Acid Rain and Its Causes

Introduction

• Burning of fossil fuels leads to acidification of rain.
• Two main sources of acid:
  • Sulfuric acid from burning coal and oil (power stations).
  • Nitric acid from motor vehicle exhaust.

Natural Rain Acidity

• Rain is naturally slightly acidic due to dissolved carbon dioxide.
• Forms carbonic acid, resulting in a natural pH of 6.
• Acid rain can have a pH as low as 3.

Acidity Increase Calculation

• pH change from 6 to 3 = 10 x 10 x 10 = 1000 times more acidic.

Sulfuric Acid Formation

• Plants need sulfur, absorbed from sulfates in soil.
• Upon dying, sulfur bonds to carbon in coal/oil (1% to 4% sulfur by weight).
• Burning fossil fuels releases sulfur as sulfur dioxide (SO2) and carbon dioxide (CO2).
• In sunlight, SO2 converts to sulfur trioxide (SO3).
• SO3 dissolves in rain forming sulfuric acid (H2SO4).

Nitric Acid Formation

• High temperatures in vehicle cylinders allow nitrogen and oxygen to react (air is 80% nitrogen).
• Vehicle exhaust contains nitrogen oxides.
• Nitrogen oxides react with oxygen and dissolve in rain forming nitric acid (HNO3).

Effects of Acid Rain

• Potential neutralization of acidity if rain falls on alkaline limestone soils.
• Harmful effects on neutral or acidic soils and vegetation.

Impact on Living Things

• Acidity affects growth:
  • Example: Bread soaked in vinegar (acid) vs. water (neutral).
  • Vinegar preserves food by inhibiting enzyme function in acidic conditions.
• Soil impact:
  • Toxic metals like aluminium become soluble in acidic soils.
  • Soluble metals replace essential nutrients like zinc and iron.
  • Hydrogen ions in acid rain wash away essential metals (K, Mg, Ca) from plant roots.

Summary

• Normal rain: slightly acidic (pH 6) due to CO2.
• Acid rain: contains sulfuric and nitric acid, making it 1000 times more acidic.
• Sources:
  • Sulfuric acid from sulfur in fossil fuels.
  • Nitric acid from combustion in vehicle engines.