c6.2

• Theories to explain the changes to the Earth's atmosphere must explain where the oxygen came from, and where the carbon dioxide went
• Photosynthesis


  1. Plants and algae absorb carbon dioxide from the atmosphere and release oxygen

  2. carbon dioxide + water → glucose + oxygen
• When plants and algae appeared

  Photosynthesis reduced the amount of carbon dioxide in the atmosphere and released oxygen
• Oxygen accumulation in the atmosphere

  1. At first, oxygen reacted with metals in rocks to produce metal oxides

  2. Later as most of the metals became oxidised, free oxygen began to accumulate in the atmosphere

  3. Eventually the oxygen-rich atmosphere that you have today developed
• It is likely that Earth's early atmosphere came from substances released by volcanoes
• The Earth is 4.54 billion years old and there was a great deal of volcanic activity during its early years
• Formation of oceans and carbon dioxide-rich atmosphere

  1. Volcanoes release huge volumes of water vapour and carbon dioxide

  2. As the Earth cooled, the water vapour condensed to form oceans, leaving an atmosphere of mostly carbon dioxide

  3. It probably contained small amounts of other gases such as ammonia and methane, but little or no oxygen
• Substances that form 99% of the Earth's atmosphere today

  • Nitrogen
  • Oxygen
  • Argon
• There are smaller percentages of many other gases, including carbon dioxide and water vapour
• Particulates


  Small particles produced in industrial processes such as metal extraction, and during incomplete combustion and in vehicle engines
• The smallest particulates settle deep in the lungs when they are breathed in, causing diseases such as bronchitis and other breathing problems, and increases the chance of heart disease
• Carbon monoxide


  Toxic gas produced during incomplete combustion of fuels that contain carbon
• Carbon monoxide poisoning


  1. Carbon monoxide attaches to the haemoglobin protein in red blood cells, reducing the amount of oxygen that the bloodstream can carry

  2. Causes drowsiness, difficulty breathing, and even death
• Many homes have carbon monoxide detectors
• Pollutants


  Substances released into the environment that may harm living things
• Atmospheric pollutants are released into the air, many as a result of burning fossil fuels, including carbon monoxide, particulates, oxides of nitrogen, and sulfur dioxide
• Greenhouse effect


  Greenhouse gases such as carbon dioxide and methane absorb infrared radiation radiated by the Earth's surface, then emit it in all directions, keeping the Earth and its atmosphere warm enough for living things to exist
• Formation of nitrogen dioxide and acid rain


  1. Nitrogen and oxygen react at the high temperatures in vehicle engines, forming nitrogen monoxide

  2. This is then oxidised in air to form nitrogen dioxide

  3. Nitrogen dioxide dissolves in the moisture in clouds, forming an acidic solution, which eventually falls as acid rain
• Acid rain erodes stonework and corrodes metals, kills trees and living things in rivers and lakes
• Formation of sulfur dioxide

  1. Fossil fuels naturally contain small amounts of sulfur compounds

  2. These impurities form sulfur dioxide when the fuel is burnt

  3. Sulfur dioxide also causes acid rain and it can cause breathing difficulties
• Enhanced greenhouse effect


  The release of extra greenhouse gases by human activities has the potential to cause an increased greenhouse effect, increasing the temperature of the Earth's atmosphere
• Enhanced greenhouse effect


  Leads to melting ice caps and rising sea levels, and to climate change, including altered weather patterns, causing flooding, and problems with farming and disease control
• Ways to reduce greenhouse gas emissions

  • Reducing the consumption of fossil fuels, for example by using biofuels
  • Using renewable energy resources such as wind and solar energy to generate electricity
  • Stopping carbon dioxide escaping when fuels are used by using carbon capture
• Expensive steps to address climate change include flood barriers, planting different crops, and designing buildings to withstand high winds
• The IPCC has considered the scientific evidence for global warming and climate change, and concluded that the main cause of global warming between 1951 and 2010 was human activity
• Scientists can't carry out a control fair test in the normal way, but they can make predictions based on observations, measurements, and computer models, though the actual outcome will only be revealed in the future
• Some people believe that we're carrying out a huge, dangerous experiment on the planet by letting CO2 levels rise
• Tap water has to meet strict safety standards, but low levels of pollutants are still found
• Sources of pollutants in tap water

  • Nitrate residues from excess fertiliser run-off into rivers and lakes
  • Lead compounds from old lead pipes
  • Pesticide residues from spraying pesticides too near to rivers and lakes
• Nitrates prevent the blood from carrying oxygen properly, especially in young babies
• Desalination


  The process of removing the high concentrations of dissolved salts from seawater to make it potable
• Small scale desalination


  Reverse osmosis using special ultrafilters is used to filter out the salts
• Large scale desalination


  Simple distillation is used
• Desalination is worthwhile where fresh water supplies are limited, or where the cost of energy resources is low
• Sources of tap water

  • Water stored in lakes, reservoirs, or aquifers
  • Rivers
  • Waste water
• Water from all these sources contains microorganisms and many different substances, including insoluble materials such as leaves, and particles from rocks and soil, and soluble substances, including salts, and pollutants such as pesticides and fertilisers
• Water purification process


  1. Filtration - a wire mesh screens out large twigs etc. and then gravel and sand beds filter out any other solid bits

  2. Sedimentation - iron sulfate or aluminium sulfate is added to the water, which makes fine particles clump together and settle at the bottom

  3. Chlorination - chlorine gas is bubbled through to kill harmful bacteria and other microbes
• Some soluble impurities that are dissolved in the water are not removed, because they can't be filtered out, including minerals which cause water hardness and some harmful chemicals