Chemistry Unit 1

Atomic Structure

In the nucleus of an atom there are protons and neutrons. On the shells of atoms there are electrons.

Protons: +1                                                   1st Shell: 2 electrons                             Electrons: -1                                                  2nd Shell: 8 electrons                             Neutrons: No charge                                     3rd Shell: 8 electrons

In a periodic table there are groups and periods. Groups share similar characteristics because they have the same number of electrons on their outer shell. Group 1 elements are very reactive, they are alkalis. When mixed with oxygen they produce metal oxides and when mixed with water they produce metal hydroxides and hydrogen. However, group 0/8 metals are very unreactive and are also known as noble gases. They have 8 electrons on their outer shell except for helium. 

Chemical bonds involve electrons from the reacting atoms. Compounds formed from metals and non-metals consist of ions. Ions are charged particles that form when atoms (or clusters of atoms) lose or gain electrons:

• metal atoms lose electrons to form positively charged ions
• non-metal atoms gain electrons to form negatively charged ions

Ionic Bonding

Ionic Bonding forms the production of ions and it is between a metal & non metal. The bond created is always strong. Examples of ionic bonding include sodium chloride and magnesium fluroide. Ionic bonding is the donation 

Covalent Bonding

Covalent bonding is between two non-metals. It involves the sharing of electrons. Covalent bonding produces simple molecular compounds (e.g. oxygen) or giant covalent structures (e.g. diamond). Some examples of covalent bonding are chlorine and carbon dioxide. Molecules are atoms that are joined by a covalent bond.

Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.

copper + oxygen → copper oxide

If we just replace the words shown above by the correct chemical formulas, we will get an unbalanced equation, as shown here:

Cu + O₂ → CuO

Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.

Here is the balanced symbol equation:

2Cu + O₂    →    2CuO

YOU MUST NEVER CHANGE THE SMALL NUMBERS BECAUSE THAT CHANGES THE SUBSTANCE COMPLETETLY

Calcium carbonate breaks down when heated strongly. This reaction is called thermal decomposition.

Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them. Indeed, not all the carbonates of group 1 metals decompose at the temperatures reached by a Bunsen burner.

Metals low down in the reactivity series, such as copper, have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show thermal decomposition. It is easily decomposed and its colour change, from green copper carbonate to black copper oxide, is easy to see.

Making calcium oxide

If calcium carbonate is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is yellow when hot, but white when cold.

CaCO₃ CaO + CO₂                    This is a thermal decomposition reaction.

Making calcium hydroxide

Calcium oxide reacts with water to form calcium hydroxide, which is an alkali. Here are the equations for this reaction:

calcium oxide + water → calcium hydroxide

CaO + H₂O → Ca(OH)₂

A lot of heat is produced in the reaction, which may even cause the water to boil.

Uses of limestone

Limestone is a type of rock, mainly composed of calcium carbonate. Limestone is quarried (dug out of the ground) and used as a building material. It is also used in the manufacture of cement, mortar and concrete.

Reactions with acids
Carbonates react with acids to produce carbon dioxide, a salt and water. For example:

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

CaCO₃ + 2HCl → CO₂ + CaCl₂ + H₂O

Since limestone is mostly calcium carbonate, it is damaged by acid rain. Sodium carbonate, magnesium carbonate, zinc carbonate and copper carbonate also react with acids: they fizz when in contact with acids, and the carbon dioxide released can be detected using limewater.

Uses of limestone (part 2)

Calcium Hydroxide

When limestone is heated strongly, the calcium carbonate it contains decomposes to form calcium oxide. This reacts with water to form calcium hydroxide, which is an alkali. Calcium hydroxide is used to neutralise excess acidity, for example, in lakes and soils affected by acid rain.

Cement, mortar and concrete

Cement is made by heating powdered limestone with clay. Cement is an ingredient in mortar and concrete:

• mortar, used to join bricks together, is made by mixing cement with sand and water
• concrete is made by mixing cement with sand, water and aggregate (crushed rock)

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 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 acid 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.

Quarrying - 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.
• Produces valuable material.

Quarrying - 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.
• Spoils landscape.
• The area used for quarrying could instead be used for other things (e.g. growing crops)

Metals are very useful. Ores are naturally occurring rocks that contain metal or metal compounds in sufficient amounts to make it worthwhile extracting them: most everyday metals are mixtures called alloys.

