C13

• What was the Earth when it first formed?

  A hot molten ball
• How did the Earth's early atmosphere form?

  Through volcanic activity as the Earth cooled
• Which gases escaped into outer space during Earth's formation?

  Hydrogen and helium
• What gases were released by volcanic activity?

  Carbon dioxide and water vapour
• What marked the beginning of the first atmosphere?

  The release of carbon dioxide and water vapour
• What did the first atmosphere mainly consist of?

  Carbon dioxide, water vapour, methane, ammonia
• What formed at the Earth's surface as it cooled?

  A thin crust
• What happened to water vapour as the temperature dropped?

  It condensed to form rain
• What did the first rain produce?

  The first rivers, lakes, seas, and oceans
• When did photosynthetic bacteria evolve?

  About 570 million years ago
• What did photosynthetic bacteria use for photosynthesis?

  Water and carbon dioxide
• What was produced by photosynthetic bacteria during photosynthesis?

  Oxygen
• What was the atmospheric oxygen level compared to today’s level?

  Only 10% of today’s level
• How did plants affect the levels of carbon dioxide and oxygen?

  They created oxygen and removed carbon dioxide
• What happened to organisms that could not tolerate oxygen?

  They declined in numbers
• What did the increasing amounts of oxygen lead to?

  The creation of the ozone layer
• What was the role of the ozone layer?

  It blocked harmful rays from the sun
• What did the ozone layer allow to happen?

  More complex organisms to evolve
• What is the chemical equation for photosynthesis?

  CO₂(g) + H₂O(l) ⇒ organic compounds + O₂(g)
• What role did photosynthetic bacteria play in the atmosphere?

  They produced oxygen and reduced carbon dioxide
• How did plants contribute to the carbon cycle?

  By fixing carbon dioxide in their tissues
• How thick is the Earth's atmosphere?

  100 km
• How much of the Earth's atmosphere is breathable air?

  Only the bottom 8 km
• What causes the Aurora's glow?

  Solar wind crashing into atmospheric gases
• What determines the different colors of the Aurora?

  Different gases in the atmosphere
• What is the chemical equation for methane combustion?

  $CH_4 +$$2O_2 \rightarrow CO_2 +$$2H_2O$
• What role do nitrifying bacteria play in the nitrogen cycle?

  They convert ammonia into nitrates
• What do denitrifying bacteria do?

  They convert ammonia into nitrogen
• What flammable gases were present in the early atmosphere?

  Methane and ammonia
• What happens when flammable gases are ignited by lightning?

  They burn with oxygen to produce water vapor
• What is the chemical equation for ammonia combustion?

  $4NH_3 +$$3O_2 \rightarrow 2N_2 +$$6H_2O$
• What is the summary of the evolution of the atmosphere?

  • Cooling planet forms oceans
  • Plants evolve and convert CO2 to O2
  • Carbon dioxide turns into carbonate rocks and fossil fuels
  • Increasing oxygen reacts with ammonia and methane
  • Present-day composition: 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide
• How did the composition of the atmosphere change over time?

  From mainly CO2, water vapor, methane, and ammonia
• What are the current percentages of gases in the atmosphere?

  Nitrogen 78%, Oxygen 21%, Argon 0.9%
• What has been the trend in atmospheric composition over the last 200 million years?

  It has been relatively constant
• What regulates the amount of carbon dioxide in the atmosphere?

  • The carbon cycle
  • Plants obtain carbon from CO2
  • Animals obtain carbon from plants
  • Photosynthesis removes CO2 from the atmosphere
• How do plants contribute to the carbon cycle?

  They take in CO2 and release oxygen
• How is carbon dioxide added to the atmosphere?

  • Respiration by plants and animals
  • Decomposition of organic matter
  • Burning of carbon-containing fuels
• What do plants and animals release during respiration?

  Carbon dioxide and water vapor
• What happens to carbon compounds during decomposition?

  They are converted into CO2 by respiration