Carbon dioxide (CO₂) is a naturally occurring gas in Earth's atmosphere, made up of one carbon atom (C) covalently bonded to two oxygen atoms (O₂)

• Physical properties: It's colourless, odourless, and non-flammable under normal conditions
• Occurrence: Found in the atmosphere at about 0.04% (420 ppm as of 2025)
• Sources: Naturally from respiration, volcanic activity, wildfires, and decomposition; anthropogenically from burning fossil fuels, deforestation, and cement production.
• Role in life: Essential for photosynthesis, where plants convert CO₂ and water into glucose and oxygen using sunlight. A greenhouse gas, trapping heat and regulating Earth's climate—but excessive concentrations drive global warming.
• Physical behavior: It doesn't support combustion and is often used in fire extinguishers. At high concentrations, it can displace oxygen and become dangerous to breathe.
• Phase change: Solid CO₂ is called dry ice, which sublimates directly into gas without a liquid phase at standard pressure.