Which statement best describes buffering in natural waters?

• A. The carbonate system amplifies pH changes.
• B. Biological uptake of carbon dioxide.
• C. The calcium carbonate buffering system resists pH changes by neutralizing acids.
• D. Only rainfall determines pH, not buffering.

Answer: (C)

Buffering in natural waters is the ability of the water to resist changes in pH when acids or bases are added. The main buffering system here is calcium carbonate. When acids introduce hydrogen ions, carbonate and bicarbonate ions react to neutralize those H+ ions, forming carbonic acid that becomes CO2 and water. This uses up the added acidity, so the pH doesn’t drop as much. If bases are added, the system can dissolve some calcium carbonate to release more carbonate ions, which again helps neutralize the base and keep the pH steadier. This buffering action keeps natural waters around a stable pH instead of swinging wildly. The idea that buffering would amplify pH changes isn’t accurate, since buffering dampens changes. While CO2 interactions with water and biological processes affect pH, buffering specifically refers to this chemical system that resists pH shifts, not just the biological uptake of CO2. Rainfall can influence pH, but buffering determines how much the pH will actually change in response to those inputs.