AQA Chemistry 1a

1) Its in the middle of the atom.

2) It contains protons and neutrons.

3) Protons are positively charged.

4) Neutrons have no charge (they're neutral).

5) The nucleus has an overall positive charge because of the protons.

6) Size-wise it is tiny compared to the rest on the atom.

1) Move around the nucleus.

2) They're negatively charged.

3) They're tiny but they cover a lot of space.

4) They occupy shells around the nucleus.

5) These shells explain the whole of chemistry.

1) Atoms have no overall charge.

2) The charge on the electrons is the same size as the charge on the protons - but opposite.

3) This means the number of protons always equals the number of electrons in an atom.

4) If some electrons are added or removed, the atom becomes charged and is then an ion.

1) Atoms can have different numbers of protons, neutrons and electrons.

2) For example, an atom with one proton in its nucleus is hydrogen and an atom with two protons is helium.

3) If a substance only contains one type of atom it's called an element.

4) There are about 100 different elements.

All the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons.

One or two letter symbol: - (shorthand for the full name)

C = Carbon

O = Oxygen

Mg = Magnesium

Most of these odd symbols come from the Latin names of the elements:

Na = Sodium

Fe = Iron

Pb = Lead

1) Elements with similar properties form columns.

2) Vertical columns are called groups and  Roman numerals are often used for them.

3) All of the elements in a group have the same number of electrons in their outershell.

e.g. Group 1 metals all react in the same way. They all react with water to form an alkaline solution and hydrogen gas, and they all react with oxygen to form an oxide.

Elements in Group 0 are noble gases. They are stable and unreactive.

The top number is the mass number. This is the total number of neutrons and protons.

The bottom number is the atomic number. This is the number of protons, which conveniently also tells you the number of electrons.

1) Electrons always occupy shells (sometimes called energy levels).

2) The lowest energy levels are always filled first - these are the ones close to the nucleus.

3) Only a certain number of electrons are allowed in each shell: 2,8,8

4) Atoms are much happier when they have full electron shells - like the noble gases in Group 0.

5) In most atoms the outer shell is not full and this makes the atom want to react to fill it.

1) When different elements react, atoms form chemical bonds with other atoms to form compounds. It's usually difficult to seperate out the two original elements again.

2) Making bonds involves atoms giving away, taking or sharing electrons. Only the electrons are involved - its nothing to do with the nuclei of the atoms at all.

3) A compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to form positive ions and the non-metal atoms gain electrons to form negative ions. The opposite charges (positive and negative) of the ions mean that they're strongly attracted to each other. This is called ionic bonding.

4) A compound formed from non-metals consists of molecules. Each atom shares an electron with another atom - this is called a covalent bond. Each atom has to make enough covalent bonds to fill up its outer shell.

5) The properties of a compound are totally different from the properties of the original elements. For example, if iron (a lustrous magnetic metal) and sulfur (a nice yellow powder) react, the compound formed (iron sulfide) is a dull grey solid lump, and doesnt behave anything like iron or sulfur.

6) Compounds can be small molecules like water, or great whopping latices like sodium chloride.

Carbon dioxide, CO2, is a compound formed from a chemical reaction between carbon and oxygen. It contains 1 carbon atom and 2 oxygen atoms.

The formula of sulfuric acid is H2SO4. So, each molecule contains 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms.

There might be brackets in a formula, e.g. calcium hydroxide Ca(OH)2. The little number outside the bracket applies to everything inside the brackets.

Balanced symbol equations show the reactant atoms and the product atoms and how they're arranged.

The mass of the reactants equals the mass of the products.

6g of magnesium and 4g of oxygen reacts to form 10g of magnesium oxide.

TIPS FOR BALANCING:

1) Find an element that doesn't balance and pencil in a number to try and sort it out.

2) See where it gets you. It may create another imbalance - so if so, just pencil in another number and see where it gets you.

Carry on chasing unbalanced elements and it'll sort itself out pretty quickly.

