Which one of the following is an amphoteric oxide?
Al2O3
The oxide that exhibits amphoteric behavior among the given options is Al₂O₃ (aluminium oxide). Amphoteric oxides are unique because they can react both with acids and bases, a property that places them between acidic and basic oxides in terms of chemical reactivity. This dual character is typically exhibited by metal oxides of metalloids or elements close to the metal–nonmetal boundary in the periodic table.
🔬 Aluminium Oxide (Al₂O₃): An Amphoteric Oxide
Aluminium, a post-transition metal, forms Al₂O₃, a white, crystalline, high-melting-point solid. It is insoluble in water but reacts with both acids and bases, showing amphoteric properties.
Reactions Demonstrating Amphoteric Behavior
With Acids (acts as a base):
\(\mathrm{Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O}\)
- Aluminium oxide neutralizes hydrochloric acid to form aluminium chloride and water.
With Bases (acts as an acid):
\(\mathrm{Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]}\)
- In this reaction, aluminium oxide reacts with sodium hydroxide (a base) to form sodium aluminate, indicating acidic behavior.
This capacity to behave both as an acid and as a base makes Al₂O₃ a classic example of an amphoteric oxide.
⚖️ Comparative Analysis of Given Options
To further clarify why only Al₂O₃ is amphoteric, here's an analysis of each oxide:
| Oxide | Chemical Name | Type of Oxide | Explanation |
|---|---|---|---|
| MgO | Magnesium oxide | Basic oxide | Formed from a Group 2 metal. Reacts with acids but not with bases. |
| P₄O₁₀ | Tetraphosphorus decoxide | Acidic oxide | A non-metal oxide. Reacts with bases to form salts but not with acids. |
| Na₂O | Sodium oxide | Strongly basic oxide | An alkali metal oxide. Reacts violently with water and acids, but not with bases. |
| Al₂O₃ | Aluminium oxide | ✅ Amphoteric | Reacts with both acids and bases — classic amphoteric behavior. |
⚗️ Summary of Oxide Behaviors
| Oxide | Metal / Non-metal Origin | Water Solubility | Reacts with Acids | Reacts with Bases | Oxide Nature |
|---|---|---|---|---|---|
| MgO | Metal (Group 2) | Slightly soluble | ✅ Yes | ❌ No | Basic |
| P₄O₁₀ | Non-metal (Group 15) | Reacts to form acid | ❌ No | ✅ Yes | Acidic |
| Na₂O | Metal (Group 1) | Reacts to form NaOH | ✅ Yes | ❌ No | Strongly basic |
| Al₂O₃ | Metal (borderline) | Insoluble | ✅ Yes | ✅ Yes | Amphoteric |
🌍 Real-World Relevance of Al₂O₃
Aluminium oxide has important industrial and environmental applications due to its amphoteric nature:
- Used in water purification systems as an adsorbent.
- Serves as a catalyst support in chemical reactions.
- In metallurgy, helps extract pure aluminium through the Hall–Héroult process.
- Forms a protective oxide coating on aluminium, preventing corrosion.
✅ Conclusion
Al₂O₃ (aluminium oxide) is the only compound among the given options that qualifies as an amphoteric oxide, because it reacts both with acids and bases. This behavior reflects aluminium's chemical position near the metalloid line in the periodic table and makes Al₂O₃ a scientifically and industrially important compound.
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