Which one of the following oxides shows both acidic and basic behaviour?
Zinc oxide
Understanding Oxide Behaviour: Acidic, Basic, and Amphoteric
Oxides are chemical compounds that contain at least one oxygen atom and one other element in their chemical formula. Based on their chemical properties, oxides can be classified as acidic, basic, neutral, or amphoteric.
- Acidic oxides: These oxides react with bases to form salt and water. They are usually oxides of non-metals (like CO\(_2\), SO\(_2\), P\(_4\)O\(_{10}\)). Some metal oxides in higher oxidation states can also be acidic.
- Basic oxides: These oxides react with acids to form salt and water. They are typically oxides of metals (like Na\(_2\)O, K\(_2\)O, MgO, CaO).
- Neutral oxides: These oxides do not react with either acids or bases (like CO, NO, N\(_2\)O).
- Amphoteric oxides: These are special oxides that exhibit both acidic and basic characteristics. They can react with acids to form salt and water, and they can also react with bases to form salt and water.
Analyzing the Given Options
Let's examine the behaviour of the oxides provided in the options:
- Zinc oxide (ZnO): Zinc is a metal. However, zinc oxide is known to react with both acids and bases. This suggests it has amphoteric properties.
- Copper oxide (CuO): Copper is a metal. Copper oxide is typically a basic oxide, reacting with acids but not significantly with bases.
- Magnesium oxide (MgO): Magnesium is an alkaline earth metal. Magnesium oxide is a strong basic oxide, readily reacting with acids to form salt and water.
- Calcium oxide (CaO): Calcium is an alkaline earth metal. Calcium oxide, also known as quicklime, is a strong basic oxide, reacting vigorously with acids and water.
Zinc Oxide: An Amphoteric Example
To confirm that zinc oxide is amphoteric, we can look at its reactions with an acid and a base.
Reaction with an Acid (e.g., Hydrochloric Acid, HCl):
Zinc oxide reacts with hydrochloric acid to form zinc chloride and water. In this reaction, ZnO behaves like a basic oxide:
\(\text{ZnO(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{O(l)}\)
Reaction with a Base (e.g., Sodium Hydroxide, NaOH):
Zinc oxide reacts with a strong base like sodium hydroxide to form sodium zincate (or tetrahydroxozincate) and water. In this reaction, ZnO behaves like an acidic oxide:
\(\text{ZnO(s)} + 2\text{NaOH(aq)} + \text{H}_2\text{O(l)} \rightarrow \text{Na}_2[\text{Zn(OH)}_4]\text{(aq)}\)
or in a simplified anhydrous form:
\(\text{ZnO(s)} + 2\text{NaOH(aq)} \rightarrow \text{Na}_2\text{ZnO}_2\text{(aq)} + \text{H}_2\text{O(l)}\)
Since zinc oxide reacts with both acids and bases, it demonstrates both basic and acidic characteristics. Therefore, zinc oxide is an amphoteric oxide.
Conclusion
Based on the reactions and properties discussed, zinc oxide is the only oxide among the given options that exhibits both acidic and basic behaviour.
Revision Table: Oxide Behaviour Summary
| Oxide | Metal/Non-metal | Typical Behaviour | Reacts with Acid? | Reacts with Base? | Classification |
|---|---|---|---|---|---|
| Zinc oxide (ZnO) | Metal | Amphoteric | Yes | Yes | Amphoteric |
| Copper oxide (CuO) | Metal | Basic | Yes | No significant reaction | Basic |
| Magnesium oxide (MgO) | Metal | Basic | Yes | No significant reaction | Basic |
| Calcium oxide (CaO) | Metal | Basic | Yes | No significant reaction | Basic |
Additional Information on Amphoteric Oxides
Amphoterism is often observed in oxides of elements that are located near the metalloid region of the periodic table, or metals with variable oxidation states. Besides zinc oxide, other common amphoteric oxides include:
- Aluminum oxide (Al\(_2\)O\(_3\))
- Lead(II) oxide (PbO)
- Tin(II) oxide (SnO)
- Beryllium oxide (BeO)
The amphoteric nature allows these oxides to dissolve in both acidic and alkaline solutions, which is a property utilized in various chemical processes, such as the extraction of aluminum from bauxite.
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