Unit 9 · Topic 9.7 Beta

Ocean Acidification

The ocean absorbs part of the CO₂ people add to the air.

Practice 2: Visual RepresentationsPractice 6: Mathematical Routines

Part 1 · Hook

Why this matters

Around 2007, oyster hatcheries in Oregon and Washington began losing most of their baby oysters. The larvae could not build their first shells. The culprit was not disease or pollution from a pipe, but seawater that had absorbed so much carbon dioxide from the air that it was eating away at shell-building.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What happens to the pH of a solution when its hydrogen ion concentration increases?

pH decreases
pH increases
pH does not change
pH becomes exactly 7

Show the answer

More hydrogen ions means a lower pH, more acidic.

Correct: pH decreases:
pH increases:
pH does not change:
pH becomes exactly 7:

2. How did atmospheric CO₂ change at Mauna Loa from 1960 to 2023?

It rose from about 317 to 421 ppm
It fell from 421 to 317 ppm
It stayed near 280 ppm
It rose and then fell

Show the answer

The Keeling curve rose every year.

Correct: It rose from about 317 to 421 ppm:
It fell from 421 to 317 ppm:
It stayed near 280 ppm:
It rose and then fell:

Part 4 · See it

See it first

Carbon dioxide from the air dissolves in the surface ocean. CO2 + H2O forms H2CO3, carbonic acid, which releases a hydrogen ion and a bicarbonate ion (H+ + HCO3-): more hydrogen ions, so pH falls. Hydrogen ions also combine with carbonate ions (H+ + CO3 2- gives HCO3-), so fewer carbonate ions are left for corals and shellfish to build calcium carbonate. A pH scale below shows average surface ocean pH of about 8.2 around 1750, about 8.1 today and about 7.8 by 2100 if emissions stay high. — How dissolved CO₂ lowers pH and takes carbonate ions away from shell-builders, with average surface ocean pH past, present and projected. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

Burning fossil fuels raises CO₂ in the airthe ocean absorbs about a quarter of it
Dissolved CO₂ reacts with water to form carbonic acidit releases hydrogen ions, so pH falls
Hydrogen ions combine with carbonate ionsless carbonate is left for building calcium carbonate
Shell-builders such as corals, oysters and some plankton strugglereefs weaken and food webs that depend on them suffer

Part 6 · Key ideas

Key ideas

Ocean acidification: seawater pH falls as it absorbs CO₂. Average surface pH has dropped from about 8.2 to about 8.1 since the 1700s and could reach about 7.8 by 2100 if emissions stay high.
Chemistry: $\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$ (carbonic acid) $\rightarrow \text{H}^+ + \text{HCO}_3^-$ ; then $\text{H}^+ + \text{CO}_3^{2-} \rightarrow \text{HCO}_3^-$ removes carbonate.
The pH scale is logarithmic: hydrogen ions multiply by $10^{\text{drop}}$ . A drop of 0.1 is about 26% more H⁺; a drop of 0.3 is about twice as much.
The ocean is still basic (pH above 7); "acidification" names the direction of change.

Part 7 · Misconception

A common mistake

The wrong idea: Ocean acidification is caused by acid rain, and it means the ocean is now an acid.

What actually happens: It is caused by CO₂ dissolving straight from the air, not by the SO₂ and NOx of acid rain. Seawater is still basic, around pH 8.1; it is becoming less basic.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. When CO₂ dissolves in seawater, which product makes the water more acidic?

Oxygen gas released by the reaction
Hydrogen ions from carbonic acid
Calcium carbonate dissolving out of the water
Nitrogen gas from the air

Show the answer

pH measures hydrogen ions: more H⁺ means lower pH and more acidic water.

Oxygen gas released by the reaction: The reaction does not release oxygen.
Correct: Hydrogen ions from carbonic acid: Right: CO₂ + H₂O → H₂CO₃, which releases H⁺. More H⁺ means lower pH.
Calcium carbonate dissolving out of the water: Calcium carbonate is what shells are made of; acidification makes it harder to build.
Nitrogen gas from the air: Nitrogen is not part of the reaction.

