The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.

The earth is made up of:

Oxygen: 21%

Nitrogen: 78%

Argon: 0.9%

CO2: 0.037% 

The early atmosphere

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources: for example, volcanoes on other planets release high quantities of carbon dioxide or nitrogen and iron-based compounds which are present in very old rocks that could have formed only if there was little or no oxygen.

So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?

The proportion of oxygen went up because of photosynthesis by plants.

The proportion of carbon dioxide went down because:

• it was locked up in sedimentary rocks such as limestone, and in fossil fuels
• it was absorbed by plants for photosynthesis
• it dissolved in the oceans.

The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.

A simple carbon cycle

The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.

Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion both release carbon dioxide into the atmosphere.These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same

Human influences

Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.

The rising human population is adding to atmospheric carbon dioxide in other ways too. When land is cleared for timber and farms (deforestation), there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. Not only are there then fewer trees to absorb carbon dioxide, but the burning of the trees releases carbon dioxide out the same.

Igneous rock

Igneous rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms. When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion and weathering. Intrusive rock will have large crystals as it has cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly.

Examples of igneous rocks include basalt and granite.

Sedimentary rocks

A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.

The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.

The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.

These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.

These are the different processes in order:

sedimentation → compaction → cementation

Sedimentary rock formation

The pieces of rock stick together through the process of cementation

The river transports bits of rock, and deposits them on the bottom of the river bed.

Metamorphic rock

Metamorphic rock has been subjected to tremendous heat and / or pressure, which caused it to change into another type of rock. It is usually resistant to weathering and erosion and is therefore very hard wearing.

Examples of metamorphic rock

Examples include marble - which originates from chalk or limestone, slate - which originates from clay, and schists formed from sandstone or shale (sedimentary rocks).

Limestone is mainly calcium carbonate, CaCO₃. When heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide. Limestone and its products have many uses: for example, in mortar, cement, concrete and glass.

Advantages and disadvantages of various building materials:

Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or "pointed". Pollution from burning fossil fuels makes the rain more acidic than it should be, and this rain makes these problems worse.

Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.

Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, the use of too much glass can make buildings very hot in the summer.

Advantages: Limestone is a valuable natural resource, used to make things such as glass and concrete.

Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry.

Disadvantages: Limestone quarries are visible from long distances and may permanently disfigure the local environment.

Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life.

Quarrying limestone is big business but the need for limestone has to be balanced against the economic, environmental and social effects. Some factors that have to be considered include:

• effect on employment – increased job opportunities
• pollution – noise, sound and air
• traffic levels
• visual effects of having a quarry

Acids have a variety of applications for the industrial and domestic markets. Acids can be neutralised using an alkali or base and used to make salts

All acids:

• have a low pH (1-6) – the lower the number the stronger the acid
• react with bases to form neutral compounds
• are corrosive when they are strong
• are an irritant when they are weak.

Indigestion remedies

Hydrochloric acid is used in the body to help digestion and kill bacteria.

However too much acid can cause indigestion and we use indigestion remedies to neutralise excess acids.

An indigestion remedy contains a base such as magnesium hydroxide, which reacts to form a neutral compound and raises the pH of the stomach.

Metal hydroxides

Metal hydroxides, such as sodium hydroxide, usually dissolve in water to form clear, colourless solutions. When an acid reacts with a metal hydroxide, the only products formed are a salt plus water. Here is the general word equation for the reaction:

acid + metal hydroxide → a salt + water

You usually observe these things during the reaction:

• there is a temperature rise
• the pH of the reaction mixture changes

Metal oxides

Some metal oxides, such as sodium oxide, dissolve in water to form clear, colourless solutions. Many of them are not soluble in water, but they will react with acids. Copper(II) oxide is like this. When an acid reacts with a metal oxide, the only products formed are a salt plus water. Here is the general word equation for the reaction:

acid + metal oxide → a salt + water

You usually observe the same things during the reaction that you observe with metal hydroxides.

Metal carbonates

Although sodium carbonate can dissolve in water, most metal carbonates are not soluble. Calcium carbonate (chalk, limestone and marble) is like this. When an acid reacts with a metal carbonate, the products formed are a salt plus water, but carbon dioxide is also formed. Here is the general word equation for the reaction:

acid + metal carbonate → a salt + water + carbon dioxide

You usually observe bubbles of gas being given off during the reaction. You can show that the gas is carbon dioxide by bubbling it through limewater: this turns cloudy white when it reacts with carbon dioxide.

