Acids, Bases and the Environment
AIMS researchers have documented declines in coral calcification on the Great Barrier Reef while rising atmospheric CO2 is changing the seawater chemistry corals rely on to build their skeletons.
Printable Worksheets
Print or save as PDF, or build a custom worksheet from any module's questions.
The Great Barrier Reef is one of Australia's most iconic natural wonders. You've probably heard it is under threat. What do you think carbon dioxide in the atmosphere could have to do with damage to a coral reef in the ocean?
Acid rain has damaged forests and lakes in Europe and North America. What gases do you think cause acid rain, and where do those gases come from? Do you think this could be a problem in Australia, why or why not?
● Know
- The causes and effects of acid rain
- The connection between carbon dioxide and ocean acidification
- That Aboriginal and Torres Strait Islander Peoples have used natural indicators and understood soil chemistry
● Understand
- How human activities contribute to environmental changes in pH
- Why ocean acidification threatens marine ecosystems including the Great Barrier Reef
- How neutralisation can be used to address environmental problems
● Can do
- Explain the causes and effects of acid rain using scientific reasoning
- Describe how ocean acidification occurs and its consequences
- Communicate scientific arguments about environmental chemistry with evidence
Coral skeletons are made largely from calcium carbonate, and changes in seawater chemistry can make it harder for corals to build and maintain that material. Ocean acidification occurs when carbon dioxide dissolves in seawater and changes the carbonate system. Dissolved CO2 forms carbonic acid (H2CO3), which can release hydrogen ions and lower pH. Since pre-industrial times, average surface-ocean pH has fallen by about 0.1 units, from roughly 8.2 to 8.1. Because pH is logarithmic, that small numerical change corresponds to roughly a 26% increase in hydrogen-ion concentration.
The chemistry can be represented as linked equilibria:
CO2 + H2O <-> H2CO3
H2CO3 <-> H+ + HCO3-
HCO3- <-> H+ + CO3 2-
When hydrogen-ion concentration rises, some carbonate ions react with H+ to form bicarbonate: H+ + CO3 2- <-> HCO3-. This reduces the carbonate-ion concentration available to marine organisms. Corals, shellfish and some plankton use carbonate ions to build calcium carbonate (CaCO3) shells and skeletons.
Lower carbonate availability makes calcification more difficult. Young organisms and those in cold water can be especially vulnerable. If acidification continues, changes to calcifying organisms can affect marine food webs, fisheries and the coastal protection provided by reef structures.
Pteropods (sea butterflies) are tiny marine snails that swim using wing-like appendages. They have delicate aragonite shells that are particularly soluble in acidic water. In laboratory experiments, pteropod shells begin to dissolve within 45 days in water with pH projected for Southern Ocean conditions in 2050. Pteropods are a major food source for salmon, whales, and seabirds. Their decline could cascade through Southern Ocean food webs, affecting Australian fisheries and marine ecosystems.
Australian reef science: The Australian Institute of Marine Science (AIMS) and the ARC Centre of Excellence for Coral Reef Studies monitor ocean chemistry across the Great Barrier Reef. Their data confirms that reef waters are acidifying in parallel with atmospheric CO2 increases. Researchers at AIMS are studying whether corals can adapt to changing chemistry through natural selection, and whether assisted evolution techniques might help reefs survive. The Great Barrier Reef contributes $6.4 billion annually to the Australian economy, making acidification an economic as well as ecological threat.
Ocean acidification means the ocean will become literally acidic (pH < 7). This is false. Ocean pH is currently about 8.1, which is basic. Acidification means the pH is moving toward the acidic end of the scale, not that it has crossed into acidic territory. The term acidification refers to the direction of change, not the absolute state. However, even small pH changes matter enormously to marine chemistry because the carbonate system is highly sensitive around current pH levels.
Click each sentence that supports the claim.
Acid rain is rain with pH lower than 5.6 (the pH of pure water in equilibrium with atmospheric CO2). It forms when sulfur dioxide (SO2) and nitrogen oxides (NOx) from burning fossil fuels dissolve in atmospheric water to form sulfuric acid and nitric acid.
The reactions are:
SO2 + H2O → H2SO3 (sulfurous acid) → H2SO4 (sulfuric acid)
2NO2 + H2O → HNO3 (nitric acid) + HNO2 (nitrous acid)
Acid rain damages buildings and statues made of limestone or marble (calcium carbonate), which react with acid to form soluble calcium salts. It acidifies lakes and streams, harming fish and aquatic plants. It leaches aluminium from soils, which is toxic to tree roots.
In Australia, acid rain is less severe than in Europe or North America because Australian coal has lower sulfur content and because the vast interior lacks the industrial density that produces large emissions. However, urban areas like the Latrobe Valley and industrial zones around Port Pirie experience locally elevated acidity.
