AP Environmental Science Quiz: Ocean Acidification

What this quiz covers

This quiz focuses on Ocean Acidification, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Environmental Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Ocean acidification is driven by ocean uptake of atmospheric $\mathrm{CO_2}$ forming carbonic acid; acid rain/runoff involves strong acids and other pollutants delivered from the atmosphere or land (correct answer)
2. Both processes are identical because any pH decline in the ocean must be from acid rain
3. Ocean acidification is caused by sulfuric and nitric acids, while acid rain is caused by carbonic acid from $\mathrm{CO_2}$
4. Ocean acidification increases seawater pH, while acid rain decreases pH

Explanation: Ocean acidification and acid rain/runoff are distinct processes affecting water chemistry differently. Ocean acidification is a global, long-term process driven by atmospheric CO₂ dissolving into seawater to form carbonic acid, causing gradual pH decline that tracks CO₂ levels. Acid rain and runoff involve strong acids (sulfuric and nitric) from air pollution or land-based sources causing localized, short-term pH drops. Region 1's pattern (long-term decline tracking CO₂) indicates ocean acidification, while Region 2's pattern (short-term drops after storms) indicates acid rain/runoff effects. Option A correctly distinguishes these mechanisms, while the other options confuse or reverse the processes.

Question 2

1. Less $\mathrm{CO_2}$ dissolves into seawater, reducing acidity and lowering pH
2. More $\mathrm{CO_2}$ dissolves into seawater, forming carbonic acid that releases $\mathrm{H^+}$ and lowers pH (correct answer)
3. Ocean warming directly increases pH by creating more hydroxide ions, making seawater more basic
4. Sulfur dioxide dissolves into seawater, forming sulfuric acid that lowers pH (acid rain mechanism)

Explanation: Ocean acidification occurs when atmospheric CO₂ dissolves into seawater, forming carbonic acid (H₂CO₃). This weak acid dissociates to release hydrogen ions (H⁺), which lower the pH of seawater. The pH decline from 8.12 to 8.05 represents acidification even though the water remains basic (pH > 7). This process is directly linked to rising atmospheric CO₂ levels, as more CO₂ in the atmosphere drives more dissolution into the ocean. Option A correctly describes this mechanism, while the other options either reverse the process (C), incorrectly describe warming effects (D), or invoke acid rain rather than CO₂ absorption (B).

Question 3

1. No change in pH because oceans do not exchange gases with the atmosphere
2. Lower pH and reduced shell growth because carbonic acid formation reduces carbonate availability (correct answer)
3. Higher pH and faster shell growth because more $\mathrm{CO_2}$ provides more carbonate
4. Lower pH only if sulfur dioxide is present, since $\mathrm{CO_2}$ cannot affect seawater pH

Explanation: In the mesocosm experiment, Tank Y with elevated CO₂ will experience ocean acidification conditions. More CO₂ dissolves into the seawater, forming carbonic acid that lowers pH and reduces carbonate ion availability. This makes it harder for clams to build their calcium carbonate shells, resulting in reduced shell growth compared to Tank X with present-day CO₂ levels. Option B correctly predicts both the lower pH and reduced shell growth. Option A incorrectly suggests higher pH and faster growth, C wrongly claims no gas exchange occurs, and D incorrectly limits pH effects to sulfur dioxide.

Question 4

1. Bay 1 likely has more carbonic acid formation, leading to a lower pH than Bay 2 (correct answer)
2. Bay 1 likely has less carbonic acid formation, so pH should be higher than Bay 2
3. The pH difference is mainly due to acid rain directly falling into Bay 1, not CO$_2$ chemistry
4. Higher dissolved CO$_2$ directly consumes H$^+$, raising pH in Bay 1

Explanation: Ocean acidification demonstrates a clear inverse relationship between dissolved CO₂ concentration and seawater pH. When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which dissociates to release hydrogen ions (H⁺) and lower pH. Bay 1, with higher dissolved CO₂, experiences more carbonic acid formation and thus has more H⁺ ions in solution, resulting in the lower pH of 7.95. Bay 2, with less dissolved CO₂, has less carbonic acid formation and fewer H⁺ ions, maintaining a higher pH of 8.10. This relationship is fundamental to understanding ocean acidification: as atmospheric CO₂ increases and more dissolves into seawater, pH consistently decreases due to carbonic acid chemistry, not acid rain or other mechanisms.

