From the breath you exhale to the fuel in a bus, chemical reactions are constantly exchanging carbon between Earth, air, ocean and life. How do these reactions shape our climate? And how can we make them cleaner, safer and more sustainable?
Australia produces millions of tonnes of carbon dioxide each year from burning coal, driving cars and clearing land. At the same time, plants absorb CO₂ through photosynthesis.
Write down your answers before reading on:
How carbon moves through Earth systems
Carbon is the backbone of life, and chemical reactions keep it cycling between air, water, land and organisms.
Photosynthesis removes CO₂ from the atmosphere. Plants use sunlight to convert carbon dioxide and water into glucose and oxygen:
carbon dioxide + water → glucose + oxygen
Cellular respiration returns CO₂ to the atmosphere. Plants, animals and microorganisms break down glucose using oxygen:
glucose + oxygen → carbon dioxide + water + energy
Combustion of fossil fuels also releases CO₂. When coal, oil or natural gas burn, the carbon that was stored underground for millions of years is rapidly released:
fuel + oxygen → carbon dioxide + water + energy
When you burn wood from a recently grown tree, the CO₂ released was captured by that tree within the last few decades. When you burn coal, the CO₂ was captured by ancient forests millions of years ago. This "old" carbon increases the total amount of CO₂ in the atmosphere today.
Reducing the environmental impact of burning
Scientists and engineers are developing ways to burn fuels more cleanly and to use fuels that produce less net CO₂.
Ethanol (alcohol made from fermented plant material) can be blended with petrol. The plants that produce ethanol absorbed CO₂ as they grew, so the net carbon emissions are lower than fossil fuels. In Australia, sugarcane and wheat are used to produce ethanol.
Biodiesel is made from vegetable oils or animal fats. It burns more cleanly than conventional diesel, producing less sulfur dioxide and particulate matter.
When hydrogen burns, the only product is water:
hydrogen + oxygen → water + energy
Hydrogen is a promising alternative fuel because it produces no CO₂ during combustion. However, producing hydrogen sustainably is still a challenge. Australia is investing in "green hydrogen" produced using renewable electricity to split water.
Catalytic converters in car exhausts use catalysts to speed up reactions that convert harmful gases into less harmful products. For example, they convert carbon monoxide (CO) into carbon dioxide (CO₂) and nitrogen oxides into nitrogen gas.
Chemical reactions that break materials down
When materials are discarded, chemical reactions determine whether they return safely to the environment or persist as pollution.
Biodegradable materials can be broken down by microorganisms through decomposition reactions. Examples include paper, cotton, wood and food scraps. These materials are broken into simpler substances like water, carbon dioxide and minerals.
Non-biodegradable materials resist natural decomposition. Many plastics are made from long-chain polymers that microorganisms cannot easily break apart. These materials can persist in the environment for hundreds of years, accumulating in oceans and landfills.
| Biodegradable | Non-biodegradable |
|---|---|
| Paper and cardboard | Most conventional plastics |
| Cotton and wool | Styrofoam (expanded polystyrene) |
| Food waste | Some synthetic fabrics |
| Untreated wood | Glass (persists but is inert) |
| Compostable bioplastics | Metals (rust very slowly) |
"Burning biofuels produces no CO₂ at all." No — burning biofuels does release CO₂. The advantage is that the plants recently absorbed that CO₂ from the atmosphere, so the net increase is lower than fossil fuels.
"All plastics are non-biodegradable." No — some newer bioplastics are designed to be biodegradable, but they often need specific conditions. Most everyday plastics are not biodegradable.
Aboriginal and Torres Strait Islander Peoples have practised sophisticated chemical knowledge for tens of thousands of years, developed through careful observation of Country and deep understanding of materials.
Cool burning demonstrates controlled combustion chemistry. By burning at low intensity when conditions are right, Traditional Custodians manage fuel loads without reaching the temperatures that destroy mature trees. This practice reduces the risk of catastrophic hot fires — a chemistry-informed land management strategy.
Tool making involves chemical reactions. Heating certain rocks changes their properties through thermal decomposition, making them easier to shape into stone tools. Resins and plant saps are heated to create adhesives for hafting spear points.
