Developing the Atmosphere | Chemistry for Grade 10 PDF Download

The early atmosphere was mainly carbon dioxide and water vapour. Water vapour condensed to form the oceans. Photosynthesis caused the amount of carbon dioxide to decrease and oxygen to increase.

The Earth’s early atmosphere

Where did the atmosphere come from?

• a large amount of carbon dioxide
• little or no oxygen
• small amounts of other gases, such as ammonia and methane

Volcanic activity also released water vapour, which condensed as the Earth cooled to form the oceans. Nitrogen was probably also released by volcanoes which gradually built up in the atmosphere because it is unreactive.
A volcano in iceland

The modern atmosphere

The composition of the modern atmosphere

Example: Name the most abundant gas in the modern atmosphere.

Nitrogen.

How oxygen increased

carbon dioxide + water → glucose + oxygen
6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
Scientists think that algae first evolved approximately 2.7 billion years ago, and soon after this oxygen began to exist in the atmosphere. Photosynthesis by primitive plants and algae released oxygen, which gradually built up in the atmosphere. Eventually, the amount of oxygen present in the atmosphere enabled animals to evolve.
Green algae cells viewed using an electron microscope

How carbon dioxide decreased

Formation of sedimentary rocks

Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved to form soluble carbonate compounds so its amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, eg limestone.

Uptake by living organisms

• Carbon dioxide was also absorbed from the oceans into photosynthetic algae and plants. Many of these organisms, and the simple organisms in the food chains that they supported were turned into fossil fuels, eg crude oil, coal and natural gas, which all contain carbon.
• Coal is a fossil fuel which was formed from trees which were in dense forests in low-lying wetland areas. Flooding caused the wood from these forests to be buried in a way that prevented oxidation taking place. Compression and heating over millions of years turned the wood into coal.
• Crude oil and natural gas were formed from simple plants and tiny animals which were living in oceans and lakes. These small organisms died and their remains sank to the bottom where they were buried under sediments. The lack of oxygen prevented oxidation from occurring.
• Over millions of years, heat and pressure turned the remains of the organisms into crude oil and natural gas. Natural gas contains the smallest molecules and is often found on top of crude oil, trapped under sedimentary rock.

Example: Describe two reasons why the amount of carbon dioxide in the atmosphere decreased over time.

Carbon dioxide dissolved in the oceans, and primitive plants used it for photosynthesis.