Limestone's quarriedout of the ground - it's great for making into blocks for building with. Fine old buildings like cathedrals are often made purely from limestone blocks.

Limestone is mainly calcium carbonate - CaCO3. When it's heated it thermally decomposes to make calcium oxide and carbon dioxide. CaCO3 --> CaO + CO2 Thermal decomposition is when one substance chemically changes into at least two new substances when it's heated.

When magnesium, copper, zinc and sodium carbonates are heated, they decompose in the same way. E.g. magnesium carbonate --> magnesium oxide + carbon dioxide. A bunsen burner cannot reach high enough temperatures to decompose some carbonates of Group 1 metals.

Calcium carbonate also reacts with acid to make a calcium salt, carbon dioxide and water. CaCO3 + H2SO4 --> CaSO4 + CO2 + H2O

The type of salt produced depends on the acid - hydrochloric acid would make a chloride (CaCl2). Other carbonates that react with acids are magnesium, copper, zinc and sodium.

This reaction means that limestone is damaged by acid rain.

When you add water to calcium oxide you get calcium hydroxide. CaO + H2O --> Ca(OH)2.

Calcium hydroxide is an alkali which can be used to neutralise acidic soil in fields. Powdered limestone can be used for this too, but an advantage of calcium hydroxide is that it works much faster.

Calcium hydroxide can also be used to test for carbon dioxide. If you make a soultion of calcium hydroxide in water (called limewater) and bubble gas through it, the solution will turn cloudy if there's carbon dioxide in the gas. The cloudiness is caused by the formation of calcium carbonate.

calcium hydroxide + carbon dioxide --> calcium carbonate + water

Ca(OH)2 + CO2 --> CaCO3 + H2O

Powdered limestone is heated in a kiln with powdered clay to make cement.

Cement can be mixed with sand and water to make mortar. Mortar is the stuff that you stick bricks together with. You can also add calcium hydroxide to mortar.

Or you can mix cement with sand and aggregate (water and gravel) to make concrete.

Digging limestone out of the ground can cause environmental problems.

• It makes huge ugly holes which permenantly change the landscape.
• Quarrying processes, like blasting rocks apart with explosives, make lots of noise and dust in quiet, scenic areas.
• Quarrying destroys the habitats of animals and birds.
• The limestone needs to be transported away from the quarry - usually in lorries. This causes more noise and pollution.
• Waste materials produce unsightly tips.

Cement factories make a lot of dust, which can cause breathing problems for some people.

Energy is needed to produce cement and quicklime. The energy is likely to come from burning fossil fuels, which causes pollution.

Limestone provides things people want - like housesand roads. Chemicals used in making dyes, paints and medicines also come from limestone.

Limestone products are used to neutralise acidic soil. Acidity in lakes and rivers caused by acid rain is also neutralised by limestone products.

Limestone is also used in power station chimneys to neutralise sulfur dioxide, which is a cause of acid rain.

The quarry and associated businesses provide jobs for people and bring more money into the local economy. This can lead to local improvements in transport, roads, recreation facilities and health.

Once quarrying is complete, landsacaping and restoration of the area is normally required as part of the planning permission.

Limestone and concrete (made from cement) are used as building materials. In some cases they're perfect for the job, but in other cases they're a bit of a compromise.

• Limestone is widely available and is cheaper than granite or marble. It's also a fairly easy rock to cut.
• Some limestone is more hard-wearing than marble, but it still looks attractive.
• Concrete can be poured into moulds to make blocks or panels that can be joined together. It's a very quick and cheap way of constructing buildings - and it shows... - concrete has got to be the most hideously unattractive building material ever known.
• Limestone, concrete and cement don't rot when they get wet like wood does. They can't be gnawed away by insects or rodents either. And to top it off, they're fire-resistant too.
• Concrete doesn't corrode like lots of metals do. It does have a fairly low tensile strength though, and can crack. If it's reinforced with steel bars it'll be much stronger.

