4.9 CHEMSITRY

• Proportions of different gases in the atmosphere

  • 78% nitrogen
  • 21% oxygen
  • Small proportions of various other gases, i.e. CO2, H2O(g) and noble gases
• For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today
• The Earth's early atmosphere

  • Evidence is limited because of the time scale of 4.6 billion years and theories have changed/developed over time
  • One theory suggests that during the first billion years of the Earth's existence, there was intense volcanic activity that released gases that formed the early atmosphere
  • At the start of this period, the atmosphere may have been like the atmospheres of Mars and Venus today: mainly CO2 with little or no O2(g)
  • Volcanoes also produced nitrogen (N2) which gradually built up in the atmosphere & there may have been small proportions of methane (CH4) and ammonia (NH3)
  • Water vapour condensed to form the oceans, and CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere
• This theory is the only theory of the early atmosphere you need to know
• How oxygen increased


  1. Algae & plants produced the O2 that is now in the atmosphere by photosynthesis

  2. Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere

  3. Over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolve
• How carbon dioxide decreased

  1. Algae and plants decreased the % CO2 in the atmosphere by photosynthesis

  2. CO2 was also decreased by the formation of sedimentary rocks that contain carbon (e.g. limestone and coal) and by the production of fossil fuels from the remains of dead plants and animals when they decayed
• Greenhouse gases


  Maintain temperatures on earth high enough to support life
• Greenhouse gases


  • Water vapour
  • CO2
  • CH4
• The Greenhouse effect:

  1. The sun emits short wavelength radiation to Earth, this is not absorbed by the greenhouse gases
  2. The Earth then emits long-wavelength radiation which is absorbed by the greenhouse gases and re-radiated back towards Earth in all directions , this long-wavelength radiation is in the form of thermal radiation, this results in the warming of our earth
• Human activities increase levels of CO2 & CH4
• Human activities that increase greenhouse gases

  • Driving (CO2)
  • Consuming electricity (CO2)
  • Raising livestock (cows - CH4)
  • Decay of organic waste in landfill sites (CH4)
• Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth's atmosphere to increase at the surface and that this will result in global climate change
• It is difficult to model such complex systems as global climate change, leading to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence and which may be biased
• Global climate change


  • An increase in average global temperature is a major cause
  • Potential effects include extinction of species, raising sea levels, migration, and decrease in crop yield
• Carbon footprint


  The total amount of CO2 and other greenhouse gases emitted over the full life cycle of a product, service or event
• Reducing carbon footprint


  Reducing emissions of carbon dioxide and methane e.g. by using less electricity, reducing travel or transportation, and planting more trees
• Action to reduce carbon footprint may be limited because it's more difficult, can be more expensive and planting trees takes away land that could be used to grow crops
• Combustion of fuels


  Major source of atmospheric pollutants
• Components of fuels


  • Carbon
  • Hydrogen
  • Sulfur
• Combustion of fuels


  1. Releases carbon dioxide

  2. Releases water (vapour)

  3. Releases carbon monoxide

  4. Releases oxides of nitrogen
• Incomplete/partial combustion


  • Releases solid particles of soot (carbon)
  • Releases carbon monoxide
  • Releases unburnt fuel
• Combustion of hydrocarbons releases energy
• During combustion, carbon and hydrogen in fuels are oxidised to form carbon dioxide and water
• Carbon monoxide


  Toxic gas, colourless and odourless
• Sulfur dioxide and oxides of nitrogen

  • Cause acid rain
  • Cause respiratory problems in humans
• Particulates


  • Cause global dimming
  • Cause human health problems
• Explain how carbon monoxide is produced when petrol is burned in car engines:

  Incomplete combustion happens because there is a limited supply of oxygen
• Describe how oxides of Nitrogen are produced when petrol is burned in car engines

  Nitrogen from the atmosphere reacts with oxygen from the atmosphere at high temperatures