Question 5

1. Expansion of coral reefs due to faster CaCO$_3$ precipitation under lower pH
2. Declines in reef-building and shell-forming species that provide habitat and support food webs (correct answer)
3. No effect on marine organisms because pH changes only occur in freshwater systems
4. Ocean acidification is primarily caused by hydrochloric acid released from ships, not atmospheric CO$_2$

Explanation: Ocean acidification poses a fundamental threat to marine ecosystem structure and function through its impacts on calcifying organisms. As the chemical cascade from atmospheric CO₂ to dissolved CO₂ to carbonic acid increases H⁺ concentration, the resulting lower pH and reduced carbonate availability impair the ability of key species to build and maintain calcium carbonate structures. Reef-building corals, shell-forming mollusks, and calcareous plankton form the foundation of many marine food webs, providing critical habitat, food sources, and ecosystem services. When these organisms decline due to impaired calcification, entire ecosystems can collapse: coral reefs lose their three-dimensional structure, shellfish beds diminish, and planktonic food webs shift. This cascading effect demonstrates how a chemical change in seawater can fundamentally alter marine biodiversity and productivity, with implications for fisheries, coastal protection, and global biogeochemical cycles.

Question 6

1. A decrease of 0.1 pH units reflects a decrease in H$^+$ concentration
2. A decrease in pH indicates an increase in H$^+$ concentration (correct answer)
3. pH decreases only when acid rain directly falls into the ocean
4. pH decreases because CO$_2$ removes H$^+$ from solution

Explanation: Ocean acidification refers to the reduction in seawater pH caused by the ocean's uptake of CO2, which forms carbonic acid and elevates H+ levels. Carbonate chemistry dictates that pH is inversely logarithmic to H+ concentration; a lower pH means higher H+. This is distinct from acid rain, which has minimal impact on open ocean pH compared to CO2-driven changes. The correct statement notes that a pH decrease indicates an increase in H+ concentration, accurately reflecting the acidification mechanism. This is why ocean pH has dropped about 0.1 units since the industrial era, corresponding to a 26% H+ increase. Misconceptions like pH decreasing due to H+ removal or solely from acid rain are incorrect.

Question 7

1. Decreased $\mathrm{H^+}$, higher pH; calcifiers thrive because shells dissolve more slowly
2. Increased $\mathrm{H^+}$, lower pH; calcifiers may struggle because carbonate chemistry shifts away from $\mathrm{CO_3^{2-}}$ needed for $\mathrm{CaCO_3}$ (correct answer)
3. No change in $\mathrm{H^+}$ because oceans are too large to be affected; calcification rates always increase with higher $\mathrm{CO_2}$
4. Increased acidity primarily from nitric acid produced by lightning; calcifiers are unaffected because they live underwater

Explanation: When more atmospheric CO₂ dissolves into seawater, it shifts the equilibrium to the right: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. This increases the concentration of hydrogen ions (H⁺), thereby lowering the pH and making the ocean more acidic. The increased H⁺ concentration has a critical secondary effect: it combines with carbonate ions (CO₃²⁻) to form more bicarbonate (HCO₃⁻), reducing the availability of carbonate that calcifying organisms need to build calcium carbonate (CaCO₃) shells and skeletons. This shift in carbonate chemistry makes it energetically more difficult for organisms to precipitate CaCO₃ and can even cause existing structures to dissolve. Answer B correctly describes both the increase in H⁺ and the resulting challenge for calcifiers.

Question 8

1. Ocean acidification is mainly driven by seawater absorbing atmospheric CO$_2$, forming carbonic acid and increasing H$^+$ (correct answer)
2. Ocean acidification occurs when seawater absorbs methane, forming carbonic acid and lowering pH
3. Ocean acidification is caused by increased sunlight producing acids from saltwater
4. Ocean acidification is the same as acid rain; both are primarily sulfuric acid entering oceans from clouds

Explanation: Ocean acidification is a distinct process from acid rain, though both involve pH changes. Ocean acidification specifically refers to the absorption of atmospheric CO₂ by seawater, where it forms carbonic acid (H₂CO₃) and increases hydrogen ion (H⁺) concentration, lowering pH. This is a global phenomenon driven by rising atmospheric CO₂ levels from fossil fuel combustion and deforestation. In contrast, acid rain involves sulfuric acid (H₂SO₄) and nitric acid (HNO₃) formed from SO₂ and NOₓ emissions, primarily affecting localized areas near industrial sources. While acid rain can impact coastal waters, the widespread decline in ocean pH observed globally is overwhelmingly due to CO₂ absorption. The student's claim confuses these two processes; ocean acidification is fundamentally a CO₂-carbonic acid phenomenon affecting all ocean basins.

