Carbon dioxide is a colourless and odourless gas that can readily dissolve in water. It is commonly used to create fizzy drinks.
In the early Earth’s atmosphere, water vapour condensed to form liquid water, which eventually accumulated to form the oceans.
As the oceans developed, carbon dioxide from the atmosphere dissolved in the water, initially forming carbonic acid (H2CO3). This acid then reacted with metal ions present in the water to produce soluble carbonate compounds.
Over time, these carbonate compounds were precipitated and became part of sedimentary rocks, such as limestone. This process reduced the amount of carbon dioxide in the atmosphere.
As green plants and photosynthetic algae evolved, they absorbed significant amounts of carbon dioxide through photosynthesis. The remains of these organisms, and those from the food chains they supported, over time and under specific conditions, became the sources of fossil fuels.