A few unreactive metals like gold are found in the Earth as the metal itself, rather than as a compound.

A metal ore is a rock which contains enough metal to make it worthwhile extracting metal from it.

In many cases the ore is just an oxide of the metal. For example, the main aluminium ore is called bauxite - it's aluminium oxide (Al2O3).

Most metals need to be extracted from their ores using a chemical reaction. The economics (profitability) of metal extraction can change over time. For example:

• If the market price of a metal drops a lot, it might not be worth extracting it. If the price increases a lot then it might be worth extracting more of it.
• As technology improves, it becomes possible to extract more metal from a sample of rock than was originally possible. So it might now be worth extracting metal that wasn't worth extracting in the past.

A metal can be extracted from its ore chemically - by reduction or by electrolysis (splitting with electricity).

Some ores may have to be concentrated before the metal is extracted - this just involves getting rid of the unwanted rocky material.

Electrolysis can also be used to purify the extracted metal.

Occassionally, some metals are extracted from their ores using displacement reactions.

A metal can be extracted from its ore chemically by reduction using carbon. When an ore is reduced, oxygen is removed from it, e.g.

iron (III) oxide + carbon --> iron + carbon dioxide

The position of the metal in the reactivity series determines whether it can be extracted by reduction with carbon.

Metals higher than carbon in the reactivity series have to be extracted using electrolysis, which is expensive.

Metals below carbon in the reactivity series can be extracted by reduction using carbon. For example, iron oxide is reduced in a blast furnace to make iron.

This is because carbon can only take the oxygen away from metals which are less reactive than carbon itself.

Metals that are more reactive than carbon have to be extracted using electrolysis of molten compounds.

An example of a metal that has to be extracted this way is aluminium.

This process is much more expensive than reduction with carbon because it uses a lot of energy.

FOR EXAMPLE:

a high temperature is needed to melt aluminium oxide so that aluminium can be extracted - this requires a lot of energy, which makes it an expensive process.

Copper can be easily extracted by reduction with carbon. The ore is heated in a furnace - this is called smelting.

However, the copper produced this way is impure - and impure copper doesn't conduct electricity very well. This isn't very useful because a lot of copper is used to make electrical wiring. 

So electrolysis is also used to purify it, even though it's quite expensive.

This produces very pure copper which is a much better conductor.

You could extract copper straight from its ore by electrolysis if you wnated to, but it's more expensive than using reduction with carbon.

Electrolysis is the breaking down of a substance using electricity. It requires a liquid to conduct the electricity, called the electrolyte.

Electrolytes are often metal salt solutions made from the ore (e.g. copper sulfate) or molten metal oxides. The electrolyte has free ions - these conduct the electricity and allow the whole thing to work. Electrons are taken away by the (positive) anode and given away by the (negative) cathode. As ions gain or lose electrons they become atoms or molecules and are released.

Here's how electrolysis is used to get copper:

• Electrons are pulled off copper atoms at the anode, causing them to go to into solution as Cu2+ ions.
• Cu2+ ions near the cathode gain electrons and turn back into copper atoms.
• The impurtites are dropped at the anode as a sludge, whilst pure copper atoms bond to the cathode.

The cathode starts as a thin piece of pure copper and more pure copper adds to it. The anode is just a big lump of impure copper, which will dissolve.

1) More reactive metals react more vigorously than less reactive metals.

2) If you put a reactive metal into a solution of a dissolved metal compound, the reactive metal will replace the less reactive metal in the compound.

3) This is because the more reactive metal bonds more strongly to the non-metal bit of the compound and pushes out the less reactive metal.

4) For example, scrap iron can be used to displace copper from solution - this is really useful because iron is cheap but copper is expensive. If some iron is put in a solution of copper sulfate, the more reactive iron will "kick out" the less reactive copper from the solution. You end up with iron sulfate and solution and copper metal.

copper sulfate + iron --> iron sulfate + copper

5) If a piece of silver metal is put into a solution of copper sulfate, nothing happens. The more reactive metal (copper) is already in the solution.