Question 9

1. Increased ability of corals and pteropods to precipitate CaCO$_3$
2. Reduced calcification rates in organisms that build shells/skeletons from CaCO$_3$ (correct answer)
3. Greater formation of nitric acid from CO$_2$ leading to acid rain in the ocean
4. Immediate global warming reversal because dissolved CO$_2$ no longer traps heat in the atmosphere

Explanation: Ocean acidification fundamentally alters the carbonate system equilibrium in seawater, with profound ecological consequences. As CO₂ dissolves and forms carbonic acid, the released H⁺ ions shift the carbonate equilibrium: more H⁺ combines with carbonate ions (CO₃²⁻) to form bicarbonate (HCO₃⁻), causing CO₃²⁻ concentrations to decrease even as HCO₃⁻ increases. This reduction in carbonate ion availability directly impacts calcifying organisms that depend on CO₃²⁻ to build calcium carbonate (CaCO₃) shells and skeletons. Marine species like corals, mollusks, and pteropods (sea butterflies) experience reduced calcification rates because the thermodynamic conditions for CaCO₃ precipitation become less favorable. This can lead to thinner shells, slower growth rates, and increased dissolution of existing structures, ultimately affecting entire marine food webs that depend on these calcifying organisms.

Question 10

1. An increase in dissolved CO$_2$ in seawater during the same period as the pH decline (correct answer)
2. A decrease in atmospheric CO$_2$ above the ocean during the same period
3. A strong increase in sulfate deposition from acid rain as the dominant global driver of seawater pH trends
4. A shift toward higher carbonate ion concentration as pH declines

Explanation: Ocean acidification is characterized by a specific set of chemical changes that distinguish it from other potential causes of pH decline. The key diagnostic feature is the simultaneous increase in dissolved CO₂ concentration as pH decreases, reflecting the fundamental process where atmospheric CO₂ dissolves to form carbonic acid. This correlation between rising dissolved CO₂ and falling pH, combined with the biological observation of thinner shells, strongly indicates ocean acidification as the mechanism. The thinning shells result from reduced carbonate ion availability as more CO₃²⁻ is converted to HCO₃⁻ in acidified conditions. Other options would not support this conclusion: decreasing atmospheric CO₂ would contradict the mechanism, acid rain affects localized areas rather than showing global patterns, and carbonate ions decrease rather than increase as pH declines in ocean acidification.

Question 11

1. CO$_2$ increases pH by forming hydroxide ions, which dissolves shells
2. CO$_2$ absorption increases carbonic acid and H$^+$, making it harder for larvae to form CaCO$_3$ shells (correct answer)
3. Upwelling brings sulfuric acid from deep-sea volcanoes, which is the dominant cause of ocean acidification
4. Lower pH indicates fewer dissolved gases, so larvae lack oxygen and cannot calcify

Explanation: Ocean acidification creates particularly challenging conditions for larval shellfish through its impact on carbonate chemistry. When CO₂-rich upwelled water reaches the surface, it contains high levels of dissolved CO₂ that form carbonic acid, releasing H⁺ ions and lowering pH. This increased acidity reduces carbonate ion (CO₃²⁻) availability, which larval oysters need to precipitate calcium carbonate (CaCO₃) for their initial shell formation. Young larvae are especially vulnerable because they must rapidly build shells during critical developmental stages when their energy reserves are limited. The energetic cost of calcification increases dramatically in acidified water, as larvae must work harder to extract scarce carbonate ions. This explains why hatcheries observe reduced larval survival during upwelling events that bring acidified water to coastal areas where shellfish aquaculture operates.