Methods of extracting metals

The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To become useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in high enough concentration to make it economic to extract the metal.

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

Gold, because it is so unreactive, is found as the native metal and not as a compound. It does not need to be chemically extracted from its ore, but chemical reactions may be needed to remove other elements that might contaminate the metal.

Transition Metals

The transition metals are placed in the periodic table in a large block between groups 2 and 3. Most metals (including iron, titanium and copper) are transition metals.

Common properties of transition metals are that they are metals, they are good conductors of heat and electricity and they can be hammered or bent into shape. The transition metals are useful as construction materials. They are also useful for making objects that need to let electricity or heat travel through them easily.

Iron

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon → iron + carbon dioxide

2Fe₂O₃ + 3C → 4Fe + 3CO₂

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide will also reduce iron oxide:

iron oxide + carbon monoxide → iron + carbon dioxide

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Copper

Copper is soft and easily bent and so is a good conductor of electricity, which makes it useful for wiring. Copper is also a good conductor of heat and it does not react with water. This makes it useful for plumbing, and making pipes and tanks.

Copper Ores

Some copper ores are copper-rich – they have a high concentration of copper compounds. Copper can be extracted from these ores by heating them in a furnace, a process called smelting. The copper is then purified using a process called electrolysis.

Electricity is passed through solutions containing copper compounds, such as copper sulfate. During electrolysis, positively charged copper ions move towards the negative electrode and are deposited as copper metal.

The future of copper

We are running out of copper-rich ores. Research is being carried out to find new ways to extract copper from the remaining low-grade ores, without harming the environment too much. This research is very important, as traditional mining involves huge open-cast mines that produce a lot of waste rock.

Phytomining, bioleaching and scrap iron

Some plants absorb copper compounds through their roots. They concentrate these compounds as a result of this. The plants can be burned to produce an ash that contains the copper compounds. This method of extraction is called phytomining.

Some bacteria absorb copper compounds. They then produce solutions called leachates, which contain copper compounds. This method of extraction is called bioleaching.

Copper can also be extracted from solutions of copper salts using scrap iron. Iron is more reactive than copper, so it can displace copper from copper salts. For example:

iron + copper sulfate → iron sulfate + copper

Alluminium and Titanium

Aluminium and titanium are two metals with a low density. They have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful.

Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.

Extraction 

Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon. You do not need to know any details of how these metals are extracted, but existing methods are expensive because the processes have many stages and arge amounts of energy are needed 

Recycling

Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment.

Alloys

The properties of a metal are changed by adding other elements to it. 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.

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

• brass, used in electrical fittings, is 70 percent copper and 30 percent zinc
• 18 carat gold, used in jewellery, is 75 percent gold and 25 percent copper and other metals
• duralumin, used in aircraft manufacture, is 96 percent aluminium and 4 percent copper and other metals.

Iron 

Pure iron is soft and easily shaped because its atoms are arranged in a regular way that lets layers of atoms slide over each other. Pure iron is too soft for many uses.

Iron from the blast furnace is an alloy of about 96 percent iron, with carbon and some other impurities. It is hard, but too brittle for most uses, so most iron from the blast furnace is converted into steel by removing some of the carbon.

Steel

Carbon is removed from molten iron by blowing oxygen into it. The oxygen reacts with the carbon, producing carbon monoxide and carbon dioxide, which escape from the molten metal. Enough oxygen is used to achieve steel with the desired carbon content. Other metals are often added, such as vanadium and chromium, to produce alloys with properties suited to specific uses.

There are many different types of steel, depending on the other elements mixed with the iron.

A summary of the properties of some different steels

Type of steel                  Iron alloyed with              Properties                    Typical use  low carbon steel          about 0.25 percent      carbon easily shaped     car body panels high carbon steel         up to 2.5 percent        carbon hard                     cutting tools  stainless steel             chromium and nickel   resistant to corrosion        cutlery and sinks

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.

Alkanes

Crude oil forms naturally over millions of years from the remains of living things. Most of the compounds in crude oil are hydrocarbons. These are compounds that contain hydrogen and carbon atoms only, joined together by chemical bonds called covalent bonds. There are different types of hydrocarbon, but most of the ones in crude oil are alkanes.