Food preparation uses chemical reactions to detoxify and preserve. Some plant foods contain toxins that are broken down through leaching, fermentation or heating. These are deliberate chemical processes applied to make food safe and nutritious.
Traditional Knowledge about chemical processes on Country is the Cultural and Intellectual Property of Aboriginal and Torres Strait Islander Peoples and should be acknowledged and respected.
1. In the carbon cycle, which reaction removes carbon dioxide from the atmosphere?
2. Why does burning fossil fuels contribute more to climate change than burning recently grown wood?
3. Which word equation correctly represents the complete combustion of hydrogen?
4. A "biodegradable" plastic bottle is thrown into the ocean. After two years, it is still intact. Which statement best explains this observation?
5. A student claims that catalytic converters make cars environmentally friendly because they remove all pollutants. Which evaluation is most accurate?
1. Explain the difference between biodegradable and non-biodegradable materials. Include one example of each and describe the chemical reactions (or lack of them) that determine their environmental fate. 4 MARKS
2. Using word equations, explain how the carbon cycle moves carbon between the atmosphere, plants and animals. Then explain why burning fossil fuels disrupts this cycle. 4 MARKS
3. Aboriginal cool burning demonstrates a sophisticated understanding of combustion chemistry. Explain how controlling the fuel, temperature and oxygen supply changes the combustion reaction, and how this helps manage the Australian landscape. 4 MARKS
Go back to your Think First answer. Has your understanding changed?
B — Photosynthesis removes CO₂ from the atmosphere by converting it into glucose. Combustion and respiration both add CO₂ to the atmosphere.
C — Fossil fuels contain carbon that was removed from the atmosphere millions of years ago and stored underground. Burning them releases this "old" carbon, increasing the total CO₂ in the atmosphere today. Recently grown wood recycles carbon that was in the atmosphere within the last few decades.
A — Hydrogen + oxygen → water + energy. This is why hydrogen is considered a clean fuel — it produces no carbon dioxide.
D — Many biodegradable plastics need specific conditions like heat, moisture and microorganisms found in industrial composting facilities. The ocean does not provide these conditions, so the bottle persists.
B — Catalytic converters reduce harmful gases like carbon monoxide and nitrogen oxides, but they do not remove CO₂. Cars still produce CO₂ through fuel combustion, contributing to climate change.
Model answer: Biodegradable materials can be broken down by natural chemical reactions (decomposition) carried out by microorganisms. For example, a cotton T-shirt will decompose into simpler substances like carbon dioxide, water and minerals when buried in soil. Non-biodegradable materials resist these natural decomposition reactions. For example, a conventional plastic bottle is made of long-chain polymers that microorganisms cannot easily break apart, so it persists in the environment for hundreds of years, potentially harming wildlife and ecosystems.
Model answer: Photosynthesis removes carbon from the atmosphere: carbon dioxide + water → glucose + oxygen. Cellular respiration returns it: glucose + oxygen → carbon dioxide + water + energy. In a natural cycle, the CO₂ released by respiration was recently captured by photosynthesis, so the amount of CO₂ in the atmosphere stays relatively balanced. Burning fossil fuels disrupts this because the carbon in coal, oil and gas was removed from the atmosphere millions of years ago. Combustion releases this ancient carbon rapidly, increasing atmospheric CO₂ faster than photosynthesis can remove it, leading to climate change.
Model answer: Cool burning controls the amount of fuel (fine leaves and bark rather than heavy logs), which limits how much heat the combustion reaction can produce. With less fuel, the fire temperature stays lower, and less oxygen is consumed. This means mature trees survive because the fire does not reach the high temperatures needed to kill them. The landscape benefits because fuel loads are reduced, preventing catastrophic hot fires, and the ecosystem maintains its biodiversity. This demonstrates deep understanding of how controlling reactants (fuel and oxygen) controls the reaction.
Blast through environmental chemistry challenges! Match reactions to their impacts, spot biodegradable materials and race to save the planet.
Tick when you have finished all activities and checked your answers.