1) The supply of copper-rich ores is limited, so it's important to recycle as much copper as possible.

2) The demand for copper is growing and this may lead to shortages in the future.

3) Scientists are looking into new ways of extracting copper from low-grade ores (ores that only contain small amounts of copper) or from waste that is currently produced when copper is extracted.

4) Examples of new methods to extract copper are bioleaching and phytomining.

5) Traditional methods of copper mining are pretty damaging to the environment. These new methods of extraction have a much smaller impact, but the disadvantage is that they're slow.

This uses bacteria to seperate copper from copper sulfide. The bacteria get energy from the bond between the copper and sulfur, seperating out the copper from the ore in the process. The leachate (the solution produced by the process) contains copper, which can be extracted, e.g. by filtering.

This involves growing plants in soil that contains copper. The plants can't use or get rid of the copper so it gradually builds up in the leaves. The plants can be harvested, dried and burned in a furnace. The copper can be collected from the ash left in the furnace.

Metal extraction uses a lot of energy and is bad for the environment.

1) People have to balance the social, economic and environmental effets of mining the ore.

2) Most of the issues are exactly the same as those to do with quarrying limestone.

• So mining metal ores is good because it means that useful products can be made. It also provides local people with jobs and brings money into the area. This means services such as transport and health can beimproved.
• But mining ores is bad for the environment as it causes noise, scarring of the landscape and loss of habitats. Deep mine shafts can also be dangerous for a long time after the mine has been abandoned.

1) Mining and extracting metals takes a lot of energy, most of which comes from burning fossil fuels.

2) Fossil fuels are running out so it's important to conserve them. Not only this, but burning them contributes to acid rain, global dimming and climate change.

3) Recycling metals only uses a small fraction of the energy needed to mine and extract metal. E.g. recycling copper only takes 15% of the energy that's needed to mine and extract new copper.

4) Energy doesn't come cheap, so recycling saves money too.

5) Also, there's a finite amount of each metal in the Earth. Recycling conserves these resources.

6) Recycling metal cuts down on the amount of rubbish that gets sent to landfill. Landfill takes up space and pollutes the surroundings. If all the aluminium cans in the UK were recycled, they'd be 14 million fewer dustbins to empty each year.

1) Most of the elements are metals - so they cover most of the periodic table. Only the elements on the far right are non-metals.

2) All metals have some fairly similar basic properties:

• Metals are strong (hard to break), but they can be bent or hammered into different shapes.
• They're great at conducting heat.
• They conduct electricity well.

3) Metals (and especially transition metals, which are found in the centre block of the periodic table) have loads of everyday uses because of these properties...

• Their strength and 'bendability' makes them handy for making into things like bridges and car bodies.
• Metals are ideal if you want to make something that heat needs to travel through, like a sauce pan base.
• Their conductivity makes them great for making things like electrical wires.

• Copper is a good conductor of electricity, so it's ideal for drawing out into electrical wires. It's hard and strong but can be bent. It also doesn't react with water.
• Aluminium is corrosion-resistant and has a low density. Pure aluminium isn't particularly strong, but it forms strong hard alloys.
• Titanium is another low density metal. Unlike aluminium it's very strong. It is also corrosion-resistant.

2) Different metals are chosen for different uses because of their specific properties. 

• If you were doing some plumbing, you'd pick a metal that could be bent to make pipes and tanks, and is below hydrogen in the reactivity series so it doesnt react with water. Copper is great for this.
• If you wanted to make an aeroplane, you'd probably use metal as it's strong and can be bent into shape. But you'd also need it to be light, so aluminium would be a good choice.
• And if you were making replacement hips, you'd pick a metal that won't corrode when it comes into contact with water. It'd also have to be light too, and not too bendy. Titanium has all of these properties so it's used for this.

1) Metals are very useful structural materials, but some corrode when exposed to air and water, so they need to be protected, e.g. by painting. If metals corrode, they lose their strength and hardness.

