Chemistry Unit 1

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons. Neutrons are neutral, but protons and electrons are electrically charged: protons have a relative charge of +1 and electrons have a relative charge of -1.

Atoms and elements

All substances are made of tiny particles called atoms. An element is a substance that is made of only one sort of atom. There are about 100 different elements. These are shown in the periodic table, which is a chart with all the elements arranged in a particular way. The horizontal rows in the periodic table are called periods and the vertical columns are called groups. The elements in a group have similar properties to each other.

Metals and non-metals

The metals are shown on the left of the periodic table, and the non-metals are shown on the right. The dividing line between metals and non-metals is shown in red on the table below. You can see that most of the elements are metals.

The periodic table divided into non-metals and metals

Chemical symbols

The atoms of each element are represented by a chemical symbol. This usually consists of one or two different letters, but sometimes three letters are used for newly discovered elements. For example, O represents an oxygen atom, and Na represents a sodium atom.

The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for a magnesium atom is Mg and not mg, MG or mG.

Atomic structure

Structure of the atom

All substances are made from tiny particles called atoms. An atom has a small central nucleus made up of smaller sub-atomic particles called protons and neutrons. The nucleus is surrounded by even smaller sub-atomic particles called electrons.

Protons and electrons have an electrical charge. Both have the same size of electrical charge, but the proton is positive and the electron negative. Neutrons are neutral.

A summary of the electrical charges in sub-atomic particles

Name of particle	Electrical charge
proton	+1
neutron	0
electron	-1

The number of electrons in an atom is equal to the number of protons in its nucleus. This means atoms have no overall electrical charge.

Atomic number and mass number

The atomic number of an atom is the number of protons it contains. All the atoms of a particular element have the same atomic number (number of protons). The atoms of different elements have different numbers of protons. For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons.

The mass number of an atom is the total number of protons and neutrons it contains. The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

Full chemical symbols

You need to be able to calculate the number of each sub-atomic particle in an atom if you are given its atomic number and its mass number. The full chemical symbol for an element shows its mass number at the top, and its atomic number at the bottom.

The full symbol for a chlorine atom

This symbol tells you that the chlorine atom has 17 protons. It will also have 17 electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells you that the total number of protons and neutrons in the chlorine atom is 35. Note that you can work out the number of neutrons from the mass number and atomic number. In this example, it is 35 – 17 = 18 neutrons.

Electronic structure

The electrons in an atom occupy energy levels. These are also called shells. Each electron in an atom is found in a particular energy level. The lowest energy level (innermost shell) fills with electrons first. Each energy level can only hold a certain number of electrons before it becomes full. The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.

Electrons in the first three energy levels for the elements with atomic numbers 1 to 20

Energy level or shell	Maximum number of electrons
first	2
second	8
third	8

Writing an electronic structure

The electronic structure of an atom is written using numbers to represent the electrons in each energy level. For example, for sodium this is 2,8,1 – showing that there are:

• 2 electrons in the first energy level
• 8 electrons in the second energy level
• 1 electron in the third energy level.

You can work out the electronic structure of an atom from its atomic number or its position in the periodic table. Start at hydrogen, H, and count the elements needed to reach the element you are interested in. For sodium, it takes:

• 2 elements to reach the end of the first period (row)
• 8 elements to reach the end of the second period
• 1 element to reach sodium in the third period.

The diagram of the periodic table shows how this works.

Periodic table related to electronic structure

You need to be able to write the electronic structure of any of the first twenty elements (hydrogen to calcium).

Electronic structure diagrams

You need to be able to draw the electronic structure of any of the first twenty elements (hydrogen to calcium). In these drawings:

• the nucleus is shown as a black spot
• each energy level is shown as a circle around the nucleus
• each electron is shown by a dot or a cross.

The electronic structure of some elements

Element	Symbol	Electronic structure (written)	Electronic structure (drawn)
lithium	Li	2,1
fluorine	F	2,7
chlorine	Cl	2,8,7
calcium	Ca	2,8,8,2

Do not worry in the exam about colouring in the electrons. Just make them clear and ensure they are in the right place. You may be asked to use a cross rather than a dot for each electron.