The tall chimneys of the former Hazelwood Power Station in Victoria Latrobe Valley released sulfur dioxide and nitrogen oxides that contributed to local acid deposition. While modern pollution controls (flue gas desulfurisation) reduce emissions significantly, historical deposition acidified some local soils and waterways. Rehabilitation programs now include limestone application to neutralise accumulated soil acidity, demonstrating how acid-base chemistry is used in environmental remediation.
Australian air quality: The National Pollutant Inventory tracks emissions of acid rain precursors from Australian industrial facilities. State environment protection authorities set limits on SO2 and NOx emissions. The shift from coal to renewable energy (solar, wind) is reducing these emissions nationally. CSIRO atmospheric chemists model how Australian emissions affect local and regional air quality, informing policy decisions about power generation and transport.
Acid rain is a problem only in cold climates with lots of industry. While acid rain is most severe in industrialised temperate regions, it can occur anywhere that burns sulfur-containing fuels. Volcanic eruptions also produce acid rain - the 1991 Mount Pinatubo eruption released enough SO2 to cause measurable global cooling and acid deposition thousands of kilometres away. Even remote areas can experience acid rain from distant sources or natural events.
Addressing ocean acidification and acid rain requires reducing the emissions that cause them. While some adaptation strategies exist, prevention is far more effective than cure.
Emission reductions: Transitioning from fossil fuels to renewable energy (solar, wind, hydro) reduces CO2, SO2, and NOx emissions simultaneously. Improving energy efficiency reduces the total energy needed. Electric vehicles and public transport reduce transport emissions.
Carbon capture: Some industrial facilities capture CO2 before it enters the atmosphere and store it underground (carbon capture and storage, CCS). Australia's Gorgon CCS project has been capturing and storing CO2 underground since 2019. CCS remains technically complex and energy-intensive, and its contribution should be described with dated project information rather than as a universal solution.
Local adaptation: For reefs, reducing other stressors (overfishing, pollution, physical damage) increases resilience to acidification. Marine protected areas allow ecosystems to recover and maintain biodiversity that might include more acid-tolerant species.
International cooperation: The Paris Agreement commits signatories to limiting global warming, which implicitly limits acidification. However, current commitments are insufficient to prevent significant acidification this century.
Australia has pledged to reach net-zero emissions by 2050. In calendar year 2025, coal generated 42.7% of Australia's electricity, so reducing electricity-sector emissions remains an important part of the transition alongside transport and industry. Projects such as Snowy 2.0, large solar farms and offshore wind development can expand lower-emissions generation. The emissions avoided by one megawatt-hour of renewable electricity depend on which source would otherwise have generated that electricity and on grid conditions, so there is no single universal CO2 saving per megawatt-hour.
Australian climate policy: The Australian Government Reef 2050 Long-Term Sustainability Plan recognises climate change as the greatest threat to the Great Barrier Reef. The plan includes emissions reduction targets, water quality improvements, and crown-of-thorns starfish control. However, independent reviews have criticised the plan for insufficient emissions ambition. The Climate Council launched in September 2013, five days after the Abbott Government abolished the Climate Commission, and continues as an independent organisation advocating for climate action.
Technology will fix acidification without us needing to change behaviour. This is dangerously optimistic. While technologies like CCS and geoengineering have been proposed, none are proven at the scale needed to offset global emissions. The most reliable solution is reducing emissions through behaviour change, policy, and infrastructure transformation. Waiting for a technological silver bullet while continuing current emissions is a form of denial that risks irreversible damage to marine ecosystems.
Wrong: "Acid rain is only a problem in other countries, not Australia." While Australia has less severe acid rain than some regions, industrial areas and coal-fired power stations can still produce locally acidic precipitation. Monitoring and regulation remain important.
Right: Acid rain affects Australia too, particularly around industrial areas and coal power stations. Although the problem is less severe than in some other countries, emissions of sulfur dioxide and nitrogen oxides can make rainfall locally acidic.
Wrong: "Ocean acidification means the ocean will become actually acidic (pH below 7)." No - the ocean is still basic (pH above 7). Acidification means it is becoming less basic, moving toward neutral. The term refers to the direction of change, not the endpoint.
Right: The ocean remains basic (currently around pH 8.1) and is not becoming acidic. "Acidification" describes the direction of change, the pH is decreasing, but the ocean is still well above neutral pH 7.
Wrong: "Indigenous knowledge is not scientific because it was not developed in laboratories." No - science is a process of observation, hypothesis testing and evidence gathering. Indigenous knowledge was developed through exactly this process, applied over millennia in the natural environment.
Right: Indigenous knowledge is a form of evidence-based knowledge built through careful observation and testing over thousands of years, the same foundations as scientific inquiry. The setting is different, but the process of evidence gathering is the same.
The Great Barrier Reef Under Pressure
The Great Barrier Reef is the largest living structure on Earth, visible from space and home to more than 1,500 species of fish. It contributes approximately $6.4 billion annually to the Australian economy through tourism and fishing.
