Ocean acidification is the long-term decrease in ocean pH caused mainly by the ocean absorbing carbon dioxide from the atmosphere. Students need this cheat sheet to connect climate change, chemistry, and marine ecosystems in one clear reference. It explains how carbon dioxide changes seawater chemistry and why small pH changes can have large biological effects.
It also helps students interpret data, diagrams, and cause-and-effect relationships in environmental science.
Key Facts
- Ocean acidification happens when atmospheric CO2 dissolves in seawater and forms carbonic acid, which releases hydrogen ions.
- The main reaction pathway is CO2 + H2O -> H2CO3 -> H+ + HCO3-.
- A lower pH means a higher concentration of hydrogen ions, because pH = -log[H+].
- Since the pH scale is logarithmic, a 1 unit drop in pH means hydrogen ion concentration increases by 10 times.
- Extra H+ ions combine with carbonate ions, so H+ + CO3^2- -> HCO3-, reducing carbonate available for shells.
- Marine organisms such as corals, oysters, clams, and some plankton use Ca2+ + CO3^2- -> CaCO3 to build shells and skeletons.
- Lower carbonate ion concentration can reduce calcification rates and make calcium carbonate structures easier to dissolve.
- Reducing CO2 emissions is the main long-term solution, while protecting seagrasses, mangroves, and wetlands can help locally absorb CO2.
Vocabulary
- Ocean acidification
- Ocean acidification is the decrease in seawater pH caused mainly by the ocean absorbing extra carbon dioxide from the atmosphere.
- pH
- pH is a measure of how acidic or basic a solution is, based on the concentration of hydrogen ions.
- Carbonic acid
- Carbonic acid is the weak acid formed when carbon dioxide dissolves in water.
- Carbonate ion
- A carbonate ion is CO3^2-, an important ion that many marine organisms need to build calcium carbonate shells and skeletons.
- Calcification
- Calcification is the process of using calcium and carbonate ions to form calcium carbonate structures.
- Buffering
- Buffering is the ability of seawater to resist large changes in pH when acids or bases are added.
Common Mistakes to Avoid
- Saying the ocean is turning into a strong acid is wrong because seawater is still slightly basic, but its pH is decreasing.
- Treating a small pH change as unimportant is wrong because the pH scale is logarithmic, so small pH drops can mean large increases in H+ concentration.
- Confusing carbon dioxide absorption with oxygen loss is wrong because ocean acidification is driven by CO2 chemistry, while deoxygenation is a separate but related problem.
- Assuming only coral reefs are affected is wrong because shellfish, plankton, food webs, fisheries, and coastal economies can also be impacted.
- Thinking acidification adds carbonate ions is wrong because extra H+ ions actually reduce carbonate availability by forming bicarbonate.
Practice Questions
- 1 If seawater pH drops from 8.2 to 7.2, by what factor does the hydrogen ion concentration increase?
- 2 A seawater sample has [H+] = 1.0 x 10^-8 M. What is its pH using pH = -log[H+]?
- 3 Write the reaction that shows how calcium ions and carbonate ions form calcium carbonate.
- 4 Explain why higher atmospheric CO2 can make it harder for oysters and corals to build shells, even though CO2 itself is not a shell material.