Here are the word equations and balanced formulae equations for the reactions involving hydrochloric acid in the table:

sodium hydroxide + hydrochloric acid → sodium chloride + water

NaOH + HCl → NaCl + H₂O

copper oxide + hydrochloric acid → copper chloride + water

CuO + 2HCl → CuCl₂ + H₂O

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Here are the word equations and balanced formulae equations for the reactions involving sulfuric acid in the table:

potassium hydroxide + sulphuric acid → potassium sulfate + water

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

zinc oxide + sulfuric acid → zinc sulfate + water

ZnO + H₂SO₄ → ZnSO₄ + H₂O

sodium carbonate + sulfuric acid → sodium sulfate + water + carbon dioxide

Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂

Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.

The process of electrolysis:

Here is what happens during electrolysis:

• Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
• Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

Many substances are commonly electrolysed, but here are two examples:

Hydrochloric acid

• Produces chlorine at the positive electrode
• Produces hydrogen at the negative electrode
• If the gas produces a squeaky pop from a lighted splint, it is hydrogen
• If the gas turns blue litmus paper red then white (bleached) it is chlorine.

Water

• Produces oxygen at the positive electrode
• Produces hydrogen at the negative electrode
• If the gas relights a glowing splint, it is oxygen.

Product: Chlorine                                                                                                                    Test: Damp blue litmus paper turns red (as chlorine is acidic) then white (chlorine is a bleach)Uses: Manufacture of bleach and PVC (polyvinylchloride) Water treatment                   Problems: Toxic gas

Product: Hydrogen                                                                                                                 Test: Lighted splint gives a squeaky pop                                                                                 Uses: Many uses including as a fuel                                                                                   Problems: Flammable

Product: Sodium hydroxide                                                                                                       Test:Turns red litmus blue                                                                                                      Uses: Cleaning products                                                                                                     Problems: corrosive

Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them. The method used to extract a given metal from its ore depends upon the reactivity of the metal and so how stable the ore is. The uses of metals depend on their properties. Alloys are made by mixing a metal with another material in order to improve the properties.

Oxidation and reduction

Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide:

magnesium + oxygen → magnesium oxide

2Mg + O₂ → 2MgO

Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen:

copper oxide + hydrogen → copper + water

CuO + H₂ → Cu + H₂O

Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is.

Reactivity and extraction method

MetalReactivity

• potassium
• sodium
• calcium              
• magnesium
• aluminium

extract by electrolysis carbon

• zinc
• iron
• tin
• lead

extract by reaction with carbon or carbon monoxide hydrogen

• copper
• silver
• gold
• platinum

extracted by various chemical reactions

The method used to extract a metal from its ore depends upon the stability of its compound in the ore, which in turn depends upon the reactivity of the metal:

• the oxides of very reactive metals, such as aluminium, form stable oxides and other compounds. A lot of energy is needed to reduce them to extract the metal.
• the oxides of lesser reactive metals, such as iron, form less stable oxides and other compounds. Relatively little energy is needed to reduce them to extract the metal.

So, the method of extraction of a metal from its ore depends on the metal's position in the reactivity series.

Reactive metals such as aluminium are extracted by electrolysis, while a less-reactive metal such as iron may be extracted by reduction with carbon. Gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.

Oxidation is the gain of oxygen by a substance. For example, magnesium is oxidised when it reacts with oxygen to form magnesium oxide:

magnesium + oxygen → magnesium oxide

2Mg + O₂→ 2MgO

Reduction is the loss of oxygen from a substance. For example, copper oxide can be reduced to form copper if it is reacted with hydrogen:

copper oxide + hydrogen → copper + water

CuO + H₂→ Cu + H₂O

Many ores contain metal oxides, therefore many metals can be extracted from their ores by reduction reactions. The method used to extract a given metal depends on how reactive it is:

• very reactive metals – electrolysis
• less reactive metals - reduction

Iron and steel rust when they come into contact with water and oxygen: this is a form of corrosion. Both water and oxygen are needed for rusting to occur. Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron(III) oxide, which we see as rust.

Salt dissolved in water does not cause rusting, but it does speed it up, as does acid rain.

Aluminium does not rust (corrode) because its surface is protected by a natural layer of aluminium oxide which prevents the metal below from coming into contact with air and oxygen. Unlike rust, which can flake off the surface of iron and steel objects, the layer of aluminium oxide does not flake off.

More reactive elements are more likely to oxidise.

MetalPropertiesUses

aluminium

low density, does not corrode suitable for the bodies of planes

copper

good conductor of electricity, does not react with water

electrical wires as it is a good conductor

water pipes due to its low reactivity

gold

very good conductor of electricity, unreactive

electrical connections on circuit boards - due to its conductivity

jewellery - due to its lack of reactivity

steel

cheap and strong suitable for building material

The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

It is more difficult for layers of atoms to slide over each other in alloys

Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:

• Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc.
• 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals.
• Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.

Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.

Crude oil is a mixture of compounds called hydrocarbons. Many useful materials can be produced from crude oil. It can be separated into different fractions using fractional distillation, and some of these can be used as fuels. Unfortunately, there are environmental consequences when fossil fuels such as crude oil and its products are used.

