The Atmosphere (WJEC GCSE Chemistry)

• Theories on the development of Earth’s atmosphere have altered and developed over time as instrumental analysis has improved
  • It is difficult to gather evidence about the early atmosphere because it happened 4.6 billion years ago
• The surface of the early Earth was molten for millions of years with no atmosphere 
• As cooling slowly occurred, the molten surface began to slowly solidify into land masses
• Volcanoes formed on the land masses
• One theory of how the early atmosphere formed suggests that the volcanoes released gases from the Earth’s interior through violent eruptions
• These eruptions released large amounts of carbon dioxide and water vapour, as well as nitrogen, hydrogen, and other gases which may have included small proportions of ammonia and methane 

How volcanoes affected the atmosphere

Volcanoes spewed out water, carbon dioxide and other gases from the Earth's interior

• Earth’s gravity prevented these gases from escaping into outer space and they formed the early atmosphere
• Analysis of the minerals in the Earth's crust enables scientists to deduce the gases present billions of years ago
• It is thought that the Earth's early atmosphere was similar to that of Venus or Mars today, consisting mainly of carbon dioxide and water vapour
• There was little or no oxygen present

• The Earth's atmosphere has changed over time
• While the surface of the Earth was still very hot the large quantities of water vapour remained in the gaseous state
• When conditions cooled sufficiently, the water vapour later condensed and fell to the surface of the Earth, forming the oceans

How carbon dioxide levels decreased

• Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and dissolves readily
• When the water vapour in Earth’s early atmosphere condensed large amounts of CO₂ dissolved in the oceans
• Carbonates were precipitated during this process which later formed sediments on the seabed
• As marine life began to evolve sea creatures began to appear which used up the carbonates to form shells and skeletons
  
  • Limestone and chalk are formed from these shells and skeletons 
• Green plants and algae began to evolve and absorbed considerable amounts of carbon dioxide during photosynthesis

Photosynthesis word equation

Carbon dioxide is used during photosynthesis

• Animals fed on the plants which transferred carbon to their tissues including bones and shells
• When these organisms died, their remains formed sedimentary rocks
• Some of the living organisms were buried under layers of mud when they died
• Over millions of years, the heat and pressure turned the dead organisms into fossil fuels, such as crude oil, natural gas and coal
• The formation of sedimentary rock and fossil fuels 'locked up' the carbon from carbon dioxide in the early atmosphere
• This is how the large amounts of carbon dioxide in the early atmosphere were reduced

How oxygen levels increased

Changes in atmospheric nitrogen

• In the early atmosphere, most of the nitrogen was in the form of ammonia released from volcanoes
• In sunlight, ammonia decomposed by reacting with oxygen to form nitrogen and water 

Decomposition of ammonia

The decomposition of ammonia by sunlight reduced the level of ammonia and increased the level of nitrogen in the atmosphere 

The present composition of the atmosphere

• The present composition of gases in the atmosphere has not changed much in 200 million years
• About four-fifths of the air is nitrogen and one-fifth is oxygen
• The remaining gases include carbon dioxide, water vapour and trace quantities of the noble gases

Pie chart of the current atmosphere

The two main gases in the air are nitrogen and oxygen

• Some useful gases in the air, such as nitrogen, oxygen, neon and argon, can be extracted and used
• Fractional distillation is used to separate these gases because they have different boiling points

Levels of O₂ of CO₂ in the atmosphere

• The following processes are involved in the maintenance of oxygen and carbon dioxide levels in the atmosphere because they use oxygen and release carbon dioxide
  • Combustion of fossil fuels, e.g. methane:

CH₄ + 2O₂ → CO₂ + 2H₂O 

• • Respiration: the production of energy in living things, represented by the equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂  + 6H₂O 

• Photosynthesis uses carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and light:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

• Carbon dioxide dissolves in the water in seas and oceans and is removed by shellfish for making their calcium carbonate shells

Maintaining the level of carbon dioxide 

• Carbon as carbonate, carbon dioxide or organic carbon compounds is present in the sea, the air and under the Earth
• There is a continuous cycle of these compounds between these sources called the carbon cycle
• There is a constant amount of carbon compounds in the sea, atmosphere and under the Earth
• As long as these are balanced, the amount of carbon dioxide in the atmosphere remains constant
• In the atmosphere, the main source of carbon is carbon dioxide
• Scientists are worried that increased combustion of fossil fuels will increase the amount of atmospheric carbon dioxide
  • Carbon dioxide that was used to form fossil fuels or carbonate rocks is described as 'locked in' 
  • Therefore, the combustion of fossil fuels is releasing carbon from millions of years ago which is disturbing the current equilibrium 
  • This will lead to increased global warming and unbalance the carbon cycle

 Carbon cycle diagram

The carbon cycle shows the movement of carbon through the Earth