Working out an element's electronic structure

Here is how to use the periodic table to work out an electronic structure:

1. Find the element in the periodic table. Work out which period (row) it is in, and draw that number of circles around the nucleus.
2. Work out which group the element is in and draw that number of electrons in the outer circle – with eight for Group 0 elements – except helium.
3. Fill the other circles with as many electrons as needed. Remember – two in the first circle, and eight in the second and third circles.
4. Finally, check that the number of electrons is the same as the atomic number.

Elements in the same group in the periodic table have similar chemical properties. This is because their atoms have the same number of electrons in the highest occupied energy level. Group 1 elements are reactive metals called the alkali metals. Group 0 elements are unreactive non-metals called the noble gases.

Groups

A vertical column of elements in the periodic table is a group. The elements in a group have similar chemical properties to each other. For example, group 1 contains sodium and other very reactive metals, while group 7 contains chlorine and other very reactive non-metals. Group 0 (also known as group 8 or group 18) contains helium and other very unreactive non-metals.

The modern periodic table

Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.

Group 1

The group 1 elements are found on the left hand side of the periodic table. They are called the alkali metals because they form alkaline compounds.

Group 1 of the periodic table

Their atoms all have one electron in their highest occupied energy level (outermost shell). This gives the group 1 elements similar chemical properties to each other.

Group 1 elements

Element	Symbol	Electronic structure (written)	Electronic structure (drawn)
lithium	Li	2,1
sodium	Na	2,8,1
potassium	K	2,8,8,1

Reactions of group 1 elements with water

Potassium reacts with a lilac flame

Lithium, sodium and potassium all react vigorously with water to form a metal hydroxide and hydrogen:

metal + water → metal hydroxide + hydrogen

The metal hydroxides are strong alkalis.

The group 1 elements need to be stored under oil to prevent them reacting with oxygen and water vapour in the air.

Reactions of group 1 elements with oxygen

Lithium, sodium and potassium are easily cut with a blade. The freshly cut surfaces are silvery and shiny, but quickly turn dull as the metal reacts with oxygen in the air. The group 1 metals react vigorously with oxygen to form metal oxides. Lithium burns with a red flame, sodium with a yellow-orange flame, and potassium burns with a lilac flame.

Group 0

The group 0 elements are found on the right hand side of the periodic table. They are called the noble gases because they are very unreactive.

The noble gases - Group 0

The highest occupied energy levels (outermost shells) of their atoms are full:

• helium atoms have two electrons in their outer energy level
• atoms of the other noble gases have eight electrons in their outer energy level.

This is a neon atom it has a stable arrangement of electrons, and makes the group 0 elements unreactive.

When elements react, their atoms join with other atoms to form compounds. Chemical bonds form when this happens, which involves atoms transferring or sharing electrons.

Reactions and compounds

New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur react together to form a compound called iron sulfide.

Compounds usually have different properties from the elements they contain.

Ionic bonds

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

The ionic bond is the force of attraction between the oppositely charged ions. This animation shows how ions form when sodium atoms react with chlorine atoms to form sodium chloride.

Covalent bonds

Compounds formed from non-metals consist of molecules. The atoms in a molecule are joined together by covalent bonds. These bonds form when atoms share pairs of electrons.

Chemical formulas

The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound. For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na₂O. Notice that the 2 is written as a subscript, so Na2O would be wrong.

This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is written as CO₂.

Carbon dioxide units contain one carbon atom and two oxygen atoms

Sometimes you see more complex formulae such as Na₂SO₄ and Fe(OH)₃:

• a unit of Na₂SO₄ contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
• a unit of Fe(OH)₃ contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H

Chemical equations

You should be able to write word equations for the reactions you study in GCSE Science or GCSE Chemistry. If you are taking the Higher Tier, you should also be able to write and balance symbol equations.

Copper and oxygen reaction: getting a balanced equation

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

Take a look at this word equation for the reaction:

copper + oxygen → copper oxide

Copper and oxygen are the reactants because they are on the left of the arrow. Copper oxide is the product because it is on the right of the arrow.

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 can see that now there are two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

Remember: never change a formula to balance an equation.

Conservation of mass

No atoms are lost or made during a chemical reaction. This means that the mass is always conserved. In other words, the total mass of products after the reaction is the same as the total mass of the reactants at the start.

