Short Answer

An amphoteric substance is a chemical that can act both as an acid and as a base depending on the reaction conditions.

For example, water (H₂O), aluminum hydroxide (Al(OH)₃), and zinc oxide (ZnO) are amphoteric. Such substances can react with acids to form salts and with bases to form complex compounds, making them versatile in chemical reactions.

Detailed Explanation :

Definition of Amphoteric Substance

An amphoteric substance is a compound or ion that exhibits dual behavior — it can donate protons (act as an acid) or accept protons (act as a base). This depends on the nature of the other reactant it is reacting with.

• Acidic medium: It behaves as a base.
• Basic medium: It behaves as an acid.

Amphoteric behavior is commonly observed in metal oxides and hydroxides, as well as certain non-metallic compounds.

1. Examples of Amphoteric Substances

1. Aluminum Hydroxide (Al(OH)₃)
   • Reacts with acids:

Acts as a base.

1. • Reacts with bases:

Acts as an acid.

1. Zinc Oxide (ZnO)
   • Reacts with acids:

1. • Reacts with bases:

1. Water (H₂O)
   • Acts as a base with HCl:

1. • Acts as an acid with NH₃:

1. Lead(II) Oxide (PbO) and Chromium Hydroxide (Cr(OH)₃)
   • Show similar amphoteric properties in reactions with acids and bases.

2. Mechanism of Amphoteric Behavior

• The dual nature arises because the substance contains hydroxyl groups (–OH) or oxide ions (O²⁻).
• In acidic solutions, these groups accept protons, acting as a base.
• In basic solutions, they donate protons, acting as an acid.

This flexibility allows amphoteric substances to neutralize both acids and bases.

3. Importance of Amphoteric Substances

1. Chemical Reactions
   • Used to neutralize acids and bases in laboratory experiments.
   • Forms complex salts that are useful in chemical synthesis.
2. Industrial Applications
   • Al(OH)₃ is used in water purification and antacid tablets.
   • ZnO is used in rubber and paint industries due to its chemical stability.
3. Environmental Chemistry
   • Amphoteric compounds can buffer acidic or basic wastes, minimizing environmental harm.
4. Analytical Chemistry
   • Useful in precipitation and titration reactions because of their dual behavior.

4. Distinction from Other Substances

• Acidic substances only donate protons.
• Basic substances only accept protons.
• Amphoteric substances can do both, making them unique in chemistry.

This dual nature depends on the reactive partner, solvent, and reaction conditions.

5. Summary

• Amphoteric substances can behave as acids or bases.
• Common examples: Al(OH)₃, ZnO, H₂O, PbO.
• Their behavior depends on acidic or basic environment.
• Important for chemical synthesis, industrial applications, environmental management, and laboratory processes.

Conclusion

An amphoteric substance is a versatile chemical that responds to its environment, acting as either an acid or a base. This property allows it to neutralize acids and bases, form complex salts, and be used in industrial, environmental, and laboratory applications. Understanding amphoteric behavior is essential in predicting chemical reactions and designing effective chemical processes.