Welcome: Carbon on the Move
The carbon atoms in your body, in your lunch and in the fuel of a minibus are always moving from place to place. Air is also full of useful gases, and even water! In this chapter you will learn to draw the carbon cycle, see how scientists remove and store carbon dioxide, get gases from liquid air, harvest water from air and test for three gases. (The gases in air are in the chapter Air; pollutants and the AQHI are in Air quality.)
1. The Carbon Cycle
Carbon is found in carbon dioxide (CO2) in the air, in living things, in fossil fuels and in the oceans. The carbon cycle shows how carbon atoms move between these places again and again. This is change and constancy: the total amount of carbon stays the same, but where it is stored keeps changing.
Carbon stores
A carbon store is a place that holds carbon: the air (carbon dioxide, only about 0.04% of air), living things, dead matter and soil, fossil fuels (coal, crude oil, natural gas) and the oceans.
Six processes that move carbon
Photosynthesis
Green plants and algae take in carbon dioxide to make food. Carbon moves from the air into plants, then passes to animals along food chains.
Respiration
All living things – plants, animals and microorganisms – respire and give out carbon dioxide, returning carbon to the air. (The word equations are in the chapter Air.)
Decomposition
Decomposers (bacteria and fungi) break down dead bodies and waste, like fallen leaves on a Country Park trail. As they respire, they release carbon dioxide.
Combustion
Burning fuels such as wood, coal, natural gas and petrol releases carbon dioxide – in power stations, cars and the gas stove at home.
Fossil fuel formation
Some dead organisms are buried before they decompose. Over millions of years, heat and pressure turn them into coal, crude oil and natural gas, locking carbon underground.
Ocean absorption
Carbon dioxide dissolves in sea water, making the oceans a huge carbon store. Sea animals use carbon to build shells of calcium carbonate. The ocean can also release carbon dioxide, especially when it warms up.
Drawing the carbon cycle diagram
Draw a box for each store and a labelled arrow for each process:
Air → plants: photosynthesis
Plants → animals: feeding
Plants and animals → air: respiration
Plants and animals → dead matter: death and waste
Dead matter → air: decomposition
Dead matter → fossil fuels: fossil fuel formation
Fossil fuels → air: combustion
Air ⇄ oceans: dissolving and release
Memory aid: "One takes, three give." Photosynthesis takes carbon dioxide out of the air; respiration, decomposition and combustion give it back.
Common mistake: Plants respire too, all day and night – so draw a respiration arrow from plants to the air, not only from animals!
A balance that humans are changing
Naturally, the carbon taken out of the air was roughly balanced by the carbon put back. Today, burning fossil fuels releases carbon locked away for millions of years, and cutting down forests means less photosynthesis. Carbon dioxide is a greenhouse gas, so more of it strengthens the greenhouse effect and causes global warming. The oceans absorb more too (ocean acidification is in the chapter Rocks and minerals).
Did you know? Carbon atoms are recycled so often that some of the carbon in your body may once have been part of a dinosaur!
Key takeaway: Photosynthesis removes carbon dioxide from the air; respiration, decomposition and combustion return it. Humans are moving extra carbon into the air.
2. Removing and Storing Carbon Dioxide
The best way to slow global warming is to release less carbon dioxide. Scientists are also finding ways to remove carbon dioxide from the air and store it for a long time.
Forestation
Forestation means planting trees on new land or where forests were cut down. Trees take in carbon dioxide by photosynthesis and store carbon in their wood – think of tree planting in Hong Kong's Country Parks.
Pros: cheap, natural, gives homes to wildlife. Cons: needs much land and many years; carbon returns to the air if trees rot or burn in a hill fire.
Biochar
Biochar is made by heating plant waste (wood chips, crop leftovers) with little or no oxygen, so it does not burn but becomes a charcoal-like solid. Mixed into soil, it keeps the carbon locked up for hundreds of years.
Pros: uses waste, improves soil. Cons: needs equipment and energy; limited plant waste.
Direct air capture
Direct air capture machines use big fans to pull air through filters or chemicals that trap carbon dioxide. The gas is then stored deep underground or used to make products.
Pros: needs little land; easy to measure. Cons: carbon dioxide is a tiny part of air, so huge amounts of air must be processed; uses lots of energy and is expensive.
Carbon capture and storage (CCS)
In CCS, carbon dioxide is caught at power stations and factories before it escapes, then piped deep underground into rock layers such as old oil and gas fields.
Pros: stops a lot of carbon dioxide from big sources. Cons: expensive; needs suitable rocks; sites must be checked for leaks.
Key takeaway: Forestation and biochar use plants; direct air capture and CCS use machines. Each has pros and cons, and none replaces cutting emissions.