This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:

Carbon reacts with oxygen to form carbon dioxide:

C + O₂ → CO₂

12 g of carbon will react to form 44 g of carbon dioxide. It must react with 44 – 12 = 32 g of oxygen to do this. The animation below shows two more examples of conservation of mass.

Limestone is mainly calcium carbonate, CaCO₃, which when heated 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, including being used to make cement, mortar and concrete.

Thermal decomposition

Calcium carbonate breaks down when heated strongly. This reaction is calledthermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:

calcium carbonatecalcium oxide + carbon dioxide

CaCO₃CaO + CO₂

Other metal carbonates decompose in the same way, including:

• sodium carbonate
• magnesium carbonate
• copper carbonate

For example, here are the equations for the thermal decomposition of copper carbonate:

copper carbonate copper oxide + carbon dioxide

CuCO₃CuO + CO₂

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 areeasily 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.

Products from calcium carbonate

For your exam, you need to know how calcium hydroxide is obtained from calcium carbonate.

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.

Here are the equations for this reaction:

calcium carbonate  calcium oxide + carbon dioxide

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.

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 analkali. 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

You need to be able to evaluate some of the effects of the limestone industry.

The main advantages and disadvantages of the limestone industry

Advantages	Disadvantages
Limestone is a valuable natural resource, used to make things such as glass and concrete.	Limestone quarries are visible from long distances and may permanently disfigure the local environment.
Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry.	Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life.

Links

You may wish to read thisBBC News item (2006) about a public inquiry into a limestone quarry in the Peak District National Park. Consider the arguments for and against quarrying limestone in a national park.

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.

Ores are mined. They may need to be concentrated before the metal is extracted and purified. The economics of using a particular ore may change over time. For example, as a metal becomes rarer, an ore may be used when it was previously considered too expensive to mine.

Reactivity and extraction method

Metals are produced when metal oxides are reduced (have their oxygen removed). The reduction method depends on the reactivity of the metal. For example, aluminium and other reactive metals are extracted by electrolysis, while iron and other less reactive metals may be extracted by reaction with carbon or carbon monoxide.

Reactivity and extraction method

Metals (in decreasing order of reactivity)	Method of extraction
potassium sodium calcium magnesium aluminium	extract by electrolysis
carbon
zinc iron tin lead	extract by reaction with carbon orcarbon monoxide
hydrogen
copper silver gold platinum	extracted in various ways

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 acompound. 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.

The transition metals

Common properties

The transition metals have these properties in common:

• they are metals
• they are good conductors of heat and electricity
• they can be hammered or bent into shape easily

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 calledreduction reactions.

Blast furnace in a modern steel works

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 calledelectrolysis.

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 calledbioleaching.

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

Aluminium and titanium

Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also 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.

Block of aluminium metal - image does not show the transparent oxide layer

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
• large 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.

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

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 and steel

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.

Layers of atoms slide over each other when metals are bent or stretched

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.

Structure of alkanes

alkane	Molecular formula	Displayed formula	Molecular model
methane	CH4
ethane	C2H6
propane	C3H8
butane	C4H10

Notice that the molecular models show that the bonds are not really at 90°C, but this makes displayed formulas easier to draw.

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.

Distillation process to separate ethanol from water

Step 1 - water and ethanol solution are heated

1. Back
2. 1
3. 2
4. 3
5. Next

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

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

The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases leave at the top of the column, the liquids condense in the middle and the solids stay at the bottom.

The fractionating column

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.

Several waste products are released when fuels burn. These do not just disappear and they can harm the environment by contributing to global warming, global dimming and acid rain.

Combustion of fuels

Complete combustion

Fuels burn when they react with oxygen in the air. If there is plenty of air, complete combustion happens. Coal is mostly carbon. During complete combustion, carbon is oxidised to carbon dioxide:

carbon + oxygen → carbon dioxide

Carbon dioxide is a greenhouse gas. Increasing concentrations of it in the atmosphere contribute to global warming.