Ocean acidification is one of several threats to the reef. Since the late 18th century, the ocean has absorbed about one-third of all human-produced CO2. This has reduced the availability of carbonate ions that corals need to build their calcium skeletons. Combined with rising water temperatures causing coral bleaching, acidification makes it harder for reefs to recover from disturbances.
Australian marine scientists at organisations such as the Australian Institute of Marine Science (AIMS) are monitoring ocean pH and studying how different coral species respond to more acidic conditions. Some coral species appear more resilient than others, which may guide reef restoration efforts.
✍ Copy Into Your Books
▾Acid Rain
- Caused by SO2 and NOx from burning fossil fuels
- Forms sulfuric and nitric acid in rain
- Damages lakes, forests and buildings
- Solutions: scrubbers, catalytic converters, renewable energy
Ocean Acidification
- CO2 dissolves in seawater, forming carbonic acid
- Ocean pH has dropped from 8.2 to 8.1
- Threatens corals, shellfish and marine ecosystems
- Solution: reduce CO2 emissions
Indigenous Knowledge
- Aboriginal peoples used plants as natural indicators
- Traditional fire management demonstrates chemistry understanding
- Protected under ICIP protocols
- Valued as evidence-based knowledge
Environmental Problem-Solver
Indigenous Science and Chemistry
At the start of this lesson, the hook introduced two connected observations: AIMS has documented declining coral calcification, and rising atmospheric CO2 is changing seawater carbonate chemistry.
Now that you understand how dissolved CO2 increases hydrogen-ion concentration and reduces carbonate-ion availability, can you trace the chemistry from atmospheric CO2 to more difficult coral calcification? How does this chemical stress interact with other reef pressures, such as ocean warming and bleaching?
Q1. 1. Describe the process of ocean acidification. Include: (a) what gas is involved, (b) what acid forms, and (c) why this threatens marine ecosystems. 4 MARKS
Q2. 2. Explain how neutralisation could be used to reduce the environmental impact of acid rain on a lake. Include a word equation in your answer. 4 MARKS
Q3. 3. Evaluate the argument that Indigenous knowledge of natural indicators and soil chemistry is just as scientific as laboratory-based chemistry. Use evidence from the lesson to support your evaluation. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Were your predictions about acid rain and ocean acidification correct?
- What new connections can you make between acids, bases and the environment?
Model answers (click to reveal)
Answers
▾MCQ 1
B - Carbon dioxide dissolves in seawater to form carbonic acid, causing ocean acidification.
MCQ 2
A - Acid rain is mainly caused by sulfur dioxide and nitrogen oxides from burning fossil fuels. Normal rain is already slightly acidic from CO2, but acid rain is much more acidic due to these additional gases.
MCQ 3
D - More acidic water reduces the availability of carbonate ions, making it harder for corals and other organisms to build and maintain calcium carbonate skeletons and shells.
MCQ 4
C - Burning more coal would release more sulfur dioxide and nitrogen oxides, making acid rain worse, not better. Scrubbers, catalytic converters and renewable energy all reduce acid rain.
MCQ 5
B - The student is incorrect. Even though the ocean remains alkaline, a lower pH means a higher hydrogen-ion concentration. That change reduces carbonate-ion availability, making it harder for corals and other calcifying organisms to build calcium carbonate structures. The direction and magnitude of the pH change matter, not just whether the current pH is above 7.
Short Answer 1
Model answer: (a) Carbon dioxide is the gas involved. It is released by burning fossil fuels and dissolves in seawater. (b) Carbonic acid forms when CO2 reacts with water. (c) This threatens marine ecosystems because many organisms, including corals, shellfish and plankton, need carbonate ions to build calcium carbonate shells and skeletons. As the water becomes more acidic, fewer carbonate ions are available, making it harder for these organisms to survive and grow. This affects the entire food web.
Short Answer 2
Model answer: To reduce the impact of acid rain on a lake, a base such as calcium carbonate (lime) can be added to the water. The base neutralises the acid in the lake, raising the pH to a level where aquatic life can survive. Word equation: sulfuric acid + calcium carbonate → calcium sulfate + water + carbon dioxide. This is the same neutralisation reaction used by farmers to treat acidic soil.
Short Answer 3
Model answer: This argument is valid. Indigenous knowledge of natural indicators and soil chemistry is scientific because it is based on careful observation, testing and evidence gathered over thousands of years. For example, Aboriginal peoples observed that certain plants only grow in specific soil conditions, acting as natural indicators of pH and mineral content - the same principle that underlies synthetic indicator paper. Cultural burning demonstrates understanding of combustion chemistry and how it affects soil nutrients. The knowledge was developed through the same scientific process of observation, prediction and verification that Western scientists use in laboratories. It should be respected and protected under ICIP protocols.