3. Fractional Distillation of Liquid Air
Air is a mixture, mainly nitrogen and oxygen with a little argon. To get pure gases, air is turned into a liquid and separated by boiling point. This is fractional distillation of liquid air.
Boiling points
Nitrogen about -196 °C, argon about -186 °C, oxygen about -183 °C. The lowest boiling point boils off first: nitrogen, then argon, then oxygen. Memory aid: "N-A-O".
The steps
1. Filter – remove dust.
2. Remove water vapour and carbon dioxide – they would freeze into solids (ice at 0 °C, solid carbon dioxide at about -78 °C) and block the pipes.
3. Compress and cool the air again and again to about -200 °C, so it becomes liquid air.
4. Warm slowly in a tall fractionating column. Nitrogen boils first and is collected at the top as a gas.
5. Argon boils next (collected from the middle). Oxygen boils last and collects at the bottom.
Uses of the gases
Nitrogen: liquid nitrogen freezes food quickly and stores blood and medical samples; nitrogen gas keeps packets of potato chips fresh; used to make fertilisers.
Oxygen: hospitals, mountain climbers, steelmaking, welding and cutting metals, rocket engines (liquid oxygen).
Argon: unreactive, so it fills some light bulbs and protects hot metal during welding.
Common mistake: Oxygen does not boil off first! With negative numbers, -196 °C is colder than -183 °C, so nitrogen has the lowest boiling point and boils first.
Crude oil is also separated by fractional distillation – see Materials of the modern world.
Key takeaway: Clean, dry, carbon-dioxide-free air is cooled to about -200 °C. As the liquid air warms, nitrogen, argon and oxygen boil off in that order.
4. Harvesting Fresh Water from the Air
Air contains water vapour. When cooled, it condenses into tiny droplets; fog is a cloud of these droplets near the ground. Where rain is scarce, people collect this water.
Fog nets
Large mesh nets stand on windy, foggy hills. Fog droplets hit the threads, join into bigger drops and run into a gutter, then pipes carry the water to tanks. Villages in dry coastal mountains, such as parts of Chile and Morocco, use them.
Dew and machines
On clear nights, cold surfaces collect dew (condensed water vapour), which special sheets can channel into containers. Some machines use electricity to cool air and condense water, then clean it. A home dehumidifier works similarly on humid Hong Kong spring days – but never drink its water, as it is not clean.
Pros and cons: Fog nets and dew collectors are cheap and need no electricity, but only work in certain places and weather. Machines work in more places but use lots of energy. The water may need cleaning before drinking.
Did you know? A beetle in Africa's Namib Desert drinks fog water collected on its bumpy back, and scientists have copied the idea for fog-collecting surfaces!
Key takeaway: We can harvest fresh water from air with fog nets, dew collectors and machines.
5. Tests for Oxygen, Carbon Dioxide and Hydrogen
These three gases are all colourless, so we use chemical tests (oxygen and carbon dioxide tests also appear in Air).
The three tests
Oxygen: a glowing splint relights.
Carbon dioxide: bubbled through limewater (calcium hydroxide solution), it turns the limewater milky.
Hydrogen: a burning splint at the mouth of the tube gives a "pop" sound as hydrogen burns to form water. Hydrogen is lighter than air, so hold its tube upside down.
Hydrogen can be made by electrolysis of water (see The ocean and sea water) or by reacting some metals with dilute acid (see Rocks and minerals).
Identifying three unknown gases
1. Glowing splint into each gas – the one that relights it is oxygen.
2. Burning splint to fresh samples of the other two – a "pop" means hydrogen.
3. Bubble the last gas through limewater – milky means carbon dioxide.
Wear goggles and use only small amounts of hydrogen.
Common mistake: A burning splint going out does not prove carbon dioxide – nitrogen also puts out flames. Always use limewater.
Key takeaway: Glowing splint relights = oxygen; limewater milky = carbon dioxide; "pop" with burning splint = hydrogen.
Quick Review
1. Carbon moves between air, living things, dead matter, fossil fuels and oceans; the total stays the same.
2. Photosynthesis takes carbon dioxide from the air; respiration, decomposition and combustion return it.
3. Forestation, biochar, direct air capture and CCS remove or store carbon dioxide.
4. Liquid air (about -200 °C) separates as nitrogen (-196 °C), argon (-186 °C), then oxygen (-183 °C).
5. Fog nets, dew collectors and machines harvest water from air.
6. Oxygen relights a glowing splint; carbon dioxide turns limewater milky; hydrogen pops.
Great work – you are now a carbon cycle and gas-testing expert!