) and do not affect the litmus paper.

Examples: Sodium chloride (NaCl), Potassium nitrate (KNO₃), Calcium carbonate (CaCO₃).

Summary Table: Properties of Acids, Bases, and Salts

Property	Acids	Bases	Salts
Ions in Water	H+	OH−	Cations & Anions
Litmus Effect	Blue → Red	Red → Blue	No effect
pH	< 7	> 7	≈ 7
Taste	Sour	Bitter	Varies
Examples	HCl, H2SO4, CH3COOH	NaOH, KOH, NH4OH	NaCl, KNO3, CaCO3

Acid-Base Theories

There are five major theories that determine the nature of acids and bases. They are given below:

Arrhenius Theory
Brønsted–Lowry Theory
Lewis Theory
Lux–Flood Theory
Hard Soft Acid Base Principle
Cady-Elsey Concept of Acids and Bases