Acid Rain
Acid rain refers to any form of precipitation — rain, snow, fog, or dry deposition — with a pH lower than 5.6, resulting from the dissolution of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) in atmospheric moisture to form sulphuric and nitric acids. These pollutants are primarily emitted by coal-burning power plants, vehicle exhausts, and industrial smelters, and can travel hundreds of kilometres before depositing. Acid rain damages forests, acidifies lakes and soils, corrodes buildings and infrastructure, and harms aquatic biodiversity.
Key Formula
SO2 + H2O → H2SO3 (sulphurous acid); 2H2SO3 + O2 → 2H2SO4 (sulphuric acid)
LaTeX: SO_2 + H_2O \rightarrow H_2SO_3; \quad 2H_2SO_3 + O_2 \rightarrow 2H_2SO_4
| Symbol | Meaning | Unit |
|---|---|---|
| SO₂ | Sulphur dioxide (primary pollutant) | ppm in atmosphere |
| NOₓ | Nitrogen oxides (NO + NO₂) | ppm in atmosphere |
| H₂SO₄ | Sulphuric acid (main acidifying agent) | mol/L in rainwater |
| HNO₃ | Nitric acid (secondary acidifying agent) | mol/L in rainwater |
Worked Example
Problem
Rainwater has a hydrogen ion concentration of [H⁺] = 2.5 × 10⁻⁴ mol/L. Calculate the pH and determine if it qualifies as acid rain.
Solution
Step 1: pH = -log₁₀[H⁺]. Step 2: pH = -log₁₀(2.5 × 10⁻⁴). Step 3: pH = -(log 2.5 + log 10⁻⁴) = -(0.398 − 4) = -(-3.602). Step 4: pH = 3.60.
Answer
pH = 3.60, which is well below 5.6 — this is highly acidic rain and constitutes severe acid rain.
Effects of Acid Rain at Different pH Levels
| pH Range | Classification | Effect on Aquatic Life | Effect on Soil/Vegetation |
|---|---|---|---|
| 5.6–7.0 | Normal rain | No effect | No significant effect |
| 5.0–5.6 | Mildly acidic | Sensitive species affected | Slight nutrient leaching |
| 4.5–5.0 | Moderately acidic | Fish reproduction impaired | Aluminium mobilisation begins |
| 4.0–4.5 | Strongly acidic | Most fish eliminated | Forest dieback begins |
| < 4.0 | Severely acidic | Aquatic dead zones | Severe crop and soil damage |
Interactive Tools
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Related Terms
Eutrophication
Eutrophication is the process by which a water body becomes overly enriched with nutrients — primarily nitrogen and phosphorus — leading to excessive growth of algae and aquatic plants, depleting dissolved oxygen, and causing the death of fish and other aquatic organisms. It most commonly results from agricultural runoff, sewage discharge, and industrial effluents that introduce nutrients into lakes, rivers, estuaries, and coastal zones. The subsequent algal blooms block sunlight, and when the algae die and decompose, microbial respiration consumes oxygen, creating hypoxic or anoxic "dead zones."
Ozone Depletion
Ozone depletion is the thinning of the stratospheric ozone layer (located 15–35 km above Earth's surface) caused by catalytic destruction of ozone (O₃) molecules by anthropogenic chemicals, particularly chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs), and methyl bromide. The ozone layer shields life on Earth from harmful ultraviolet-B (UV-B) radiation; its depletion increases risks of skin cancer, cataracts, immune suppression, and damage to phytoplankton. The Montreal Protocol (1987) successfully phased out most ozone-depleting substances, and the ozone layer is gradually recovering.
Carbon Footprint
A carbon footprint is the total amount of greenhouse gases, primarily carbon dioxide and methane, emitted directly or indirectly by an individual, organisation, event, or product, expressed as CO₂ equivalent (CO₂e). It is a widely used metric to quantify human contribution to climate change, encompassing energy use, transportation, food consumption, and manufacturing processes. Reducing carbon footprints is central to climate mitigation strategies outlined in international agreements such as the Paris Accord.
The term "acid rain" was coined by Scottish chemist Robert Angus Smith in 1872 in his book "Air and Rain: The Beginnings of a Chemical Climatology," describing acidic precipitation near industrial Manchester. The phenomenon gained global attention in the 1960s–1970s when Scandinavian scientists linked it to long-range transport of SO₂ from UK industries.