The Atmosphere

Section 1 of 7The early atmosphere

Evidence is limited

• Evidence is limited The early atmosphere formed about 4.6 billion years ago,
  • so evidence is hard to find and theories have changed as analysis improved.
• Scientists study minerals in the Earth's crust to deduce which gases were present.

Volcanic origin

• Volcanic origin The molten surface cooled and solidified into land, and volcanoes released gases from the interior.
• These were mainly carbon dioxide and water vapour, with nitrogen, hydrogen and small amounts of ammonia and methane.

Little oxygen

• Little oxygen Gravity kept the gases from escaping into space.
• The early atmosphere was probably like that of Venus or Mars today, with almost no oxygen.

Section 2 of 7How carbon dioxide levels fell

1Oceans form

• Oceans form When the Earth cooled, water vapour condensed and fell as rain to make the oceans.

2CO₂ dissolves

• CO₂ dissolves Carbon dioxide is soluble, so large amounts dissolved in the oceans and carbonates were precipitated.

3Shells and sediments

• Shells and sediments Marine organisms used carbonates for shells and skeletons, which formed limestone and chalk.

4Photosynthesis

• Photosynthesis Algae and plants absorbed carbon dioxide, and dead remains formed sedimentary rock and fossil fuels.

Section 3 of 7Oxygen and nitrogen

Oxygen rises

• Oxygen rises Algae appeared about 2.7 billion years ago and later small green plants evolved.
• Photosynthesis
  • 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
  • It removed carbon dioxide and released oxygen, allowing complex life to evolve.

Nitrogen rises

• Nitrogen rises Most early nitrogen was in ammonia from volcanoes.
• In sunlight, ammonia reacted with oxygen to form nitrogen and water, so nitrogen built up.

Today's air

• Today's air The composition has changed little for about 200 million years: roughly four-fifths nitrogen and one-fifth oxygen, with small amounts of carbon dioxide, water vapour and noble gases.

Section 4 of 7Keeping oxygen and carbon dioxide balanced

Process	Equation	Effect on the atmosphere
Combustion of methane	CH4 + 2O2 → CO2 + 2H2O	Uses oxygen, releases carbon dioxide
Respiration	C6H12O6 + 6O2 → 6CO2 + 6H2O	Uses oxygen, releases carbon dioxide
Photosynthesis	6CO2 + 6H2O → C6H12O6 + 6O2	Uses carbon dioxide, releases oxygen
Dissolving in the sea	Carbon dioxide dissolves; shellfish build calcium carbonate shells	Removes carbon dioxide

Section 5 of 7The carbon cycle and useful gases

Carbon cycle

• Carbon cycle Carbon moves between the sea, the air and the ground as carbonates, carbon dioxide and organic compounds.
• When these flows are balanced, the amount of carbon dioxide in the atmosphere stays constant.

Separating air

• Separating air Nitrogen, oxygen, neon and argon are extracted from air for use.
• They are separated by fractional distillation because their boiling points differ.

Section 6 of 7Key terms

Term	Meaning
Atmosphere	The layer of gases surrounding the Earth.
Photosynthesis	The process in which plants and algae use light to turn carbon dioxide and water into glucose and oxygen.
Sedimentary rock	A rock formed from layers of sediment, such as limestone.
Fossil fuel	A fuel such as coal, oil or natural gas formed from the buried remains of living things.
Fractional distillation	Separating a mixture by differences in boiling point.

Section 7 of 7Exam technique

1

Saying the early atmosphere was mostly oxygen - it contained very little and was mostly carbon dioxide.

2

Forgetting that carbon dioxide was locked up in limestone, chalk and fossil fuels as well as dissolving in the oceans.

3

Stating that the air is 80 percent oxygen - it is about one-fifth oxygen and four-fifths nitrogen.

4

Mixing up respiration and photosynthesis - respiration releases carbon dioxide, photosynthesis uses it.