1. Acidic Oxides
Definition: Oxides that react with water to form acids or react with bases to give salt + water.
Examples:
- CO₂, SO₂, SO₃, P₂O₅, N₂O₅, Cl₂O₇
- Non-metal oxides
Reactions:
- CO₂ + H₂O → H₂CO₃
- SO₃ + 2NaOH → Na₂SO₄ + H₂O
Trend:
- Non-metallic character ↑ ⇒ acidity ↑
- Across a period: acidity increases
- Down a group: acidity decreases
2. Basic Oxides
Definition: Oxides that react with water to form bases or react with acids to give salt + water.
Examples:
- Na₂O, CaO, MgO, K₂O
- Mostly metal oxides
Reactions:
- Na₂O + H₂O → 2NaOH
- CaO + 2HCl → CaCl₂ + H₂O
Trend:
- Metallic character ↑ ⇒ basicity ↑
- Down a group: basicity increases
- Across a period: basicity decreases
3. Amphoteric Oxides
Definition: Oxides that react with both acids and bases.
Examples (VERY IMPORTANT for exams):
- Al₂O₃, ZnO, BeO, SnO, PbO, Cr₂O₃
Reactions:
- ZnO + 2HCl → ZnCl₂ + H₂O (acts basic)
- ZnO + 2NaOH → Na₂ZnO₂ + H₂O (acts acidic)
Trend:
- Found near metal–nonmetal boundary
- Small size + high charge density → amphoteric behavior
4. Neutral Oxides
Definition: Oxides that do not react with acids or bases.
Examples:
- CO, NO, N₂O
Trend:
- Mostly simple molecules
- No acidic or basic nature
IMPORTANT PERIODIC TREND (MOST ASKED)
Across a period (Left → Right):
Basic → Amphoteric → Acidic
Example (Period 3):
Na₂O → MgO → Al₂O₃ → SiO₂ → P₂O₅ → SO₃ → Cl₂O₇
Key shift:
- Metals → Non-metals
- Basic → Acidic
QUICK REVISION TABLE
| Type | Nature | Examples |
|---|---|---|
| Basic | Metal oxides | Na₂O, CaO |
| Amphoteric | Both | Al₂O₃, ZnO |
| Acidic | Non-metal oxides | SO₃, CO₂ |
| Neutral | No reaction | CO, NO |
EXAM TIPS
- Al₂O₃, ZnO → always remember (amphoteric)
- CO ≠ acidic (neutral!)
- Higher oxidation state ⇒ more acidic oxide
- Example: SO₂ < SO₃ (acidity increases)
MCQs
Q1. Arrange in increasing acidic nature:
A. CO₂ < SiO₂ < SO₃
B. SiO₂ < CO₂ < SO₃
C. SO₃ < CO₂ < SiO₂
D. CO₂ < SO₃ < SiO₂
Q2. Arrange in decreasing basic strength:
A. Na₂O > MgO > Al₂O₃
B. Al₂O₃ > MgO > Na₂O
C. MgO > Na₂O > Al₂O₃
D. Na₂O > Al₂O₃ > MgO
Q3. Arrange in increasing amphoteric character:
A. Na₂O < MgO < Al₂O₃
B. Al₂O₃ < MgO < Na₂O
C. MgO < Na₂O < Al₂O₃
D. Na₂O < Al₂O₃ < MgO
Q4. Arrange in increasing acidic strength:
A. SO₂ < SO₃ < Cl₂O₇
B. Cl₂O₇ < SO₃ < SO₂
C. SO₃ < SO₂ < Cl₂O₇
D. SO₂ < Cl₂O₇ < SO₃
Q5. Arrange in increasing basic character:
A. CaO < MgO < Na₂O
B. Na₂O < MgO < CaO
C. MgO < CaO < Na₂O
D. CaO < Na₂O < MgO
Q6. Arrange in increasing acidic nature:
A. P₂O₅ < SO₃ < Cl₂O₇
B. Cl₂O₇ < SO₃ < P₂O₅
C. SO₃ < P₂O₅ < Cl₂O₇
D. P₂O₅ < Cl₂O₇ < SO₃
Q7. Arrange in increasing basic strength:
A. BeO < MgO < CaO
B. CaO < MgO < BeO
C. MgO < BeO < CaO
D. BeO < CaO < MgO
Q8. Arrange in increasing acidic character:
A. N₂O₃ < NO₂ < N₂O₅
B. N₂O₅ < NO₂ < N₂O₃
C. NO₂ < N₂O₃ < N₂O₅
D. N₂O₃ < N₂O₅ < NO₂
ANSWERS
- B → SiO₂ < CO₂ < SO₃
- A → Na₂O > MgO > Al₂O₃
- A → Na₂O < MgO < Al₂O₃
- A → SO₂ < SO₃ < Cl₂O₇
- C → MgO < CaO < Na₂O
- A → P₂O₅ < SO₃ < Cl₂O₇
- A → BeO < MgO < CaO
- A → N₂O₃ < NO₂ < N₂O₅
SHORT TRICKS (VERY IMPORTANT)
- Across period: Basic ↓, Acidic ↑
- Down group: Basic ↑
- Higher oxidation state ⇒ more acidic
- Metal oxides → basic
- Non-metal oxides → acidic
- Border elements (Be, Al, Zn) → amphoteric
Sarvan sir- Chemistry For All 