O₃ — Ozone

Ozone (O₃) is a triatomic molecule composed of three oxygen atoms. It is a highly reactive allotrope of oxygen and plays a crucial role in the Earth's atmosphere by absorbing the majority of the sun’s harmful ultraviolet (UV) radiation. Ozone is found naturally in the stratosphere (forming the ozone layer) and near the Earth's surface (in the troposphere).

While stratospheric ozone protects life by filtering UV rays, tropospheric ozone is considered a pollutant that can harm living organisms and degrade materials. This dual nature makes ozone one of the most studied and important atmospheric compounds in environmental chemistry.

Industrially, ozone is used as a powerful oxidizing and disinfecting agent in water treatment, air purification, and organic synthesis. Its strong oxidation potential surpasses that of chlorine, making it useful for sterilization and bleaching applications.

Ozone has a bent or angular molecular geometry with a bond angle of approximately 117°. It is a resonance hybrid of two structures, meaning the double bond is delocalized across the three oxygen atoms, resulting in partial double-bond character between each pair of atoms.

The central oxygen atom is sp² hybridized, and the molecule possesses a dipole moment due to its asymmetrical shape. This polarity makes ozone slightly soluble in water and reactive with both organic and inorganic compounds.

Ozone is paramagnetic in nature and exhibits a pale blue color in the gaseous state. In liquid and solid forms, the color deepens to dark blue or violet.

Ozone can be prepared by passing dry oxygen through a high-voltage electrical discharge called a silent electric discharge or ozonizer. This method prevents the decomposition of ozone due to heat.

This process converts about 10% of oxygen into ozone. The mixture of oxygen and ozone thus obtained is called ozonized oxygen. Ozone can also be generated by photochemical reactions involving ultraviolet light, especially in the upper atmosphere.

Physical Properties: Ozone is a pale blue gas with a pungent, chlorine-like odor. It is heavier than air and slightly soluble in water. Liquid ozone is deep blue, while solid ozone is violet-black in color. It is unstable and decomposes easily, especially when heated or exposed to light.

Chemical Properties:

• Decomposition: Ozone is thermally unstable and decomposes to oxygen gas, releasing heat.

• Oxidizing Agent: Ozone is a powerful oxidizer, converting lead sulfide to lead sulfate.

• Reaction with Iodide: Ozone oxidizes potassium iodide to iodine.

Ozone is also used in the detection of unsaturated organic compounds since it reacts with double bonds (ozonolysis).

Ozone is of immense environmental importance due to its dual roles in the atmosphere:

• Stratospheric Ozone (Good Ozone): Found between 10–50 km above Earth’s surface, this layer absorbs the majority of harmful UV-B and UV-C radiation from the Sun, protecting living organisms from genetic damage, skin cancer, and crop loss.
• Tropospheric Ozone (Bad Ozone): Near ground level, ozone is a secondary pollutant formed by the photochemical reaction of nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight. It causes respiratory problems, smog formation, and damage to plants.

Depletion of the ozone layer, mainly due to chlorofluorocarbons (CFCs), allows more UV radiation to reach Earth, leading to environmental and health concerns. International efforts like the Montreal Protocol (1987) aim to phase out ozone-depleting substances globally.

Ozone’s strong oxidizing and disinfecting power makes it valuable in numerous applications:

• Water purification: Used for sterilizing drinking water and removing harmful bacteria and viruses.
• Air purification: Removes odors and pollutants in hospitals, laboratories, and industrial settings.
• Food preservation: Used to disinfect cold storage areas and prolong food shelf life.
• Bleaching agent: Ozone is used to bleach oils, waxes, textiles, and flour.
• Chemical synthesis: In organic chemistry, ozone helps break down unsaturated bonds in ozonolysis reactions to form aldehydes and ketones.

Its ability to decompose into oxygen makes ozone an eco-friendly disinfectant with minimal residual toxicity.

Although ozone is beneficial in the stratosphere, exposure to high concentrations at ground level is harmful to human health. Inhalation can irritate the respiratory tract, cause coughing, and worsen asthma or bronchitis. Prolonged exposure may lead to lung inflammation.

Industrial use of ozone requires strict safety measures, including proper ventilation and monitoring devices. Occupational exposure limits are generally set below 0.1 ppm to ensure worker safety.

Ozone is also used therapeutically in low doses in ozone therapy, though such applications remain controversial and require medical supervision.