What is the primary chemical difference between normal rain and acid rain?

Normal rain is slightly acidic (pH $approx$ 5.6) due to dissolved atmospheric carbon dioxide forming carbonic acid. Acid rain, however, has a much lower pH (typically below 5.0) because it contains strong acids like sulfuric acid ($ ext{H}_2 ext{SO}_4$) and nitric acid ($ ext{HNO}_3$), which are formed from the reaction of anthropogenic sulfur dioxide ($ ext{SO}_2$) and nitrogen oxides ($ ext{NO}_x$) with atmospheric water and oxidants. The presence of these strong acids is the key differentiator.

How do chlorofluorocarbons (CFCs) destroy ozone molecules in the stratosphere?

CFCs are very stable in the lower atmosphere but break down in the stratosphere under intense UV radiation, releasing highly reactive chlorine free radicals ($ ext{Cl} cdot$). These chlorine radicals then catalytically destroy ozone ($ ext{O}_3$) molecules. A single chlorine atom reacts with an ozone molecule to form chlorine monoxide ($ ext{ClO} cdot$) and oxygen ($ ext{O}_2$). The $ ext{ClO} cdot$ then reacts with a free oxygen atom ($ ext{O}$) to regenerate the chlorine radical ($ ext{Cl} cdot$), allowing it to destroy many more ozone molecules in a chain reaction.

What are Polar Stratospheric Clouds (PSCs) and their role in ozone depletion?

Polar Stratospheric Clouds (PSCs) are clouds that form in the extremely cold conditions of the polar stratosphere (below $-78^circ ext{C}$). They provide surfaces for heterogeneous chemical reactions that convert relatively unreactive chlorine reservoir species (like $ ext{HCl}$ and $ ext{ClONO}_2$) into more reactive forms such as molecular chlorine ($ ext{Cl}_2$) and hypochlorous acid ($ ext{HOCl}$). When sunlight returns in spring, these reactive chlorine compounds are photolyzed, releasing large quantities of active chlorine radicals that rapidly deplete ozone, leading to the 'ozone hole'.

Besides environmental damage, what are the direct health impacts of increased UV-B radiation due to ozone depletion?

Increased exposure to UV-B radiation due to ozone depletion has several direct health impacts. It significantly raises the risk of various skin cancers, including melanoma and non-melanoma types. It also contributes to the development of cataracts, a clouding of the eye's lens that impairs vision. Furthermore, UV-B radiation can suppress the human immune system, making individuals more susceptible to infections and reducing the effectiveness of vaccinations.

What is the Montreal Protocol and why is it significant?

The Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987, is an international treaty designed to protect the ozone layer by phasing out the production and consumption of numerous substances responsible for ozone depletion, primarily CFCs and halons. It is considered one of the most successful international environmental agreements, demonstrating that global cooperation can effectively address major environmental challenges. Its implementation has led to a significant reduction in ODS emissions, and the ozone layer is now showing signs of recovery.

How does acid rain affect historical monuments made of marble?

Historical monuments made of marble, which is primarily calcium carbonate ($ ext{CaCO}_3$), are particularly vulnerable to acid rain. The sulfuric acid ($ ext{H}_2 ext{SO}_4$) present in acid rain reacts with calcium carbonate to form calcium sulfate ($ ext{CaSO}_4$), water, and carbon dioxide. Calcium sulfate is more soluble in water than calcium carbonate and is easily washed away by subsequent rain, leading to erosion, discoloration, and loss of intricate details on the monument's surface, a phenomenon often referred to as 'stone leprosy'.

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Acid Rain and Ozone Layer Depletion

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Acid rain refers to any form of precipitation with acidic components, such as sulfuric or nitric acid, that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail, or even dust that is acidic. While natural rain is slightly acidic due to dissolved carbon dioxide, acid rain has a significantly lower pH, typically below 5.6, primarily caused by the emissio…

Quick Summary

Acid rain is precipitation with a pH below 5.6, primarily caused by the emission of sulfur dioxide ( $ext{SO}_2$ ) and nitrogen oxides ( $ext{NO}_x$ ) from burning fossil fuels. These gases react in the atmosphere to form sulfuric acid ( $ext{H}_2\text{SO}_4$ ) and nitric acid ( $ext{HNO}_3$ ).