Question 12

1. Calcifying organisms (e.g., corals, oysters) have more difficulty building CaCO$_3$ shells/skeletons (correct answer)
2. All marine organisms gain thicker shells because carbonate becomes more available at lower pH
3. Ocean acidification primarily occurs when nitric acid from car exhaust dissolves into seawater
4. Lower pH reduces H$^+$ concentration, shifting equilibrium to produce less bicarbonate

Explanation: Ocean acidification fundamentally alters seawater carbonate chemistry through the reaction sequence shown: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. As more CO₂ dissolves and forms carbonic acid, the increased H⁺ ions react with carbonate ions (CO₃²⁻) to form bicarbonate (HCO₃⁻), reducing carbonate availability. Calcifying organisms like corals and oysters require carbonate ions to build their calcium carbonate (CaCO₃) shells and skeletons through the reaction Ca²⁺ + CO₃²⁻ → CaCO₃. With fewer carbonate ions available at lower pH, these organisms struggle to maintain normal calcification rates. This makes shell and skeleton formation energetically more costly and can lead to thinner, weaker structures that are more vulnerable to dissolution and predation.

Question 13

1. SO$_2$ dissolves into seawater to form sulfuric acid, lowering pH primarily near industrial coasts
2. CO$_2$ is absorbed by seawater and forms carbonic acid, increasing H$^+$ and lowering pH (correct answer)
3. Higher CO$_2$ causes more photosynthesis, removing H$^+$ and raising ocean pH
4. Ozone reacts with seawater to form strong acids that directly dissolve carbonate minerals

Explanation: Ocean acidification occurs when atmospheric CO₂ dissolves in seawater, forming carbonic acid (H₂CO₃) through the reaction CO₂ + H₂O → H₂CO₃. This weak acid dissociates to release hydrogen ions (H⁺), which directly lowers the pH of seawater. The observed pH decline from 8.15 to 8.05 represents a significant increase in H⁺ concentration since pH is a logarithmic scale. This process is distinct from acid rain, which involves sulfuric and nitric acids from industrial emissions. The correlation between increased atmospheric CO₂ and decreased ocean pH confirms that CO₂ absorption and carbonic acid formation is the primary mechanism driving ocean acidification globally.

Question 14

1. It decreases by a factor of 10
2. It increases by a factor of about $10^{0.10}\approx1.26$ (correct answer)
3. It does not change because pH remains above 7
4. It increases by a factor of 10

Explanation: Ocean acidification causes pH drops as CO2 forms carbonic acid, increasing H+ concentration. pH is -log[H+], so a 0.1 unit decrease means H+ increases by 10^0.1 ≈1.26 times. This logarithmic scale means small pH changes reflect significant H+ shifts. The correct answer calculates this increase factor accurately. pH above 7 still allows changes, and decreases don't mean H+ drops. This exemplifies the scale of observed ocean changes.

Question 15

1. CaCO$_3$ structures are more likely to dissolve or form more slowly (correct answer)
2. Seawater pH increases because H$^+$ is removed
3. Acid rain becomes the primary driver of open-ocean pH trends
4. CaCO$_3$ structures form faster because more carbonate is available

Explanation: Ocean acidification lowers pH through CO2 forming carbonic acid and increasing H+, which reduces CO3^2- and thus the saturation state of CaCO3. In carbonate chemistry, a lower saturation state means CaCO3 is less stable, promoting dissolution and slowing formation. Therefore, structures like shells are more likely to dissolve or form more slowly. This is the likely outcome, contrary to faster formation or pH increases. Acid rain is not the primary driver in open oceans. Understanding saturation state is key to predicting impacts on marine calcifiers.

Question 16

1. Increased H$^+$ reduces CO$_3^{2-}$, lowering CaCO$_3$ saturation and promoting shell dissolution (correct answer)
2. Lower pH increases CO$_3^{2-}$ and should prevent dissolution
3. Acid rain is the main source of acidity in offshore waters, not CO$_2$
4. Increased CO$_2$ raises pH and dissolves shells by making water more basic

Explanation: Ocean acidification lowers pH via CO2 forming carbonic acid and increasing H+, which reduces CO3^2- and CaCO3 saturation. This promotes shell dissolution in organisms like pteropods, as lower saturation makes CaCO3 unstable. The correct explanation connects increased H+ to reduced CO3^2- and dissolution. Acid rain is minor in offshore waters, and lower pH decreases, not increases, CO3^2-. CO2 lowers, not raises, pH. This mechanism explains observed shell damage in acidified areas.

