CO₂ & carbonic acid

You twist open the cap of a soda bottle and it hisses. Little bubbles rise, and your tongue tingles. What escapes there is a gas you already know: carbon dioxide, CO₂ for short. You breathe out the same gas with every breath, it is produced when wood and petrol burn, and plants need it to grow. Time to get to know this molecule properly.

CO₂ in soda: carbonic acid

A CO₂ molecule consists of one carbon atom and two oxygen atoms. In the soda bottle, CO₂ is dissolved in the water under pressure. A small part of it reacts with the water to form carbonic acid, H₂CO₃, while the bulk of it simply stays as CO₂ molecules between the water molecules. The carbonic acid makes the soda slightly acidic, and together with the bursting bubbles that gives you the fresh tingle on your tongue. When you open the bottle, the pressure drops: that much gas can no longer stay dissolved, part of it escapes, and that is exactly the hissing and bubbling.

Breathing, burning, photosynthesis

CO₂ is produced wherever carbon reacts with oxygen. In your body's cells, sugar burns very slowly with oxygen, the energy released keeps you going, and what is left over is water and the CO₂ you breathe out. At a campfire and in a car engine, essentially the same thing happens, just faster and hotter. Plants go the opposite way: during photosynthesis they capture CO₂ from the air and use it with water and sunlight to build sugar, releasing oxygen. This creates a cycle in which carbon constantly travels between air, plants and living things.

CO₂ and the climate

CO₂ has another important property: it holds heat back. The Earth constantly gives off heat as invisible infrared radiation, and CO₂ molecules catch part of it and radiate it back down. This is called the greenhouse effect: in much the same way, the glass roof of a greenhouse holds the heat in. Without this effect, the Earth's average temperature would be about −18 °C instead of around 15 °C. Since humans began burning large amounts of coal, oil and gas, the share of CO₂ in the air has risen from about 280 to more than 420 particles per million particles of air, and the average temperature has risen with it. That is why it matters to understand where CO₂ is produced and how it can be captured again.