The alkanes are a family of hydrocarbons that share the same general formula. This is:

C_nH_2n+2

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH₄ and ethane is C₂H₆.

Alkane molecules can be represented by displayed formulas. In a displayed formula, each atom is shown as its symbol (C or H) and each covalent bond by a straight line. This table shows four different alkanes.

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from burning or combustion, which is their reaction with oxygen in the air.

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.

The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.

The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.

This is the sequence of events in distillation:

heating → evaporating → cooling → condensing

Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.

The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Oil Fractions

As you go up the fractionating column, the hydrocarbons have:

• lower boiling points
• lower viscosity (they flow more easily)
• higher flammability (they ignite more easily).

This means that in general hydrocarbons with small molecules make better fuels than hydrocarbons with large molecules.

Fractions that are produced by the distillation of crude oil can go through a process called cracking, a chemical reaction which produces smaller hydrocarbons, including alkanes and alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can be used to make polymers.

Cracking

Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient: they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules.

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either, passed over a hot catalyst, or mixed with steam and heated to a very high temperature.

These processes break chemical bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes (another type of hydrocarbon).

Some of the smaller hydrocarbons formed by cracking are used as fuels, and the alkenes are used to make polymers in plastics manufacture.

The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula: C_nH_2n

The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C₂H₄ and propene is C₃H₆.

Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that: they can be used to make ethanol and polymers (plastics) - two crucial products in today's world.

Testing for unsaturation

Bromine water is a dilute solution of bromine, normally orange-brown in colour. It becomes colourless when shaken with an alkene, but its colour remains the same when it is shaken with alkanes.

Monomers and polymers

Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers.

Alkenes can act as monomers because they are unsaturated (they have a double bond) for example ethene can polymerise to form poly(ethene), also called polythene.

Polymer molecules are very large compared with most other molecules, so the idea of a repeating unit is used when drawing a displayed formula. When drawing one, starting with the monomer:

• change the double bond in the monomer to a single bond in the repeating unit
• add a bond to each end of the repeating unit.

Examples of polymers and their uses:

• Polyethene - plastic bags and bottles
• Polypropene - crates and ropes
• Polychloroethene - water pipes and insulation on electricity cables

Polymers have properties that depend on the chemicals they are made from, and the conditions in which they are made. For example, there are two main types of poly(ethene): LDPE, low-density poly(ethene), is weaker than HDPE, high-density poly(ethene), and becomes softer at lower temperatures.

Modern polymers have many uses, including:

• new packaging materials
• waterproof coatings for fabrics (such as for outdoor clothing)
• fillings for teeth
• dressings for cuts
• hydrogels (for example for soft contact lenses and disposable nappy liners)
• smart materials (for example shape memory polymers for shrink-wrap packaging).

One of the useful properties of polymers is that they are unreactive, so they are suitable for storing food and chemicals safely. Unfortunately, this property makes it difficult to dispose of polymers. They can cause litter and are usually sent to landfill sites.

Biodegradable plastics

Most polymers, including poly(ethene) and poly(propene) are not biodegradable, so they may last for many years in rubbish dumps. However, it's possible to include substances such as cornstarch that cause the polymer to break down more quickly. Carrier bags and refuse bags made from such degradable polymers are available now.

Recycling

Many polymers can be recycled. This reduces the disposal problems and the amount of crude oil used. But the different polymers must be separated from each other first, and this can be difficult and expensive to do

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

Making ethanol from ethene and steam

Ethanol can be made by reacting ethene (from cracking crude oil fractions) with steam. A catalyst of phosphoric acid is used to ensure a fast reaction.

Notice that ethanol is the only product. The process is continuous – as long as ethene and steam are fed into one end of the reaction vessel, ethanol will be produced. These features make it an efficient process, but there is a problem. Ethene is made from crude oil, which is a non-renewable resource. It cannot be replaced once it is used up and it will run out one day.

Fermentation

Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen. Unlike ethene, sugar from plant material is a renewable resource.

Vegetable oils are obtained from plants. They are important ingredients in many foods, and can be hardened through a chemical process to make, for example, margarine. They can also be used as fuels, for example as biodiesel. Emulsifiers are food additives that prevent oil and water mixtures in food from separating.

