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How does water take part in the carbon dioxide cycle?

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water rapids, Niagara Falls, Canada — water Water is the most plentiful compound on Earth and is essential to life. Although water molecules are simple in structure (H₂O), the physical and chemical properties of water are extraordinarily complicated.

Water plays a fundamental role in the carbon dioxide cycle through both chemical and biological processes.

Chemical Weathering and Transport

Water acts as a solvent for atmospheric carbon dioxide ( $C O_{2}$ ), forming carbonic acid ( $H_{2} C O_{3}$ ). This weak acid facilitates the chemical weathering of continental rocks, a process that breaks down minerals and releases ions, including calcium, into the water. These dissolved products are transported via rivers to the oceans, where they can combine to form calcium carbonate ( $C a C O_{3}$ ). This mineral eventually settles on the seafloor to form limestone, effectively sequestering carbon in the Earth's crust until it is returned to the atmosphere through volcanic activity or tectonic processes.123

Biological Processes

In biological systems, water is essential for photosynthesis. Plants and marine phytoplankton use water as a source of hydrogen atoms to convert $C O_{2}$ into organic compounds, releasing oxygen as a byproduct. Additionally, water is vital for the transport of carbon dioxide in the blood of animals, where it reacts to form bicarbonate ions, a process catalyzed by the enzyme carbonic anhydrase.451