Hydrocarbons

Most of the compounds in crude oil are hydrocarbons. This means that they only contain hydrogen and carbon atoms, joined together by chemical bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.

Structure of alkanes

alkaneformulachemical structureball-and-stick model methane CH₄ ethane C₂H₆ propane C₃H₈ butane C₄H₁₀

Boiling point and state at room temperature

Hydrocarbons have different boiling points, and can be either solid, liquid or gas at room temperature:

• small hydrocarbons with only a few carbon atoms have low boiling points and are gases
• hydrocarbons with between five and 12 carbon atoms are usually liquids
• large hydrocarbons with many carbon atoms have high boiling points and are solidsstrong.

Distillation

Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.

This is the sequence of events in distillation:

heating → evaporating → cooling → condensing

Complete combustion

Fuels burn when they react with oxygen in the air. The hydrogen in hydrocarbons is oxidised to water (remember that water, H₂O, is an oxide of hydrogen). If there is plenty of air, we get complete combustion and the carbon in hydrocarbons is oxidised to carbon dioxide:

hydrocarbon + oxygen → water + carbon dioxide

The test to show carbon dioxide is limewater it turns from clear to cloudy in the presence of carbon dioxide.

Incomplete combustion

If there is insufficient air for complete combustion, we get carbon monoxide. Particles of carbon, seen as soot or smoke, are also released. Carbon monoxide is a problem because it reduces the amount of oxygen that the haemoglobin part of the blood can carry around the body. Every year many people are admitted to hospital due to carbon monoxide poisoning and some die.

The combustion of a fuel may release several gases into the atmosphere, including:

• water vapour
• carbon dioxide
• carbon monoxide
• particles
• sulfur dioxide

The fossil fuels include coal, oil and natural gas. Various factors need to be considered when deciding how to use a fossil fuel. These include:

• the energy value of the fuel in J/g of fuel
• the availability of the fuel
• how the fuel can be stored
• the cost of the fuel
• the toxicity of the fuel - whether it is poisonous
• any pollution caused when the fuel is used, such as acid rain
• how easy it is to use the fuel

Sulfur dioxide

Sulfur dioxide is produced when fuels that contain sulfur compounds burn. It is a gas with a sharp, choking smell. When sulfur dioxide dissolves in water droplets in clouds, it makes the rain more acidic than normal. This is called acid rain.

Effects of acid rain

Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result. Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

Reducing acid rain

Sulfur dioxide can be removed from waste gases after combustion of the fuel. This happens in power stations. The sulfur dioxide is treated with powdered limestone to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.

1. The Sun’s rays enter the Earth’s atmosphere
2. Heat is reflected back from the Earth’s surface
3. Heat is absorbed by greenhouse gases, such as carbon dioxide, and as a result becomes trapped in the Earth’s atmosphere.
4. The Earth becomes hotter as a result

A rise of just a few degrees in world temperatures will have a dramatic impact on the climate:

• global weather patterns will change, causing drought in some places and flooding in others.
• polar ice caps will melt, raising sea levels and causing increased coastal erosion and flooding of low-lying land – including land where major cities lie

Scientists are trying to control the amount of carbon dioxide in the atmosphere by:

• iron seeding of oceans
• converting carbon dioxide into hydrocarbons

However, some scientists do not believe that the global temperature increase and the carbon dioxide increase are caused by human activities.

Biofuels come from the products of living organisms, such as methane biogas from decaying manure and sewage. Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil.

Advantages of biofuel:

Biofuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis.

This helps to reduce global warming.

However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.

Ethanol

Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also useful as a fuel. For use in cars and other vehicles it is usually mixed with petrol.

Ethanol can be made by a process called fermentation. This converts sugar from sugar cane or sugar beet into ethanol and carbon dioxide. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen:

C₆H₁₂O₆2C₂H₅OH + 2CO₂

Hydrogen is often seen as an environmentally friendly alternative to fossil fuels and biofuels. When hydrogen burns, the only product formed is water:

hydrogen + oxygen → water

2H₂ + O₂ → 2H₂O

Making hydrogen

At the moment, most hydrogen is made by reacting steam with coal or natural gas, which are non-renewable resources.

Hydrogen can also be made by passing electricity through water. Unfortunately, most electricity is generated using coal and other fossil fuels: any pollution from burning these fuels just happens at the power station instead of at the vehicle itself.

Handling hydrogen

Hydrogen gas is very flammable and may explode if handled incorrectly. It must be compressed and chilled, then stored in tough, insulated tanks. It is not as convenient as petrol and diesel.

Problems with hydrogen

Some hydrogen-powered vehicles fitted with hydrogen fuel cells have already been made and are on the road, but there are few of them because of difficulties involved in making and handling hydrogen.