Vegetable oils

Vegetable oils are natural oils found in seeds, nuts and some fruit. These oils can be extracted. The plant material is crushed and pressed to squeeze the oil out. Olive oil is obtained this way. Sometimes the oil is more difficult to extract and has to be dissolved in a solvent. Once the oil is dissolved, the solvent is removed by distillation, and impurities such as water are also removed, to leave pure vegetable oil. Sunflower oil is obtained in this way.

Vegetable oils have higher boiling points than water. This means that foods can be cooked or fried at higher temperatures than they can be cooked or boiled in water. Food cooked in vegetable oils:

• cook faster than if they were boiled
• have different flavours than if they were boiled.

However, vegetable oils are a source of energy in the diet. Food cooked in vegetable oils releases more energy when it is eaten than food cooked in water. This can have an impact on our health. For example, people who eat a lot of fried food may become overweight.

Saturated and unsaturated fats and oils

The fatty acids in some vegetable oils are saturated: they only have single bonds between their carbon atoms. Saturated oils tend to be solid at room temperature, and are sometimes called vegetable fats instead of vegetable oils. Lard is an example of a saturated oil.

The fatty acids in some vegetable oils are unsaturated: they have double bonds between some of their carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are useful for frying food. They can be divided into two categories Unsaturated fats are thought to be a healthier option in the diet than saturated fats.

Vegetable oils do not dissolve in water. If oil and water are shaken together, tiny droplets of one liquid spread through the other liquid, forming a mixture called an emulsion.

Emulsions are thicker (more viscous) than the oil or water they contain. This makes them useful in foods such as salad dressings and ice cream. Emulsions are also used in cosmetics and paints. There are two main types of emulsion:

• oil droplets in water (milk, ice cream, salad cream, mayonnaise)
• water droplets in oil (margarine, butter, skin cream, moisturising lotion).

Emulsifiers

If an emulsion is left to stand, eventually a layer of oil will form on the surface of the water. Emulsifiers are substances that stabilise emulsions, stopping them separating out. Egg yolk contains a natural emulsifier. Mayonnaise is a stable emulsion of vegetable oil and vinegar with egg yolk.

Emulsifier molecules have two different ends:

• a hydrophilic end - 'water-loving' - that forms chemical bonds with water but not with oils
• a hydrophobic end - 'water-hating' - that forms chemical bonds with oils but not with water.

Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different substances. A molecular model of one of these substances is seen in the diagram.

The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the water and oil droplets become unable to separate out.

Unsaturated vegetable oils contain double carbon-carbon bonds. These can be detected using bromine water (just as alkenes can be detected). Bromine water becomes colourless when shaken with an unsaturated vegetable oil, but it stays orange-brown when shaken with a saturated vegetable fat.

Bromine water can also be used to determine the amount of unsaturation of a vegetable oil. The more unsaturated a vegetable oil is, the more bromine water it can decolourise.

Saturated vegetable fats are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry. Unsaturated vegetable oils can be ‘hardened’ by reacting them with hydrogen, a reaction called hydrogenation.

During hydrogenation, vegetable oils are reacted with hydrogen gas at about 60ºC. A nickel catalyst is used to speed up the reaction. The double bonds are converted to single bonds in the reaction. In this way unsaturated fats can be made into saturated fats – they are hardened.

The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet.

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth’s mantle. These are driven by the heat produced by the natural decay of radioactive elements in the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places known for having earthquakes.

Before Wegner

The theory of plate tectonics and continental drift was proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before Wegener developed his theory, it was thought that mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth’s crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case.

After Wegener

Wegener suggested that mountains were formed when the edge of a drifting continent collided with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia. It took more than 50 years for Wegener’s theory to be accepted. One of the reasons was that it was difficult to work out how whole continents could move: it was not until the 1960s that enough evidence was discovered to support the theory fully.

Volcanic activity

Where tectonic plates meet, the Earth’s crust becomes unstable as the plates slide past each other, push against each other, or ride under or over one another. Earthquakes and volcanic eruptions happen at the boundaries between plates. Magma (molten rock) is less dense than the crust. It can rise to the surface through weaknesses in the crust, forming a volcano.

