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Environmental Science
Ocean Acidification
CO2 absorption, falling pH, and shell-building life
Related Worksheets
Ocean acidification is the ongoing decrease in ocean pH caused mainly by the ocean absorbing extra carbon dioxide from the atmosphere. It matters because seawater chemistry controls how easily corals, shellfish, and some plankton build shells and skeletons. As human activities add more CO2 to the air, the ocean takes up a large share of it, slowing climate change but changing marine habitats. Even small pH changes can stress organisms because the pH scale is logarithmic.
Key Facts
- CO2(g) ⇌ CO2(aq)
- CO2 + H2O ⇌ H2CO3
- H2CO3 ⇌ H+ + HCO3-
- HCO3- ⇌ H+ + CO3^2-
- pH = -log10[H+]
- Ca^2+ + CO3^2- ⇌ CaCO3, the mineral used by many shells and coral skeletons
Vocabulary
- Ocean acidification
- Ocean acidification is the decrease in seawater pH caused by absorption of carbon dioxide from the atmosphere.
- pH
- pH is a logarithmic measure of hydrogen ion concentration, where lower pH means a more acidic solution.
- Carbonic acid
- Carbonic acid is the weak acid formed when dissolved carbon dioxide reacts with water.
- Carbonate ion
- Carbonate ion, CO3^2-, is a dissolved ion needed by many marine organisms to build calcium carbonate shells and skeletons.
- Calcification
- Calcification is the biological process of forming calcium carbonate structures such as coral skeletons and mollusk shells.
Common Mistakes to Avoid
- Saying the ocean becomes acidic, not just more acidic, is misleading because average seawater is still usually above pH 7 but has shifted toward lower pH.
- Treating a pH drop of 0.1 as tiny is wrong because pH is logarithmic, so a 0.1 decrease means about a 26 percent increase in hydrogen ion concentration.
- Assuming acidification only affects corals is wrong because shellfish, plankton, fish behavior, food webs, and local fisheries can also be affected.
- Forgetting carbonate ion availability is a mistake because lower pH reduces CO3^2-, making it harder for organisms to form CaCO3 even if calcium is present.
Practice Questions
- 1 A seawater sample changes from pH 8.2 to pH 8.1. By what factor does the hydrogen ion concentration increase? Use [H+] ratio = 10^(pH_initial - pH_final).
- 2 If the ocean absorbs 30 percent of 40 billion metric tons of human-produced CO2 in a year, how many billion metric tons of CO2 enter the ocean?
- 3 Explain why adding CO2 to seawater can make it harder for corals to build skeletons, even though CO2 itself is not the material in the skeleton.