C1.1: How has the Earth's atmosphere changed over time, and why?

A: What types of substance are found in the atmosphere?
1. The atmosphere forms a layer around the planet, and is essential to life. There would be no oxygen to breathe and the temperature would be too cold for life without the atmosphere
  1. The atmosphere is a mixture of elements and compounds, and is around 100km thick, but 75% of it's mass is in the first 11km
2. The air is made of mostly the elements nitrogen and oxygen
  1. It also contains compounds, water vapour and carbon dioxide.
  2. Oxygen is found in the atmosphere as a gas
3. Key Words:
  1. Element: Made of one type of atom
  2. Compound: Made from two or more elements that are chemically combined
  3. Mixture: Different elements which are not chemically combined
  4. Atmosphere: A layer of a mixture of gases around the planet
  5. Molecule: A group of atoms which are chemically combined
4. H2O can be found in a liquid state (water), a gas state (water vapour) and a solid state (ice)
  1. H2O evaporates and moves to the atmosphere
  2. H2O in the form of water vapour, condenses and falls as rain
    1. These are physical changes
5. When coal burns, the carbon atoms chemically mix with the oxygen atoms in the air, and forms carbon dioxide, which then enters the atmosphere
  1. This is a chemical change
B: What determines the state of a substance on Earth?
1. Key Words:
  1. Sublimation: When a substance turns from a solid into a gas, without becoming a liquid in between
  2. Melting Point: The temperature at which a solid substance turns into a liquid
  3. Boiling Point: The temperature at which a liquid substance turns into a gas
2. Carbon dioxide can be a solid on Mars, when the temperature plummets so low that the gas freezes
3. The melting point and the boiling point can determine the state of a substance at a given temperature
  1. Question: The melting point of methane is -182C, and it's boiling point is -162C. What is the state of methane at -180C?
    1. Below the melting point, the substance is solid
    2. In-between the melting point and the boiling point, the substance is a liquid
    3. Above the boiling point, the substance is a gas
    4. ANSWER: Methane is a liquid at -180C
4. Substances have different boiling points because the particles in the substance have a different force of attraction to each other than other substances
  1. When a liquid is heated, the particles move faster because they have more energy, soon they gain enough energy to overcome the forces of attraction between the particles and can move further apart, which changes the substance into a gas
5. Water molecules are very strongly attracted to each other, which makes the boiling point much higher than other substances of the same size.
  1. Water is a liquid when most other substances are in the gas state, without the strong attraction between water molecules, it all would have evaporated and Earth would not be suitable for life
C: How did the atmosphere form?
1. The early Earth had an atmosphere which mainly consisted of CO2 and water vapour, with smaller amounts of other gases. These probably came from the volcanic eruptions. And if the planet had stayed that way, it would be as unsuitable for life as Venus
  1. Then temperatures began to cool, and water vapour condensed to form the oceans and the seas
2. Scientists believe that oxygen levels increased about 2.3 billion years ago, however, new evidence suggests that oxygen was first released by photosynthesising bacteria
  1. This oxygen was thought to have reacted with other substances before it entered the atmosphere, then plants developed and released larger amounts of oxygen
3. There was a large decrease in CO2 levels, which is explained by the carbon cycle.
  1. CO2 was removed from the atmosphere and dissolved into the oceans, as there was an increase in plant life, more CO2 was taken to photosynthesise with, and when those plants died, they locked carbon in the ground and formed fossil fuels which are used today
    1. However, the burning of those fossil fuels today is adding the CO2 back into the atmosphere
4. Key Words:
  1. Carbon Cycle: The natural processes which recycle carbon through the living and non-living parts of the ecosystem
  2. Peer Review: Scientists who are experts in their field evaluate another scientist's work before and after publication
5. All new research needs to be peer-reviewed before publication to make sure the work is valid, original and significant to the scientific community
  1. The work can then be published, and should inspire new/further research to be made in the fields
D: How do human activities affect the atmosphere?
1. Gas is released into the atmosphere when a car is driven, or a light is turned on. they are released by the car/power station where the electricity is generated. They are released because the fossil fuels which are burnt, react with the oxygen in the air, and create air pollutants
2. Key Words:
  1. Air Pollutant: Harmful substances which are released into the atmosphere
  2. Oxidation: Reactions where oxygen is gained by a reactant
  3. Combustion: An oxidation reaction where energy/light is released
  4. Hydrocarbons: Compounds of hydrogen and carbon only
  5. Particulates: Tiny part of a solid
  6. Incomplete Combustion: When there is an insufficient amount of oxygen to produce CO2 whilst burning
3. Most power stations are fuelled by coal or natural gas.
  1. When coal combusts CO2 is released and enters the atmosphere
  2. Natural gas, petrol, diesel and oil are made of hydrocarbon molecules
    1. When burnt, these produce CO2 and water vapour which are released into the atmosphere
  3. If coal/natural gas are burnt with insufficient oxygen (incomplete combustion), they produce CO and unburnt particles of carbon (particulates)
    1. If the coal contains sulphur impurities, then SO2 will be released
  4. High temperatures in a furnace cause nitrogen and oxygen in the air to react and produce NO
    1. NO is oxidised further to form NO2
    2. These are the nitrogen oxides
4. Vehicles burn petrol/diesel, which are hydrocarbons
  1. Oxygen and nitrogen in the air enter the engine, CO2, H2O, CO and NO are produced
    1. The formation of carbon particulates and CO is called incomplete combustion
E; How are scientists working to reduce air pollution?
1. Concentration is the amount of pollutants in a certain volume of air, if there is a low concentration, this indicates good air quality
  1. If there is a high concentration of pollutants, this indicates poor air quality
2. If a scientist measures the concentration of a substance in the air, they will get varying results, this may be because they used the equipment differently, or there were differences in the equipment itself
  1. It is better to take more measurements as it is easier to estimate the true value, which is what the measurement should really be. The accuracy of a result depends on how close it is to the true value
3. Engineers are working to improve the efficiency of car engines, a more efficient engine will burn less fuel to travel to the same distance, which therefore reduces the amount of pollutants produced
  1. This benefits car owners, as they do not need to buy so much fuel
  2. Scientists have developed a catalytic converter, which changes harmful pollutants into less harmful ones, all cars are now fitted with a catalytic converter in the exhaust system
    1. CO and NO are changed into less harmful substances, CO2 and nitrogen
      1. Although these are less harmful to humans directly, CO2 has a large impact on climate change, and the only way to produce less CO2 is to burn less fossil fuels
        So, some cars run on low-sulfur fuels, which reduces the amount of SO2, which helps reduce acid rain
        Other cars are electric, and use batteries. This means the pollutants are not released in a large city area, but at the power station, if they burn fossil fuels
F: Representations of chemical reactions
1. The chemicals before the arrow are the reactants of the reaction
2. The chemicals after the arrow are the products of the reaction
3. The word equation for this reaction is: carbon + oxygen = carbon dioxide
4. The atoms at the beginning of the reaction are still there at the end, no atoms are destroyed, and no new ones are created. This is the conservation of atoms
  1. All atoms have mass, as the atoms are conserved, so is the mass of the products. This is the conservation of mass

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C1.1: How has the Earth's atmosphere changed over time, and why?

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