Geologists study volcanoes to try to predict future eruptions. Volcanoes can be very destructive, but some people choose to live near them because volcanic soil is very fertile.

Earthquakes

The movement of tectonic plates can be sudden and disastrous, causing an earthquake. It is difficult to predict exactly when and where an earthquake will happen, even when a lot of data is available.

Heat can be transferred from place to place by conduction, convection and radiation. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.

The morden atmosphere

The Earth’s atmosphere has remained much the same for the past 200 million years.

The two main gases are both elements and account for about 99 percent of the gases in the atmosphere. They are:

• about ⁴/₅ or 80 percent nitrogen (a relatively unreactive gas)
• about ¹/₅ or 20 percent oxygen (the gas that allows animals and plants to respire and for fuels to burn)

The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

The percentage of oxygen in the air can be measured by passing a known volume of air over hot copper and measuring the decrease in volume as the oxygen reacts with it. Here are the equations for this reaction:

copper + oxygen → copper oxide

2Cu + O₂ → 2CuO

Gas syringes are used to measure the volume of gas in the experiment. The starting volume of air is often 100 cm³ to make the analysis of the results easy, but it could be any convenient volume. In the simulation, there is 100 cm³ of air at the start.

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.

There is evidence that the first living things appeared on Earth billions of years ago. There are many scientific theories to explain how life began. One theory involves the interaction between hydrocarbons, ammonia and lightning.

The Miller-Urey experiment

Stanley Miller and Harold Urey carried out some experiments in 1952 and published their results in 1953. The aim was to see if substances now made by living things could be formed in the conditions thought to have existed on the early Earth.

The two scientists sealed a mixture of water, ammonia, methane and hydrogen in a sterile flask. The mixture was heated to evaporate water to produce water vapour. Electric sparks were passed through the mixture of water vapour and gases, simulating lightning. After a week, contents were analysed. Amino acids, the building blocks for proteins, were found.

The Miller-Urey experiment supported the theory of a ‘primordial soup’, the idea that complex chemicals needed for living things to develop could be produced naturally on the early Earth.

The Earth’s early atmosphere is believed to have been mainly carbon dioxide with little or no oxygen gas. The Earth’s atmosphere today contains around 21 percent oxygen and about 0.04 percent carbon dioxide. So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?

Increasing oxygen 

Plants and algae can carry out photosynthesis. This process uses carbon dioxide from the atmosphere (with water and sunlight) to produce oxygen (and glucose). The appearance of plants and algae caused the production of oxygen, which is why the proportion of oxygen went up.

Photosynthesis by plants and algae used carbon dioxide from the atmosphere, but this is not the only reason why the proportion of carbon dioxide went down. These processes also absorb carbon dioxide from the atmosphere:

• dissolving in the oceans
• the production of sedimentary rocks such as limestone
• the production of fossil fuels from the remains of dead plants and animals

Today, the burning of fossil fuels (coal and oil) 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, contributing to global warming. It also means that the oceans are becoming more acidic as they dissolve increasing amounts of carbon dioxide. This has an impact on the marine environment, for example making the shells of sea creatures thinner than normal.

You will recall that about 78 percent of the air is nitrogen and 21 percent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies:

• water vapour condenses, and is removed using absorbent filters
• carbon dioxide freezes at –79ºC, and is removed
• oxygen liquefies at –183ºC
• nitrogen liquefies at –196ºC.

The liquid nitrogen and oxygen are then separated by fractional distillation.

The liquefied air is passed into the bottom of a fractionating column. Just as in the columns used to separate oil fractions, the column is warmer at the bottom than it is at the top.

The liquid nitrogen boils at the bottom of the column. Gaseous nitrogen rises to the top, where it is piped off and stored. Liquid oxygen collects at the bottom of the column. The boiling point of argon - the noble gas that forms 0.9 percent of the air - is close to the boiling point of oxygen, so a second fractionating column is often used to separate the argon from the oxygen.

Uses of nitrogen and oxygen

• liquid nitrogen is used to freeze food
• food is packaged in gaseous nitrogen to increase its shelf life
• oil tankers are flushed with gaseous nitrogen to reduce the chance of explosion
• oxygen is used in the manufacture of steel and in medicine.