Evolution of the Atmosphere
Phase 1 - Volcanoes gave out Gases
The Earths surface was originally molten for millions of years, so hot that any atmosphere would 'boil away' into space. It cooled down a bit, and a thin crust formed, but volcanoes kept erupting. The volcanoes gave out lots of gas, including: carbon dioxide (CO2(g)), Water vapour (H2O(g)) and nitrogen (N(g)). The early atmosphere was mostly CO2 and water vapour, with virtually no oxygen. (Oceans formed when the water vapour condensed when Earth cooled).
Phase 2 - Green Plants Evolved and Produced Oxygen
Plants evolved over much of Earth, happy in the CO2 atmosphere. Lots of the early CO2 dissolved into the oceans. Green plants replaced CO2 from the airwith O2 (photosythesis). When plants died & were buried under layers of sediment, the carbon they had removed from the air (as CO2) became trapped in sedimentary rocks as insoluble carbonates and fossil fuels.
Phase 3 - Complex Organisms Evolved
The O2 build up in the atmosphere killed off some early organisms that couldn't tolerate it, and allowed more complex organisms to evolve. There's virtually no CO2 left now.
The Atmosphere Today
The Atmosphere Contains Various Chemicals
The Earth is surrounded by a mixture of gases - the atmosphere (air). The atmosphere is mostly made up of: Nitrogen - 78%, Oxygen - 21%, Argon - 1%.
These figures are rounded slightly - the atmosphere also contains small amounts of CO2 and other gases, and varying amounts of water vapour.
Human Activity is Changing the Atmosphere
The above concentrations of gases are fairly constant, but humans are adding small amounts of pollutants to the air. These pollutants include: Carbon Dioxide, Carbon Monoxide, Particulates (tiny particles that are in the air), Sulfur Dioxide and Nitrogen oxides. Pollutants come from many different sources e.g. burning fuels in power stations and vehicles, nature (e.g. volcanoes).
Some pollutant gases are directly harmful to humans - can cause disease or death in people who breath in large enough quantities e.g. carbon monoxide reduces the amount of O2 that the blood can carry which leads to flu-like symptoms and death. Pollutants can also harm us indirectly, by damaging our environment e.g. SO2 causing acid rain & CO2 -> climate change.
- All substances are made from tiny particles called atoms.
- When atoms are joined together they make molecules.
- Chemical reactions = atoms rearranging themselves to make new substances.
- Almost all chemical reactions involve atoms changing places - the atoms you start off with (the reactants) rearrange themselves to form different chemicals. The new chemicals are called the products. For Example:
No atoms 'disappear' during the reaction. You start with 1 Carbon, 4 Hydrogens and 4 Oxygens, and you end up with the same number of each element - just in a different order. This means that no mass is lost during the chemical reaction - the mass of the reactants = the mass of the products.
Reactants and products often have very different properties. E.g. carbon is a black solid and O2 is a colourless gas (@room temp). But when they react (coal burning) the product is CO2 - which is also a colourless gas but has very different properties from O2 (it's heavier for a start and can be toxic)
- Fossil Fuels are Hydrocarbons - contains only two elements, Hydrogen and Carbon.
- They're formed from the remains of dead plants and animals over millions of years.
- These fuels e.g. crude oil, are drilled out of the Earth and then refined to make useful products like petrol and diesel.
- The only difference between fuels (e.g. petrol, diesel fuel, fuel oil) is the size of the hydrocarbons they contain.
- Coal isn't a hydrocarbon, because even though it contains impurities it's mostly just carbon.
Burning Fuels is an Example of Oxidation. Burning (also known as combustion) is a type of chemical reaction. When a hydrocarbon burns, the hydrogen atoms in the fuel combine with oxygen atoms in the air to make hydrogen oxide (H2O/water). And the carbon atoms in the fuel combine with oxygen atoms in the air to make carbon dioxide. Any reaction where oxygen is added is called an oxidation reaction. Reactions where oxygen is lost are called reduction reactions. Combustion is an example of an oxidation reaction. The O2 needed to burn hydrocarbons can come from the air, or it can be in the form of pure O2. Hydrocarbon fuels burn more rapidly in pure O2. Pure O2 is obtained from the air and put in pressurised cylinders. It can then be used, for exmaple, in oxy-fuel welding torches. The O2 allows the fuel to burn more rapidly so a higher temperature can be reached.
Air Pollution - Carbon
Different forms of carbon pollution cause different problems. Carbon based products of burning fossil fuels often pollute the environment.
All fossil fuels contain large amounts of Carbon, so they produce a lot of carbon pollutants. If it's burnt where there's lots of O2 available then nearly all the carbon ends up as CO2. But if there isnt (e.g. car engine) then small amounts of CO and small particles of carbon are produced as well. These products are all pollutants.
Carbon Dioxide (CO2). It will stay in the atmosphere, causing problems until it's removed (like any other atmospheric pollutant). It can be removed naturally - plants use up CO2 from the air when they photosynthesise, it also dissolves in rainwater and in seas, lakes & rivers. Despite this, the CO2 level can still increase if human activity (e.g. burning fuels) adds extra CO2 into the atmosphere. Increased CO2 level increases the greenhouse effect, which is warming up the Earth - causing lots of problems e.g. sea level rise.
Air Pollution - Carbon 2
Carbon Monoxide (CO). Only one oxygen atom attached to a carbon atom. It's produced when there's not enough oxygen available when fuels burn - incomplete burning. It's poisonous. Carbon monoxide reduces the amount of O2 that the blood can carry which leads to flu-like symptoms and death.
Particulate Carbon. Tiny particles of carbon are produced by incomplete burning. If they escape into the atmosphere, they just float around - and eventually fall back to the ground as a black dust called soot. A lot falls onto buildings and makes them look very dirty.