GED Science TestEarth and Space ScienceHard
A geochemist is studying the carbon cycle and its interaction with Earth's oceans. She is specifically interested in the process by which atmospheric carbon dioxide dissolves into seawater and can eventually lead to the formation of marine shells. What is the correct sequence of chemical reactions involved in this process?
- ACaCO3 → CO2 + H2O; CO2 + H2O → H2CO3; H2CO3 → H+ + HCO3-
- BH2CO3 → CO2 + H2O; CO32- + Ca2+ → CaCO3; CaCO3 → HCO3-
- CCO2 + H2O → H2CO3; H2CO3 → H+ + HCO3-; HCO3- → H+ + CO32-
- DCO2 + H2O → HCO3-; HCO3- + Ca2+ → CaCO3; CaCO3 → H2CO3
Show answer & explanationAnswer & explanation
Correct answer: C. CO2 + H2O → H2CO3; H2CO3 → H+ + HCO3-; HCO3- → H+ + CO32-
This sequence correctly shows atmospheric CO2 dissolving into water to form carbonic acid (H2CO3), which then dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). The bicarbonate can further dissociate into another hydrogen ion and a carbonate ion (CO32-), which is essential for forming calcium carbonate (CaCO3) shells.
Why the other options are wrong
- A. This starts with the breakdown of CaCO3, not the formation, and the sequence is scrambled.
- B. This sequence is incorrect; H2CO3 does not directly form CO2 + H2O in this context, and CaCO3 doesn't form HCO3- in this way.
- D. This is incorrect; CO2 + H2O forms H2CO3 first, not directly HCO3-.
Ocean Carbon Chemistry
The series of chemical reactions involving carbon dioxide dissolving in seawater, forming carbonic acid, bicarbonate, and carbonate ions, which impacts ocean pH and marine life.
- CO2 + H2O ⇌ H2CO3 (carbonic acid)
- H2CO3 ⇌ H+ + HCO3- (bicarbonate)
- HCO3- ⇌ H+ + CO32- (carbonate)
- Carbonate ions are used by organisms to form calcium carbonate shells.
Memory trick: Carbon's Ocean Chemistry: Carbonic Creates Carbonate.