2) Metals can get 'tired' when stresses and strains are repeatedly put on them over time. This is known as metal fatigue and leads to the metals breaking, which can be very dangerous, e.g. in planes.

1) 'Iron' straight from the blast furnace is only 96% iron. The other 4% is impuritites such as carbon.

2) This impure iron is used as cast iron. It's handy for making ornamental railings, but it doesnt have many other uses as it's brittle.

3) So all the impurities are removed from most of the blast furnace iron. This pure iron has a regular arrangement of identical irons. The layers of atoms can slide over each other, which makes the iron soft and easily shaped. This iron is far too bendy for most uses.

Most of the pure iron is changed into alloys called steels. Steels are formed by adding small amounts of carbon and sometimes other metals to the iron.

Low Carbon Steel (0.1% carbon):

• easily shaped
• car bodies

High Carbon Steel (1.5% carbon):

• very hard, inflexible
• blades for cutting tools, bridges

Stainless Steel (chromium added, and sometimes nickel):

• corrosion-resistant
• cutlery, containers for corrosive substances

1) Different elements have different sized atoms. So when an element such as carbon is added to pure iron, the smaller carbon atom will upset the layers of pure iron atoms, making it more difficult for them to slide over each other. So alloys are harder.

2) Many metals in use today re actually alloys. E.g:

Bronze = Copper + Tin. Bronze is harder than copper. Its good for making medals and statues from. Cupronickel = Copper + Nickel. This is hard and corrosion resistant. It's used to make "silver" coins. Gold alloys are used to make jewellery. Pure gold is too soft. Metals such as zinc, copper, silver, palladium and nickel are used to "harden" the gold. Aluminium alloys are used to make aircraft. Aluminium has a low density, but it's alloyed with small amounts of other matals to make it stronger.

3) In the past, the development of alloys was by trial and error. But nowadays we understand much more about the properties of metals, so alloys can be designed for specific uses.

1) A mixture consist of two (or more) elements or compounds that arent chemically bonded to each other.

2) Crude oil is a mixture of many different compounds. Most of the compounds are hydrocarbon molecules.

3) Hydrocarbons are basically fuels such as petrol and diesel. They're made of just carbon and hydrogen.

4) There are no chemical bonds between the different parts of a mixture, so the different hydrocarbon molecules in the crude oil aren't chemically bonded to one another.

5) This means that they keep all of their original properties, such as their condensing points. The properties of a mixture are just a mixture of the properties of the seperate parts.

6) The parts of a mixture can be seperated out by physical methods, e.g. crude oil can be split up into its seperate fractions by fractional distillation. Each fraction contains molecules with a similar number of carbon atoms to each other.

The fractional distilation column works continously, with heated crude oil piped in at the bottom. The vapourised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.

Crude oil is formed from the buried remains of plants and animals - it's a fossil fuel.

Technology is constantly advancing, so one day it's likely we will be able to extract oil that's too difficult and expensive to extract at the moment.

• The different fractions of crude oil have different properties because of their structures.
• All the fractions of crude oil are hydrocarbons called alkanes.
• Alkanes are made up of chains of carbon atoms surrounded by hydrogen atoms.
• Different alkanes have chains of different lengths.
• The first four alkanes are methane (natural gas), ethane, propane and butane.

Methane CH4, Ethane C2H6, Propane C3H8, Butane C4H10

(In the diagrams, each straight line shows a covalent bond.)

Carbon atoms form four bonds and hydrogen atoms form one bond. The diagrams show that all the atoms have formed bonds with as many other atoms as they can - this means they're saturated.

Alkanes all have the general formula CnH2n+2.

1) The shorter the molecules, the more runny the hydrocarbon is - that is, the less viscous (gloopy) it is.

2) The shorter the molecules, the more volatile they are. "More volatile" means they turn into a gas at a lower temperature. So the shorter the molecules, the lower the temperature at which that fraction vaporises and condenses - and the lower the boiling point.

