CO₂ — Carbon Dioxide

Carbon dioxide is a naturally occurring chemical compound composed of one carbon atom and two oxygen atoms, represented by the formula \( CO_2 \). It is a colorless and odorless gas that exists in trace amounts in the Earth's atmosphere, currently around 0.04% by volume. Despite its low concentration, it plays a crucial role in maintaining the planet's temperature through the greenhouse effect and is essential for the process of photosynthesis in plants.

Carbon dioxide is produced naturally through respiration, volcanic eruptions, and the decay of organic matter, and artificially through the combustion of fossil fuels and industrial processes. It can exist as a solid known as dry ice, which sublimates directly into gas without becoming liquid.

Carbon dioxide occurs both naturally and as a result of human activities. Major sources include:

• Natural Sources: Animal respiration, volcanic eruptions, forest fires, decomposition of organic matter, and oceanic release.
• Anthropogenic Sources: Burning of fossil fuels such as coal, petroleum, and natural gas, cement manufacturing, and deforestation.

In the atmosphere, carbon dioxide acts as a greenhouse gas, trapping heat and contributing to global warming. Oceans absorb a large portion of atmospheric CO₂, forming carbonic acid \( H_2CO_3 \), which plays a key role in regulating oceanic pH levels.

Laboratory Preparation:

Carbon dioxide can be easily prepared in the laboratory by the reaction between dilute hydrochloric acid and calcium carbonate (marble chips):

The gas evolved is collected by upward displacement of air since it is denser than air.

Industrial Preparation:

In industries, carbon dioxide is obtained as a byproduct during fermentation, the production of lime from limestone, and from ammonia synthesis plants. It is purified, compressed, and liquefied for commercial use in soft drinks and refrigeration systems.

Physical Properties:

• Carbon dioxide is a colorless, odorless, and non-flammable gas.
• It is heavier than air and does not support combustion.
• Solid CO₂ (dry ice) sublimates directly to gas without melting.
• It is moderately soluble in water, forming weak carbonic acid \( H_2CO_3 \).

Chemical Properties:

• Reaction with Water: Forms carbonic acid:

  \( CO_2 + H_2O \rightleftharpoons H_2CO_3 \)

• Reaction with Bases: Reacts with sodium hydroxide to form sodium carbonate:

  \( 2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O \)

• Reaction with Lime Water: Turns lime water milky due to formation of calcium carbonate:

  \( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \)

  On passing excess CO₂, the milkiness disappears because of formation of soluble calcium bicarbonate:

  \( CaCO_3 + CO_2 + H_2O \rightarrow Ca(HCO_3)_2 \)

• Combustion Role: CO₂ does not burn nor support combustion, making it useful for fire extinguishers.

• Photosynthesis: Plants use CO₂ in photosynthesis to produce glucose and oxygen:

  \( 6CO_2 + 6H_2O \xrightarrow{light, chlorophyll} C_6H_{12}O_6 + 6O_2 \)

• Fire Extinguishers: CO₂ is used to extinguish fires because it displaces oxygen and cools the burning surface.
• Carbonated Beverages: Used to make soft drinks fizzy by dissolving CO₂ under pressure.
• Dry Ice: Solid CO₂ serves as a refrigerant for transporting frozen food and biological samples.
• Industrial Applications: Employed in welding, chemical synthesis, and as an inert atmosphere for certain reactions.

Carbon dioxide is essential for maintaining the Earth's temperature and for plant life, but excessive CO₂ emissions are a major contributor to climate change. Human activities have led to an increase in atmospheric CO₂ levels from about 280 ppm in the pre-industrial era to over 420 ppm today.

This increase enhances the greenhouse effect, trapping more heat in the atmosphere and causing global warming. It also leads to ocean acidification as CO₂ dissolves in seawater, reducing marine biodiversity.

To mitigate its impact, efforts such as afforestation, renewable energy use, and carbon capture and storage technologies are being implemented worldwide.