Question 17

1. Higher atmospheric CO$_2$ leads to more CO$_2$ dissolved in seawater, increasing carbonic acid and lowering pH (correct answer)
2. Higher atmospheric CO$_2$ causes seawater to become more basic because carbonic acid removes H$^+$
3. The pH decline is most likely due to increased acid rain, which is identical to ocean acidification
4. The pH decline shows that fewer acids are present, so carbonate shells should form more rapidly

Explanation: Ocean acidification demonstrates a direct causal relationship between atmospheric CO₂ levels and seawater pH through well-understood chemical processes. When atmospheric CO₂ increases, more CO₂ dissolves into seawater following Henry's Law, forming carbonic acid (H₂CO₃) that dissociates to release hydrogen ions (H⁺). The pH scale is logarithmic and inversely related to H⁺ concentration, so as H⁺ increases, pH decreases. The observed change from pH 8.12 in Period 1 (lower CO₂) to pH 8.02 in Period 2 (higher CO₂) represents approximately a 26% increase in H⁺ concentration. This pattern is consistent with global observations showing that ocean pH has declined by about 0.1 units since pre-industrial times due to anthropogenic CO₂ emissions. The data clearly supports the mechanism of CO₂-driven ocean acidification rather than other potential causes.

Question 18

1. Tuna, because their gills dissolve in acidic water
2. Corals, because they build calcium carbonate skeletons that require sufficient carbonate ions (correct answer)
3. Kelp, because it cannot photosynthesize when pH decreases slightly
4. Jellyfish, because they build calcium carbonate shells

Explanation: Ocean acidification most directly threatens organisms that build calcium carbonate (CaCO₃) structures because they depend on adequate carbonate ion availability. Corals are particularly vulnerable as they build extensive calcium carbonate skeletons for their reef structures. When ocean acidification reduces carbonate ion concentrations, corals struggle to maintain their skeletons and grow new structures. Option B correctly identifies corals as most threatened. Jellyfish (A) don't build calcium carbonate shells, tuna gills (C) don't dissolve in slightly acidic water, and kelp (D) can photosynthesize across a range of pH values typical of ocean acidification.

Question 19

1. Build taller smokestacks so acids fall farther from the ocean (acid rain control)
2. Increase the use of high-sulfur coal to reduce atmospheric $\mathrm{CO_2}$
3. Add nitrogen fertilizer to coastal waters to raise pH by stimulating algal blooms
4. Reduce $\mathrm{CO_2}$ emissions to slow ocean uptake of $\mathrm{CO_2}$ and carbonic acid formation (correct answer)

Explanation: Ocean acidification is driven by atmospheric CO₂ dissolving into seawater, so reducing CO₂ emissions addresses the root cause by slowing the rate of CO₂ uptake by oceans. This would slow carbonic acid formation and the resulting pH decline, protecting coral reefs from acidification stress. Option A correctly identifies this direct link. Option B would worsen the problem through more CO₂ emissions, option C addresses acid rain (not ocean acidification), and option D could cause harmful algal blooms without addressing the CO₂ problem. Reducing atmospheric CO₂ is the only action that directly targets the source of ocean acidification.

Question 20

1. A 0.1 pH decrease means $\mathrm{H^+}$ concentration decreases by about 10%, reducing stress on calcifiers.
2. A 0.1 pH decrease means $\mathrm{H^+}$ concentration increases by about 26%, which can increase stress on calcifiers by reducing carbonate availability. (correct answer)
3. A 0.1 pH decrease means $\mathrm{H^+}$ concentration increases by a factor of 10, immediately dissolving all coral reefs.
4. A 0.1 pH decrease is caused mainly by acid rain and does not involve dissolved $\mathrm{CO_2}$ or carbonic acid.

Explanation: The pH scale is logarithmic, meaning each 0.1 unit decrease represents approximately a 26% increase in hydrogen ion (H⁺) concentration. When ocean pH drops from 8.2 to 8.1 due to CO₂ absorption and carbonic acid formation, this seemingly small change actually represents a significant increase in acidity. The increased H⁺ concentration shifts the carbonate equilibrium, reducing the availability of carbonate ions that calcifying organisms need. This 26% increase in H⁺ can measurably stress calcifiers by making it more energetically costly to build and maintain calcium carbonate structures. Option B correctly quantifies this relationship and its implications for marine calcifiers, while the other options either underestimate or overstate the magnitude of change.