Its effects include corrosion of buildings, acidification of aquatic ecosystems, damage to forests, and soil degradation. Ozone layer depletion refers to the thinning of the stratospheric ozone layer, which protects Earth from harmful UV radiation.

This depletion is mainly caused by human-made chemicals like chlorofluorocarbons (CFCs). CFCs release chlorine radicals in the stratosphere, which catalytically destroy ozone molecules. The 'ozone hole' over Antarctica is a prominent example, exacerbated by Polar Stratospheric Clouds (PSCs).

Consequences include increased skin cancer, cataracts, and harm to ecosystems due to higher UV-B exposure. International efforts like the Montreal Protocol aim to phase out ozone-depleting substances.

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Key Concepts

Formation of Sulfuric Acid in Acid Rain

Sulfur dioxide ( $ext{SO}_2$ ), released from burning sulfur-containing fossil fuels, undergoes oxidation in…

Chlorine Radical Catalytic Cycle in Ozone Depletion

Chlorofluorocarbons (CFCs) are photolyzed by UV radiation in the stratosphere, releasing a chlorine radical…

Impact of Acid Rain on Marble Structures

Marble is predominantly calcium carbonate ( $ext{CaCO}_3$ ). When acid rain, containing sulfuric acid ($…

Acid Rain pH —

< 5.6

(Normal rain

approx 5.6

due to

ext{CO}_2

)

Acid Rain Precursors —

ext{SO}_2

(from S-containing fossil fuels),

ext{NO}_x

(from high-temp combustion)

Acid Rain Reactions

- $ext{SO}_2 xrightarrow{\text{Oxidation}} \text{SO}_3 xrightarrow{\text{H}_2\text{O}} \text{H}_2\text{SO}_4$ - $ext{NO}_x xrightarrow{\text{Oxidation}} \text{NO}_2 xrightarrow{\text{OH} cdot, \text{H}_2\text{O}} \text{HNO}_3$ - Marble erosion: $ext{CaCO}_3(\text{s}) + \text{H}_2\text{SO}_4(\text{aq}) \to \text{CaSO}_4(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$

Ozone Layer — Stratosphere (10-50 km), absorbs UV-B/C.

Ozone Formation (Chapman) —

ext{O}_2 + \text{UV} \to 2\text{O}

;

ext{O} + \text{O}_2 + \text{M} \to \text{O}_3 + \text{M}

Ozone Depleting Substances (ODS) — CFCs, Halons,

ext{CCl}_4

ext{CH}_3\text{Br}

(contain

ext{Cl}

ext{Br}

)

Ozone Depletion Catalytic Cycle

- $ext{CF}_2\text{Cl}_2 + \text{UV} \to \text{CF}_2\text{Cl} cdot + \text{Cl} cdot$ - $ext{Cl} cdot + \text{O}_3 \to \text{ClO} cdot + \text{O}_2$ - $ext{ClO} cdot + \text{O} \to \text{Cl} cdot + \text{O}_2$

Ozone Hole — Over Antarctica, facilitated by Polar Stratospheric Clouds (PSCs).

Effects of UV-B — Skin cancer, cataracts, immune suppression, ecosystem damage.

Solution — Montreal Protocol (phase-out ODS).

For Acid Rain Pollutants: Strong Noxious Acids. S for Sulfur oxides ( $ext{SO}_x$ ), N for Nitrogen oxides ( $ext{NO}_x$ ).

For Ozone Depleting Substances (ODS): Can Harm Cool Molecules. C for CFCs, H for Halons, C for Carbon tetrachloride, M for Methyl bromide.

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