3) Also, the shorter the molecules, the more flammable (easier to ignite) the hydrocarbon is.

1) The volatility helps decide what the fraction is used for. The refinery gas fraction has the shortest molecules, so it has the lowest boiling point - in fact its a gas at room temperature. This makes it ideal for using as bottled gas. It's stored under pressure as liquid in 'bottles'. When the tap on the bottle is opened, the fuel vaporises and flows to the burner where it's ignited.

2) The petrol fraction has longer molecules, so it has a higher boiling point. Petrol is a liquid which is ideal for storing in the fuel tank of a car. It can flow to the engine where it's easily vaporised to mix witht the air before it's ignited.

3) The viscosity also helps to decide how the hydrocarbons are used. The really gloopy, viscous hydrocarbons are used for lubricating engine parts and for covering roads.

1) Crude oil fractions burn cleanly so they make good fuels. Most modern transport is fuelled by a crude oil fraction, e.g. cars, boats, trains and planes. Parts of crude oil are also burned in central heating systems in homes and in power stations to generate electricity.

2) There's a massive industry with scientists working to find oil reserves, take it out of the ground, and turn it into useful products. As well as fuels, crude oil also providesraw materials for making various chemicals, including plastics.

3) Often alternatives to using crude oil fractions as fuel are possible. E.g. electricity can be generated by nuclear power or wind power, there are ethanol-powered cars, and solar energy can be used to heat water.

4) But things tend to be set up for using oil fractions. For example, car are designed for petrol or diesel and it's readily available. There are filling stations all over the country, with stroage facilities and pumps specifically designed for these crude oil fractions. So crude oil fractions are often the easiest and cheapest thing to use.

5) Crude oil fractions are often more reliable too - e.g. solar and wind power won't work without the right weather conditions. Nuclear energy is reliable, but there are lots of concerns about its safety and the storage of radioactive waste.

1) Most scientists think that oil will run out - it's a non-renewable fuel.

2) No one knows exactly when it'll run out but there have been heaps of different predicitions - e.g. about 40 years ago, scientists predicted that it'd all be gone by the year 2000.

3) New oil reserves are discovered from time to time and technology is constantly improving, so it's now possibble to extract oil that was once too difficult or expensive to extract.

4) In the worst-case scenario, oil may be pretty much gone in about 25 years.

5) Some people think we should immediately stop using oil for things like transport, for which there are alternatives, and keep it for things that it's absolutely essential for, like some chemicals and medicines.

6) It will take time to develop alternative fuels that will satisfy all our energy needs. It'll also take time to adapt things so that the fuels can be used on a wide scale. E.g. we might need different kinds of car engines, or special storage tanks built.

7) One alternative is to generate energy from renewable sources - these are sources that won't run out. Examples of renewable energy sources are wind power, solar power and tidal power.

8) So however long oil does last for, it's a good idea to start conserving it and finding alternatives now.

1) Oil spills can happen as the oil is being transported by tanker - this spells disaster for the local environment. Birds get covered in the stuff and are poisoned as they try to clean themselves. Other creatures, like sea otters and whales are poisoned too.

2) You have to burn oil to release the energy from it. But burning oil is thought to be a major cause of global warming, acid rain and global dimming.

1) Power stations burn huge amounts of fossil fuels to make electricity. Cars are also a major culprit in burning fossil fuels.

2) Most fuels, such as crude oil and coal, contain carbon and hydrogen. During combustion, the carbon and hydrogen are oxidised so that carbon dioxide and water vapour are released into the atmosphere. Energy (heat) is also produced. Hydrogen can also be used as a fuel and it only produces water vapour when burnt.

Hydrocarbon + Oxygen --> Carbon Dioxide + Water Vapour

3) If the fuel contains sulfur impuritites, the sulfur will be released as sulfur dioxide when the fuel is burnt. Oxides of nitrogen will also form if the fuel burns at a high temperature.

4) When there's plenty of oxygen, all the fuel burns - this is called complete combustion.

