Short Answer

The Arrhenius theory of acids and bases explains how acids and bases behave in water. According to this theory, an acid is a substance that produces hydrogen ions (H⁺) in water, while a base is a substance that produces hydroxide ions (OH⁻) in water. This release of ions helps identify whether a substance is acidic or basic.

The theory focuses only on aqueous (water) solutions. It helps us understand why acids turn blue litmus red and why bases turn red litmus blue. Though later theories expanded these ideas, the Arrhenius theory is still important because it was one of the first scientific explanations of acids and bases.

Detailed Explanation :

Arrhenius Theory of Acids and Bases

The Arrhenius theory was proposed by the Swedish scientist Svante Arrhenius in 1887. It was one of the earliest and simplest scientific explanations used to describe the behaviour of acids and bases. Although the theory has limitations, it formed the foundation for modern chemistry by explaining how substances ionise in water and how their acidic or basic nature depends on the ions produced.

Arrhenius introduced this idea while studying the electrical conductivity of solutions. He noticed that substances behave differently when dissolved in water because they split into ions. This understanding helped explain why acids and bases show unique properties in aqueous solutions.

Definition of Acids According to Arrhenius

According to Arrhenius, an acid is a substance that produces hydrogen ions (H⁺ ions) when dissolved in water. These hydrogen ions are responsible for the acidic nature of the solution. For example:

• Hydrochloric acid (HCl) → H⁺ + Cl⁻
• Nitric acid (HNO₃) → H⁺ + NO₃⁻

Because acids release hydrogen ions, they taste sour, change blue litmus to red, and react with metals to form hydrogen gas.

However, hydrogen ions (H⁺) cannot remain free in water. They immediately combine with water molecules to form hydronium ions (H₃O⁺). But for simplicity, Arrhenius theory uses H⁺ to explain acidity.

Definition of Bases According to Arrhenius

According to Arrhenius, a base is a substance that produces hydroxide ions (OH⁻ ions) when dissolved in water. These hydroxide ions are responsible for the basic nature of the solution. For example:

• Sodium hydroxide (NaOH) → Na⁺ + OH⁻
• Potassium hydroxide (KOH) → K⁺ + OH⁻

Because bases release hydroxide ions, they feel slippery, taste bitter, and turn red litmus blue. They also neutralise acids to form salt and water.

Why Arrhenius Theory Was Important

The Arrhenius theory was extremely useful because:

1. It clearly explained how acids and bases ionise in water.
2. It linked acidity and basicity to the presence of H⁺ and OH⁻ ions.
3. It helped explain why acids and bases conduct electricity.
4. It introduced the idea of ionisation, which became central in modern chemistry.

Before this theory, acids and bases were identified only by taste or simple reactions. Arrhenius gave a scientific explanation that could be measured and tested.

Neutralisation According to Arrhenius

One of the strengths of the Arrhenius theory is its explanation of neutralisation.
According to the theory:

Acid + Base → Salt + Water

The reaction occurs because hydrogen ions from acids combine with hydroxide ions from bases to form water (H₂O). For example:

HCl + NaOH → NaCl + H₂O
Here, H⁺ from HCl reacts with OH⁻ from NaOH to form water. This explains why the solution becomes neutral after the reaction.

Limitations of Arrhenius Theory

Even though the Arrhenius theory was useful, it has some limitations:

1. Applies only to water solutions:
   Substances that act as acids or bases in non-water solvents cannot be explained using this theory.
2. Cannot explain all bases:
   Some bases, like ammonia (NH₃), do not contain OH⁻ ions but still behave like bases. Arrhenius theory cannot explain this.
3. Narrow definitions:
   The theory only focuses on the formation of H⁺ and OH⁻ ions and does not consider proton transfer or other chemical behaviours.

Because of these limitations, new theories such as the Bronsted–Lowry theory and Lewis theory were developed later.

Examples Explained Using Arrhenius Theory

• Hydrochloric acid produces H⁺ ions, so it is an Arrhenius acid.
• Sodium hydroxide produces OH⁻ ions, so it is an Arrhenius base.
• Ammonia, however, does not produce OH⁻ directly, so Arrhenius theory does not classify it as a base, even though it behaves like one.

This shows both the usefulness and limitations of the theory.

Conclusion

The Arrhenius theory of acids and bases states that acids produce hydrogen ions in water and bases produce hydroxide ions in water. This theory helped scientists understand why acids and bases show certain properties, how they ionise in water, and how neutralisation occurs. Although later theories expanded these ideas, the Arrhenius theory remains an important foundation in chemistry because it was the first clear scientific explanation of acid and base behaviour.