2. Why does ocean acidification make it harder for corals, clams and some plankton to build shells and skeletons?

Acid water is too cold for shells to form
Dissolved CO₂ blocks sunlight from reaching the reef
Hydrogen ions bind carbonate, the shell material
Acid water has too much carbonate for shells to use

Show the answer

Fewer carbonate ions mean shell-builders spend more energy making calcium carbonate, and existing shells can start to dissolve.

Acid water is too cold for shells to form: Acidification is a chemical change, not a temperature change.
Dissolved CO₂ blocks sunlight from reaching the reef: CO₂ in water does not block light.
Correct: Hydrogen ions bind carbonate, the shell material: Right: H⁺ + CO₃²⁻ → HCO₃⁻ removes the carbonate these animals need.
Acid water has too much carbonate for shells to use: It has less carbonate, not more.

Data

Atmospheric CO₂ and seawater pH at one ocean station

Yearly averages of atmospheric CO₂ and surface seawater pH at an open-ocean station (invented data for practice, shaped like long-running station records)
Year	Atmospheric CO₂ (ppm)	Seawater pH
1990	354	8.110
1995	361	8.101
2000	370	8.093
2005	380	8.084
2010	390	8.075
2015	401	8.064
2020	414	8.053

3. Describe the relationship between atmospheric CO₂ and seawater pH in the table.

As CO₂ rose, pH fell
As CO₂ rose, pH rose
CO₂ and pH both stayed the same
pH fell until 2005, then rose

Show the answer

An inverse relationship: more CO₂ in the air, more CO₂ dissolved, more carbonic acid, lower pH.

Correct: As CO₂ rose, pH fell: Right: CO₂ went from 354 to 414 ppm while pH fell from 8.110 to 8.053.
As CO₂ rose, pH rose: pH fell in every five-year step.
CO₂ and pH both stayed the same: Both changed in every step.
pH fell until 2005, then rose: pH kept falling through 2020.

4. By how much did the seawater pH change from 1990 to 2020? Give the change as later minus earlier.

Type a number. Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$8.053 - 8.110 = -0.057$ pH units.

Answer: -0.057

5. Write the setup for the percent change in atmospheric CO₂ from 1990 to 2020 in the table, then the answer.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

$\frac{(414 - 354)}{354} \times 100$ $\approx 16.9\%$

Setup: $\frac{414 - 354}{354} \cdot 100$ ; answer 16.95 %

6. The pH scale is logarithmic: hydrogen ion concentration multiplies by 10 for each drop of 1 pH unit, so it multiplies by 10 raised to the drop. Using the table, by what factor did hydrogen ion concentration increase from 1990 to 2020?

Type a number. Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$10^{(8.110 - 8.053)}$ $= 10^{0.057}$ $\approx 1.14$ : about 14% more hydrogen ions in 30 years.

Answer: 1.14

7. Which action would do the most to slow ocean acidification worldwide?

Adding fertilizer to the open ocean
Cutting CO₂ emissions from burning fossil fuels
Phasing out CFCs
Banning sulfur in gasoline

Show the answer

Acidification is driven by CO₂ itself, so the global fix is cutting CO₂ emissions.

Adding fertilizer to the open ocean: Fertilizer causes algal blooms and dead zones.
Correct: Cutting CO₂ emissions from burning fossil fuels: Right: less CO₂ in the air means less dissolving into the ocean.
Phasing out CFCs: CFCs destroy ozone; they are not the cause of acidification.
Banning sulfur in gasoline: That reduces acid rain, not ocean acidification.

Part 9 · Summary

Summary

The ocean absorbs part of the CO₂ people add to the air. Dissolved CO₂ forms carbonic acid, which releases hydrogen ions and lowers pH; those ions also bind carbonate, so corals, oysters and some plankton struggle to build calcium carbonate. pH has fallen from about 8.2 to 8.1; because the scale is logarithmic, that is about 26% more hydrogen ions. Cutting CO₂ emissions is the fix.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Question set for this topic