AP Syllabus focus:
‘Ocean acidification is a decrease in ocean pH, mainly due to higher atmospheric CO2, and it can be represented using chemical equations.’
Ocean acidification describes a major shift in seawater chemistry driven by atmospheric carbon dioxide. Understanding the definition of pH and the core carbonate reactions helps explain why adding CO2 lowers ocean pH.
What “ocean acidification” means
Ocean acidification: A long-term decrease in ocean pH caused primarily by increased uptake of atmospheric CO2 by seawater.
“Ocean acidification” does not mean the ocean becomes acidic (pH below 7) overall; it means seawater becomes less basic (pH decreases). Because the pH scale is logarithmic, even small pH changes reflect meaningful shifts in seawater hydrogen ion concentration.
pH: A logarithmic measure of acidity defined by the concentration of hydrogen ions in solution.
Why atmospheric CO2 is central
The specification emphasises that ocean acidification is mainly due to higher atmospheric CO2. CO2 in the air and CO2 dissolved in surface ocean water are linked by ongoing gas exchange; when atmospheric CO2 rises, more CO2 tends to dissolve into seawater, altering chemical equilibria that control pH.
Long-term observations near Hawaiʻi show atmospheric CO rising alongside seawater pCO, while seawater pH trends downward. The figure visually connects increasing CO uptake by the ocean with measurable changes in ocean chemistry over time. Source
The key chemistry idea (equilibria)
Ocean acidification can be represented using chemical equations because seawater contains a set of reversible reactions that shift in response to added dissolved CO2. These reactions are commonly grouped as the carbonate system.
Diagram of the net carbonate-system reaction showing dissolved CO reacting with water and carbonate ions to form bicarbonate. It highlights a key consequence of ocean acidification chemistry: carbonate ions are consumed, which reduces the pool of carbonate available for calcification (e.g., shells and coral skeletons). Source
pH as the chemical “signal” of acidification
= acidity scale (unitless)
= hydrogen ion concentration (mol/L)
A decrease in pH corresponds to an increase in . This relationship is why acidification is defined and tracked using pH.
Carbon dioxide reactions in seawater
= dissolved carbon dioxide in seawater
= carbonic acid (formed in water)
= bicarbonate ion
= water (reactant/solvent in seawater)
= carbonate ion
As more CO2 dissolves, the equilibria shift in ways that typically increase , lowering pH.
Bjerrum plot illustrating how dissolved inorganic carbon partitions among CO(aq), bicarbonate (HCO), and carbonate (CO) as pH changes. The shaded region marks typical modern surface-ocean pH conditions, emphasizing that small pH decreases shift carbon away from carbonate toward CO/bicarbonate. Source
The exact distribution among , , and depends on conditions such as temperature and alkalinity, but the AP-level takeaway is that added CO2 pushes the system toward producing more .
Interpreting “decrease in ocean pH” scientifically
What changes (and what doesn’t)
Changes: average seawater pH decreases; increases; the balance among dissolved inorganic carbon forms shifts.
Doesn’t necessarily change: the ocean does not need to cross pH 7 for “acidification” to be real; it is defined by the direction of pH change.
What students should be able to do with the chemistry
State the definition: ocean acidification = decreasing ocean pH, mainly from higher atmospheric CO2.
Use the pH relationship to connect lower pH with higher .
Read the carbonate equations as reversible equilibria and explain, qualitatively, that adding CO2 shifts reactions in a way that increases .
FAQ
Because pH is based on $-\log_{10}([H^+])$, a small pH drop represents a proportionally larger rise in $[H^+]$.
This makes modest-looking pH changes chemically significant.
Dissolved $CO_2$ is molecular $CO_2(aq)$. Carbonic acid is $H_2CO_3$, formed when $CO_2$ reacts with water.
They interconvert and are often grouped together in discussions of dissolved inorganic carbon.
The double arrow indicates reversible equilibrium. Forward and reverse reactions occur simultaneously, and the proportions shift when conditions (like dissolved $CO_2$) change.
Alkalinity is the seawater capacity to neutralise acid, largely linked to bases such as $HCO_3^-$ and $CO_3^{2-}$.
It influences how strongly pH changes when extra $CO_2$ is absorbed.
They often measure multiple carbonate-system variables, for example:
total dissolved inorganic carbon (DIC)
total alkalinity
partial pressure of $CO_2$ ($pCO_2$)
These combined measurements better constrain the underlying chemistry than pH alone.
Practice Questions
Define ocean acidification and identify its main cause. (2 marks)
Defines as a decrease in ocean pH / seawater becoming less basic (1)
States main cause is increased atmospheric dissolved into the ocean (1)
Using chemical equations, explain how increased atmospheric can lead to a lower ocean pH. (5 marks)
Correctly writes or describes (1)
Correctly writes or describes dissociation producing : (1)
Links increased dissolved to shifting equilibria towards producing more (1)
Uses pH concept: lower pH corresponds to higher (1)
Clear, logically connected explanation using the equations (1)