5) If there's not enough oxygen, some of the fuel doesn't burn - this is called partial combustion. Under these conditions, solid particles (called particulates) of soot (carbon) and unburnt fuel are released. Carbon monoxide (a poisonous gas) is also released.

1) Sulfur dioxide is one of the gases that causes acid rain.

2) When the sulfur dioxide mixes with clouds it forms dilute sulfuric acid. This then falls as acid rain.

3) In the same way, oxides of nitrogen cause acid rain by forming dilute nitric acid in clouds.

4) Acid rain causes lakes to become acidic and many plants and animal die as a result.

5) Acid rain kills trees and damages limestone buildings and ruins some statues.

6) Links between acid rain and human health problems have been suggested.

7) The benefits of electricity and travel have to be balanced against the evnvironmental impacts. Governments have recognised the importane of this and international agreements have been put in place to reduce emissions of air pollutants such as sulfur dioxide.

1) Most of the sulfur can be removed from fuels before they're burnt, but it costs more to do it.

2) Also, removing sulfur from fuels takes more energy. This usually comes from burning more fuel, which releases more of the greenhouse gas carbon  dioxide.

3) However, petrol and diesel are starting to be replaced by low-sulfur versions.

4) Power stations now have Acid Gas Scrubbers to take the harmful gases out before they release their fumes into the atmosphere.

5) The other way of reducing acid rain is simply to reduce our usage of fossil fuels.

1) The level of carbon dioxide in the atmosphere is increasing - because of the large amounts of fossil fuels humans burn.

2) There's a scientific consensus that this extra carbon dioxide has caused the average temperature of the Earth to increase - global warming.

3) Global warming is a type of climate change and causes other types of climate change, e.g. changing rainfall patterns. It could also cause severe flooding due to the polar ice caps melting.

1) In the last feww years, some scientists have been measuring how much sunlight has been reaching the surface of the Earth and comparing it to records from the last 50 years.

2) They have been amazed to find that in some areas nearly 25% less sunlight has been reaching the surface compared to 50 years ago. They have called this global dimming.

3) They think that it is caused by particles of soot and ash that are produced when fossil fuels are burnt. These particles reflect sunlight back into space, or they can help to produce more clouds that reflect the sunlight back into space.

4) There are may scientists who don't believe the change is real and blame it on inaccurate recording equipment.

Some alternative fuels have already been developed, and there are others in the pipeline.

Many of them are renewable fuels so, unlike fossil fuels, they won't run out.

None of them are perfect - they all have pros and cons.

Ethanol can be produced from plant material so is known as a biofuel. It's made by fermentation of plants and is used to power cars in some places. It's often mixed with petrol to make a better fuel.

PROs:

The CO2 released when it's burnt was taken in by the plant as it grew, so it's carbon neutral. The only other product is water.

CONs:

Engines need to be converted before they'll work with ethanol fuels. And ethanol fuel isn't widely available. There are worries that as the demand for it increases farmers will switch from growing food crops to growing crops to make ethanol - this will increase food prices.

Biodiesel is another type of biofuel. It can be produced from vegetable oils such as rapeseed oil and soybean oil. Biodiesel can be mixed with ordinary diesel fuel and used to run a diesel engine.

PROs:

Biodiesel is 'carbon neutral'. Engines don't need to be converted. It produces much less sulfur dioxide and 'particulates' than ordinary diesel or petrol.

CONs:

We can't make enough to completely replace diesel. It's expensive to make. It could increase food prices like using more ethanol could.

Hydrogen gas can also be used to power vehicles. You get the hydrogen from the electrolysis of water - there's plenty of water about but it takes electrical energy to split it up. This energy can come from a renweable source, e.g. solar.

PROs:

Hydrogen combines with oxygen in the air to form just water - so it's very clean.

CONs:

You need a special, expensive engine and hydrogen isn't widely available. You still need to use energy from another source to make it. Also, hydrogen is hard to store.