Core Science Chemistry Topics 1-4

C1.1 -- The early atmosphere:

• Volcanoes today mostly release carbon dioxide and and water vapour, along with small amounts of methane, amonia and nitrogen; scientists therefore think that these gases could have been present in the early atmosphere as well.
• There are two main theories for how Earth's early atmosphere looked: either that it was like Titan, a moon of Jupiter, whose atmosphere is 98% nitrogen, or that it was like Mars of Venus, both of whose atmospheres are mainly carbon dixoide. Since Earth has a rocky interior like Mars and Venus, as opposed to an icy one like Titan, it is now felt that the latter theory is more likely to be true.
• Scientists are more sure that the early atmosphere did not contain oxygen, since volcanoes do not give out oxygen and the earliest rocks found contain iron compounds which could not have formed in the presence of oxygen. 
• As the Earth cooled, water vapour in the atmosphere condensed to form the oceans.

C1.2 -- A changing atmosphere:

• Oxygen was added to the atmosphere around one billion years ago, as organisms with the ability to photosynthesis emerged. This also resulted in the removal of carbon dioxide from the atmosphere.
• Carbon dioxide in the atmosphere also dissolved in the oceans, and some marine organisms used this CO2 to make calcium carbonate shells; these shells then became sediment once they died, and over millions of years these sediments became sedimentary rocks such as limestone.

C1.3 -- Oxygen in the atmosphere (N/A: Practical):

C1.4 -- The atmosphere today:

• Today, the atmosphere consists of around 79% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dixoide. There are also trace amounts of other gases. However, these amounts can vary due to natural causes (volcanoes giving out sulphur dioxide and lightening producing nitrogen oxides) or due to human activities like deforestation (resulting in fewer trees to take CO2 out of the atmosphere and add oxygen to it in the process of photosynthesis), buring fossil fuels (adding more CO2), using engines and furnaces (which can release nitrogen oxides) and the rearing of cattle and the upkeep of rice fields (both of which release methane).
• There are two main theories as to why nitrogen now dominates in the atmosphere; either nitrogen was added via volcanic eruptions whilst the Earth was young, or nitrogen was added gradually due to the reactions of nitrogen-containing compounds released from volcanoes.

C1.5 -- Rocks and their formation:

• Igneous rocks form either when magma (molten rocks inside the Earth) erupts onto the surface of the Earth as lava and then cools, or when it cools whilst still in the Earth. These rocks have interlocking crystals; the size of the crystals depends on whether the rock cooled quickly (such as in rhyolite, whose crystals cannot be seen with the naked eye) or slowly (such as in granite, whose crystals are clearly visible).
• Sedimentary rocks are formed when sediments from other rocks are eroded and then transported and compacted over millions of years to…