Evolution of the Atmosphere (Edexcel GCSE Chemistry)

Revision Note

Stewart Hird

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Earth´s Early Atmosphere

  • Theories on the development of Earth’s atmosphere have developed over time as instrumental analysis has improved

  • The surface of the early Earth was molten for millions of years during which time there was no atmosphere surrounding the planet

  • Eventually cooling began to take effect and allow for molten materials to slowly solidify forming land masses

  • Volcanoes formed on the land masses and released gases from the Earth’s interior through violent eruptions

  • Earth’s gravity prevented the gases from escaping into outer space and they formed the atmosphere

  • Analysis of the minerals in the earth's crust enables scientists to deduce the gases present millions of years ago that helped form those minerals

  • It is thought that the atmosphere at that moment in Earth’s history was similar to that of Venus or Mars today which consist mainly of CO2

  • During a period of intense volcanic activity, large amounts of carbon dioxide and water vapour were released, as well as nitrogen, hydrogen, ammonia, methane and other gases.

  • The early atmosphere therefore contained mainly CO2 and water vapour

  • There was little or no oxygen present

Early-Earth-Volcanoes, IGCSE & GCSE Chemistry revision notes

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

  • 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

  • Carbon dioxide is a water soluble gas (it is the gas used in fizzy drinks) and dissolves readily

  • When the water vapour condensed large amounts of CO2 dissolved in the oceans

  • Carbonate substances were precipitated during this process which later formed sediments

Examiner Tips and Tricks

For this topic you should be able to describe how the atmosphere and oceans came about.

How Carbon Dioxide 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

  • Green plants and algae began to evolve and absorbed considerable amounts of carbon dioxide during photosynthesis

Photosynthesis, downloadable IGCSE & GCSE Chemistry revision notes

Photosynthesis gradually removed large amounts of carbon dioxide from the atmosphere

  • When these organisms died, their remains locked the atmospheric carbon into the rocks

  • This is how the large amounts of carbon dioxide in the early atmosphere were reduced

Examiner Tips and Tricks

Exhaled air contains about 4% carbon dioxide and 16% oxygen which is why mouth-to-mouth resuscitation can save someone who has stopped breathing.

How Oxygen Increased

  • Primitive plants and algae began photosynthesizing which used up carbon dioxide from the atmosphere and released oxygen:

Photosynthesis word equation, IGCSE & GCSE Biology revision notes

Carbon dioxide and water producing glucose and oxygen during photosynthesis

  • Through these processes over a long period of time, the amount of O2 in the atmosphere increased  and the amount of CO2 decreased

  • Algae first evolved around 2.7 billion years ago and during the next billion years or so small green plants began to appear

  • As more and more plants began to appear the levels of oxygen began to increase which allowed for more complex life forms to evolve

  • This trend continued until around 200 million years ago the composition of the atmosphere reached similar characteristics as today: around 20% oxygen, 80% nitrogen and tiny amounts of other gases

  • Marine algae are throught to be responsible for about 90% of all the atmospheric oxygen produced

Investigating the percentage of oxygen in air

  • The percentage of oxygen in the air can be investigated by passing a known quantity of air over a metal

  • The oxygen in the air will react with the metal, forming a metal oxide

  • The oxygen will be removed from the air and the volume of the air with the oxygen removed can be measured

  • An example of the apparatus that can be used to investigate this is shown below:

JR_ujZoJ_10-2-1-composition-of-oxygen-in-air

Method:

  • Heat the copper using a Bunsen burner

  • Push the plunger of the syringe containing air, forcing the air into the other plunger until all of the air has transferred

  • Push the air back from the now filled plunger to the other plunger

  • Repeat this several times for about 3 minutes

  • The copper will turn black as copper reacts with the oxygen in the air and copper oxide is produced

  • Allow the apparatus to cool

  • Ensure all the gas is in one syringe and record the volume of gas

  • The percentage of oxygen in the air can be calculated from the results

Worked Example

In the experiment above, the starting volume of air was 50.0 cm3 of air and the final volume of air was 39.5 cm3

Calculate the percentage of oxygen in the air.

Answer:

  • Step 1 - calculate the volume of oxygen in 50.0 cm3 of air

            volume of oxygen in air = starting volume - final volume 

                         = 50.0 - 39.5 = 10.5 cm3

  • Step 2 - calculate the percentage of oxygen in air

Percentage of oxygen in air = fraction numerator volume space of space oxygen space in space air over denominator starting space volume space of space air end fraction space cross times space 100

                                                                                = fraction numerator 10.5 over denominator 50.0 end fraction space cross times space 100     

                                                                                = 21.0 %

The Test for Oxygen

  • The test for oxygen consists of placing a glowing splint inside a test tube of gas

  • If the gas is oxygen the splint will relight

Test for Oxygen, IGCSE & GCSE Chemistry revision notes

Diagram showing the test for oxygen gas

Examiner Tips and Tricks

Sometimes the splint does not relight, but it glows very brightly, which is also a positive result. In an exam, however, it is best to state it relights the glowing splint.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.