Short Answer

Water (H₂O) is considered amphoteric because it can act both as an acid and as a base depending on the chemical reaction.

• As a base: it reacts with acids by accepting protons.
• As an acid: it reacts with bases by donating protons.
  This dual behavior makes water very versatile in chemical reactions, especially in acid-base chemistry, biological systems, and environmental processes.

Detailed Explanation :

Definition of Amphoteric Behavior

An amphoteric substance is one that can behave as both an acid and a base. Water is a classic example because it can either donate a hydrogen ion (proton) or accept a hydrogen ion. This behavior depends on the other reactant present in the solution.

1. Water Acting as a Base

• When water reacts with a strong acid, it accepts a proton:

• Here, H₂O accepts a proton (H⁺) from hydrochloric acid (HCl) and forms the hydronium ion (H₃O⁺).
• This shows water’s basic behavior, as it neutralizes acids.

Example: In stomach acid, water can act as a base in certain reactions, helping regulate pH.

2. Water Acting as an Acid

• When water reacts with a strong base, it donates a proton:

• In this reaction, water donates a hydrogen ion (proton) to ammonia (NH₃), forming ammonium (NH₄⁺) and hydroxide ions (OH⁻).
• This shows water’s acidic behavior, as it reacts with bases to neutralize them.

Example: In biological systems, water helps balance pH by donating protons to bases.

3. Self-Ionization of Water

• Water can ionize itself, forming H⁺ (or H₃O⁺) and OH⁻ ions:

• This demonstrates water’s ability to act simultaneously as an acid and a base.
• The self-ionization is the basis of the pH scale, showing water’s amphoteric nature.

4. Importance of Water’s Amphoteric Nature

1. In Chemistry
   • Water participates in acid-base reactions, acting either as an acid or a base depending on the reactant.
2. In Biology
   • Maintains pH balance in cells and blood.
   • Essential for enzyme activity and chemical signaling.
3. In Environmental Chemistry
   • Neutralizes acidic or basic pollutants.
   • Stabilizes pH in rivers, lakes, and soil.
4. In Industry
   • Used in chemical synthesis, titrations, and neutralization reactions due to its amphoteric property.

5. Why Water is Special

• Water’s amphoteric behavior is due to its polar structure.
• The oxygen atom has lone pairs that can accept protons.
• The hydrogen atoms can donate protons.
• This makes water a universal solvent and key participant in chemical and biological reactions.

6. Summary

• Water acts as a base with acids and as an acid with bases.
• Examples:
  • With HCl → acts as base → H₃O⁺ formed
  • With NH₃ → acts as acid → NH₄⁺ formed
• Self-ionization of water demonstrates its simultaneous acid-base capability.
• Amphoteric behavior is crucial in chemical, biological, and environmental processes.

Conclusion

Water is considered amphoteric because it can donate and accept protons, depending on the reaction. This dual nature allows water to neutralize acids and bases, maintain pH balance, and support life processes. Understanding water’s amphoteric property is essential in chemistry, biology, environmental science, and industry, making it one of the most important and versatile substances on Earth.