Hydrocarbon fuels contain carbon and hydrogen. During combustion, hydrogen is oxidised to water (remember that water, H2O, is an oxide of hydrogen). In general:

hydrocarbon + oxygen → carbon dioxide + water

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

Clouds of smoke and other combustion products are emitted from chimneys

Incomplete combustion

If there is insufficient air for complete combustion, incomplete combustion (also called partial combustion) happens. Hydrogen is still oxidised to water, but carbon monoxide forms instead of carbon dioxide. Carbon monoxide is a toxic gas, so adequate ventilation is important when burning fuels.

Solid particles (particulates) are also released. These contain carbon and are seen as soot or smoke. Particulates cause global dimming. They reduce the amount of sunlight reaching the Earth’s surface.

Acidic oxides

Carbon dioxide dissolves in water in the atmosphere to form a weakly acidic solution. This means that rainwater is naturally slightly acidic. However, some of the products from burning fuels make rainwater more acidic than normal. This is acid rain.

Acid rain reacts with metals and rocks such as limestone, causing damage to buildings and statues. Acid rain damages the waxy layer on the leaves of trees. This makes it more difficult for trees to absorb the minerals they need for healthy growth and they may die. Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

Sulfur dioxide

Coal and most hydrocarbon fuels naturally contain some sulfur compounds. When the fuel burns, the sulfur it contains is oxidised to sulfur dioxide:

sulfur + oxygen → sulfur dioxide

This gas dissolves in water to form an acidic solution. It is a cause of acid rain.

Sulfur can be removed from fuels before they are used. ‘Low sulfur’ petrol and diesel are widely available at filling stations to use in vehicles. In power stations, sulfur dioxide can be removed from the waste gases before they are released from chimneys. The waste gases are treated with powdered limestone. The sulfur dioxide reacts with it to form calcium sulfate. This can be used to make plasterboard for lining interior walls, so turning a harmful product into a useful one.

The process of removing sulfur dioxide

Oxides of nitrogen

At the high temperatures found in an engine or furnace, nitrogen and oxygen from the air can react together. They produce various oxides of nitrogen, often called NO_x. These also cause acid rain.

Biofuels

Coal and crude oil are non-renewable resources. They take so long to form that they cannot be replaced once they have all been used up. This means that these fossil fuels are likely to become more expensive as they begin to run out. Petrol, diesel and other fuels produced from crude oil make a range of harmful substances when they are burned, including:

• carbon dioxide
• carbon monoxide
• water vapour
• particulates (solid particles)
• sulfur dioxide
• oxides of nitrogen or NO_x.

Biofuels are fuels produced from plant material. They have some advantages and disadvantages compared to fossil fuels.

Biodiesel

Biodiesel is made from rapeseed oil and other plant oils. It can be used in diesel-powered vehicles without needing any modifications to the engine.

Bioethanol

Ethanol, C2H5OH, is not a hydrocarbon because it contains oxygen as well as hydrogen and carbon. However, it is a liquid fuel that burns well. Bioethanol is made by fermenting sugars from sugar cane, wheat and other plants. It cannot be used on its own unless the engine is modified. However, modern petrol engines can use petrol containing up to 10 percent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol.

Ethical concerns

There are ethical issues surrounding the use of biofuels. For example, crops that could be used to feed people are used to provide the raw materials for biofuels instead. This could cause food shortages or increases in the price of food. There are other economic issues surrounding the use of biofuels, including:

• human resources -more people are needed to produce biofuels than are needed to produce petrol and diesel
• increased income - for farmers
• lower fuel prices - biofuels limit the demand for fossil fuels, helping to reduce increases in fuel prices.

There are environmental issues surrounding the use of biofuels. Biodiesel naturally contains little sulfur. For example, it may be said that they are carbon neutral – the amount of carbon dioxide released when they are used is the same as the amount absorbed by the plants as they grew. If so, this would reduce the production of this greenhouse gas. However, while biofuels produce less carbon dioxide overall, they are not carbon neutral. This is because fossil fuels are used in their production, for example in making fertilisers for the growing plants.

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 and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.

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.

Alkenes

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.

Alkene molecules can be represented by displayed formulas in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

Structure of alkenes

alkene	Formula	Displayed formula	Molecular model
ethene	C2H4
propene	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.

The bromine water test is a test for unsaturation.

Polymers have many applications and new uses are being developed. However, they can be difficult to dispose of.

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.

