Question

The correct order of acidic strength is:
1. SO₂ < P₂O₃ < SiO₂ < Al₂O₃
2. SiO₂ < SO₂ < Al₂O₃ < P₂O₃ 
3.  Al₂O₃ < SiO₂ < SO₂ < P₂O₃
4.  Al₂O₃ < SiO₂  < P₂O₃ < SO₂ 

Answer 1

Correct Answer - Option 4 :  Al₂O₃ < SiO₂  < P₂O₃ < SO₂ 

Concept:

• Oxides are compounds formed by the reaction of metals, non-metals with oxygen. The types of oxides are:
• Basic oxide: It is a complex chemical substance oxide, which forms a salt with a chemical reaction with acids or acidic oxides and does not react with bases or basic oxides.
• E.g. Copper oxide (Cu2O), Magnesium oxide (MgO)
• Acidic oxide: It is a complex chemical substance oxide, which forms a salt with the chemical reactions with bases or basic oxides and does not react with acidic oxides.
• E.g. Carbon dioxide (CO2), Silicon dioxide (SiO2)
• Amphoteric oxide: It is a complex chemical substance, also oxides, which form a salt with the chemical reactions with acids (or acid oxides) and with bases (or basic oxides).
• E.g. Copper, Zinc, Tin, Lead, Aluminum, and Beryllium.

​
Explanation:

• Nature of oxides of the elements:
  • Metals form basic oxides and Non-metals form acidic oxides.
  • As we move from left to right across a period, the metallic character decreases and the non-metallic character increases.
  • Hence, oxides become more acidic as we move from left to right across a period.
  • On the other hand, as we move down along a group, the metallic character increases and the non-metallic character decreases.
  • Hence, moving down a group, the basic character of oxides increases and the acidic character of the oxides decreases.
• Aluminium, Silicon, phosphorus and sulphur are elements of period 3.
• As we move from Aluminium to Sulphur, the non-metallic character increases.
• Thus, the acid character of oxides also increases, hence SO₂ is the most acidic and Al₂O₃ is the least acidic.

Hence, the correct order if acidity is  Al2O3 < SiO2  < P2O3 < SO2 ​.