Carbon Cycle Chemistry

The carbon cycle is tested in the Cambridge 0620 environmental chemistry section. Questions focus on the chemical reactions that move carbon between reservoirs and how human activity has disrupted the natural balance.

Carbon reservoirs

Carbon is stored in four main reservoirs:

Reservoir	Form of carbon	Example
Atmosphere	Carbon dioxide (CO₂), methane (CH₄)	Air (~0.04% CO₂)
Biosphere	Organic compounds	Glucose in plants, fats and proteins in animals
Hydrosphere	Dissolved CO₂, carbonates	Ocean water, coral reefs
Lithosphere	Carbonates, fossil fuels	Limestone (CaCO₃), coal, crude oil, natural gas

Processes that add CO₂ to the atmosphere

Combustion

Burning fossil fuels or wood releases CO₂:

CH₄ + 2O₂ → CO₂ + 2H₂O (burning methane/natural gas)

C + O₂ → CO₂ (burning coal/charcoal)

This is the largest human contribution to atmospheric CO₂.

Respiration

All living organisms (plants, animals, bacteria) respire:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Respiration releases CO₂ continuously, day and night.

Thermal decomposition of carbonates

Heating limestone in cement production and lime kilns:

CaCO₃ → CaO + CO₂

This is a significant industrial source of CO₂.

Volcanic activity

Volcanoes release CO₂ from deep within the Earth. This is a natural process but contributes a relatively small amount compared to human activities.

Processes that remove CO₂ from the atmosphere

Photosynthesis

Green plants absorb CO₂ and convert it to glucose using sunlight:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This is the main natural mechanism for removing CO₂ from the atmosphere. Tropical rainforests, including those in Malaysia (which has one of the oldest rainforests in the world), are vital carbon sinks.

Dissolving in oceans

CO₂ dissolves in seawater to form carbonic acid:

CO₂ + H₂O → H₂CO₃

The oceans absorb about 25% of human CO₂ emissions. However, this is causing ocean acidification, which threatens coral reefs and marine life — an issue of particular concern for Malaysia’s marine ecosystems.

Formation of sedimentary rocks

Over millions of years, carbon is locked into limestone (CaCO₃) and other carbonate rocks through the accumulation of shells and skeletons of marine organisms.

Formation of fossil fuels

Dead organisms buried under layers of sediment over millions of years form coal (from plants), crude oil, and natural gas (from marine organisms). This process removes carbon from the active cycle for very long periods.

The balanced cycle

In a balanced carbon cycle:

• CO₂ added to the atmosphere (by combustion, respiration, decomposition) equals CO₂ removed (by photosynthesis, dissolving in oceans)
• Atmospheric CO₂ levels remain roughly constant

Human disruption

Two main human activities have disrupted the balance:

1. Burning fossil fuels

Fossil fuels contain carbon that has been locked away for millions of years. Burning them releases this “ancient” carbon as CO₂, adding carbon to the atmosphere faster than natural processes can remove it.

2. Deforestation

Cutting down and burning forests reduces the number of trees available for photosynthesis (less CO₂ removed) and releases the carbon stored in the wood (more CO₂ added). Both effects increase atmospheric CO₂.

The result: atmospheric CO₂ has increased from about 280 ppm (pre-industrial) to over 420 ppm today.

Consequences

Increased atmospheric CO₂ enhances the greenhouse effect, leading to:

• Rising global temperatures
• Ice cap melting and sea level rise
• More extreme weather events
• Ocean acidification

Worked exam question

Describe how carbon in a coal deposit can end up as carbon dioxide in the atmosphere and then become part of a plant. [3]

Coal is burned (combustion) and the carbon reacts with oxygen to form CO₂, which is released into the atmosphere [1]. The CO₂ in the atmosphere is absorbed by green plants during photosynthesis [1]. The plant uses the CO₂ (with water and sunlight) to make glucose, incorporating the carbon into organic molecules in the plant [1].

Common exam mistakes

1. Confusing photosynthesis and respiration — photosynthesis removes CO₂ from the atmosphere; respiration adds it. Plants do both, but overall they remove more CO₂ than they produce (net absorbers).
2. Saying only animals respire — plants also respire, 24 hours a day.
3. Forgetting to mention deforestation alongside fossil fuel combustion when discussing human impact on the carbon cycle.
4. Saying the carbon cycle is “broken” — it is not broken, but human activity has shifted the balance so that more CO₂ is added than removed.