In general: lots of monomer molecules → a polymer molecule.

This animation shows how several chloroethene monomers can join end-to-end to make poly(chloroethene), also called PVC.

Alkenes can act as monomers because they are unsaturated (they have a double bond):

• ethene can polymerise to form poly(ethene), also called polythene
• propene can polymerise to form poly(propene), also called polypropylene.

Displayed formulas of polymers

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.

Addition polymerisation

Uses of polymers

Different polymers have different properties, so they have different uses. The table below gives some examples.

Examples of polymers and their uses

Polymer	Typical use
polythene	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).

Problems with polymers

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

Watch

You may wish to view thisBBC News item (2002) about degradable carrier bags.

Watch

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

Polymers have recycling symbols like this one for PVC to show what they are

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

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.

Structure of ethanol

Ethanol molecules contain carbon, hydrogen and oxygen atoms.

Structure of ethanol

Formula	Displayed formula	Molecular model
C2H5OH

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.

ethene + steam → ethanol

C₂H₄ + H₂O → C₂H₅OH

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.

C₆6H₁₂O₆ → 2C₂H₅OH + 2CO₂

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.

Structure of vegetable oils

Molecules of vegetable oils consist of glycerol and fatty acids. In the diagram below you can see how three long chains of carbon atoms are attached to a glycerol molecule to make one molecule of vegetable oil.

The structure of a vegetable oil molecule

You do not need to know any details about the structure of vegetable oil molecules for the exam.

Vegetable oils in cooking

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:

• monounsaturated fats have one double bond in each fatty acid
• polyunsaturated fats have many double bonds.

Unsaturated fats are thought to be a healthier option in the diet than saturated fats.

Emulsions

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.

Emulsifiers- Higher tier

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.

Emulsifier molecules

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.

Double bonds and hydrogenation

Bromine water test

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.

Hydrogenation- Higher tier

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 structure of part of a fatty acid

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 structure of the Earth

The Earth’s crust, its atmosphere and oceans are the only sources of the resources that humans need.

Cross section showing structure of the Earth

The Earth is almost a sphere. These are its main layers, starting with the outermost:

1. crust (relatively thin and rocky)
2. mantle (has the properties of a solid, but can flow very slowly)
3. core (made from liquid nickel and iron)

The radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core. The Earth’s atmosphere surrounds the Earth.

Plate tectonics

The Earth's crust and upper part of the mantle are broken into large pieces calledtectonic 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 calledcontinental drift.

Plate boundaries

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.

Watch

You may wish to view thisBBC News item (2006) about the 100th anniversary of San Francisco’s great earthquake.

Watch

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

Wegener

Before Wegener

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.

Wegener’s theory

Alfred Wegener (1880 - 1930)

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.

This slideshow explains Wegener's theory.

Earth around 200 million years ago, at the time of Pangaea

1. Back
2. 1
3. 2
4. 3

Volcanoes and earthquakes

There are two main types of tectonic plate:

• Oceanic plates occur under the oceans.
• Continental plates form the land.

Oceanic plates are denser than continental plates. They are pushed down underneath continental plates if they meet.

Volcanic activity

Volcano

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

Emergency services searching for survivors

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.

Now try a Test Bite.

Heat can be transferred from place to place by conduction, convection andradiation. 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 modern atmosphere

You need to know the proportions of the main gases in the atmosphere.

The Earth’s atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.

The composition of air

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.

Oxygen in the air

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.

Note that there is some air in the tube with the copper turnings. The oxygen in this air will also react with the hot copper, causing a small error in the final volume recorded. It is also important to let the apparatus cool down at the end of the experiment, otherwise the final reading will be too high.

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.

Mars and Venus today

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.

The table shows the proportions of the main gases in their atmospheres.

Gas	Mars today	Venus today
carbon dioxide	95.3	96.5
nitrogen	2.7	3.5
argon	1.6	trace
oxygen, water vapour and other gases	trace	trace

Life on Earth

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 - Higher tier

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

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.

Oxygen and carbon dioxide

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.

Decreasing carbon dioxide

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.

Now try a Test Bite.

Fractional distillation of liquid air - Higher tier

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 (note that you do not need to recall the boiling points of the different gases):

• 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.

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.

Fractional distillation

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.