[III] Chemistry - C3

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Moving continents

Geologists looking at the Earth's surface see changes that are occuring in rocks caused by the movement of TECTONIC PLATES. Plates of the Earth's crust are continually sliding past each other, moving apart or pushing together causing mountains to be lifted up. Geologists see evidence of the same processes in rocks that formed over hundreds of millions of years ago, including some of those in Britain. Across Britain, there are lots of of different types of rocks, from the white chalk along the South coast to the dark volcanic basalts in western Scotland. By studying the rocks, geologists can see evidence of how, when and where rocks were formed. They can tell that the land which makes up Britain today has, over many millions of years, moved across the surface of the Earth. 

The variety of different rocks found in Scotland gave a confusing picture. This has now been explained by the theory of PLATE TECTONICS and confirmed by readings of magnetism left in the rocks. When molten rock such as volcanic lava solidifies, IGNEOUS rocks are formed. As they solidify, the magnetic material in them records the direction towards the magnetic north. When the rocks move, this direction changes. Also, the position of magnetic north changes (even reverses) over millions of years. Rocks in Scotland that formed at different times and in different places have different magnetic directions. Geologists have been able to detect and compare the magnetic fields of rocks. They concluded that Scotland was at one time, in five chunks. The success of the theory has meant that predictions can be made about where similar rocks/fossils can be found.

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Making Britain

600 million years ago Britain wasn't even in one piece. England and Wales were separated from Scotland by an ocean and were parts of different continents. Both of these continents were close to the South Pole. The evidence comes from the different sea creatures trapped in the oldest rocks of each continent. Gradually, the continents drifted towards each other and the ocean between them shrank. The continents crashed together to form a SUPERCONTINENT joining Scotland to England and building a range of mountains.

Over the next 300 million years, the supercontinent moved northwards. The climate changed - there were deserts that formed red sandstone, tropical swamps with rainforests that produced coal, and chalk formed in the warm seas. 

About 60 million years ago, the North Atlantic Ocean began to open up taking North America away from Europe and causing volvanoes to erupt along the western edge of Scotland. Ice ages during the last 3 million years brought glaciers down from the north eroding the highlands of Scotland, northern England and Wales.

SUPERCONTINENT: very large land mass.

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Useful rocks

The north-west of England is still one of the most important areas of Britain for the chemical industry. The industry began nearly 200 years ago near the River Mersey, between Liverpool and Manchester. There are a few reasons for this: there was a market for the products in the textile factories in Lancashire, the port of Liverpool and the canal system gave good transport links to the rest of the country and there was coal in south Lancashire, salt in Cheshire and limestone in the Peak District - the three resources needed most for chemical factories.

350 million years ago, the north-west of England was covered by a sea. Calcium compounds in the water reacted with carbon dioxide in the air to form calcium carbonate. This, and dead shellfish, formed SEDIMENT on the seafloor, which hardened into the SEDIMENTARY ROCK limestone. Other rocks were laid down on top and then, 300 million years ago, tectonic plate movements pushed the limestone up to form mountains. Gradually the rocks above were ERODED away until the limestone was exposed.

About 290 million years ago, Lancashire was a warm, wet swamp on the edge of a sea where trees and other plants grew in abundance. When they died, they became buried under sediment and their carbon content gradually turned into coal. More sediment covered the coal forming more layers of rocks. 200 million years ago, Cheshire was covered by a shallow sea. Rivers flowing into the sea carried salts DISSOLVED from rocks. The climate grew hot and dry so the sea EVAPORATED leaving the salt mixed with sand blown in from the surrounding deserts. Later, other sediments were deposited over the rock sand, buring it.

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What geologists observe

We know how the limestone, coal and salt in the north-west of England were formed because geologists have found evidence.

- Coal contains fossils of the plants that formed it.

- Limestone contains bits of shell and fossils of other sea creatures that lived in the sea when the sediment was forming.

- Rock salt contains small, rounded sand grains which tell geologists that they were carried by the wind, with lots of collisions between the particles. Grains of rock carried by water tend to be larger and rougher.

They may observe other clues. For example, ripples in the sediment made by rivers or by waves on a seashore sometimes remain when the sediment dries or hardens. Geologists recognise these patterns when they look at layers of rock and can tell that they were formed under water. We need some salt in our food to stay healthy, but it is mainly used as flavouring. Many foods contain salt to boost the flavour, but many people add more table salt - particularly to vegetables. Salt is also a preservative and has been used for centuries to make foods last longer - especially meat and fish. Much of the salt we buy is "sea salt". It is made by collecting sea water and evaporating water off either using the heat of the sun of artificial heat.

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Getting at salt

Every winter, tonnes of rock salt are spread on roads. Salt water has a lower freezing point than pure water so even if the temperature falls below 0oC, the salty water on the roads will not freeze. The sand in the rock salt also helps vehicles to grip the road. Rock salt has to be mined from underground deposits. There is just one salt mine in Britain - it is in Cheshire and a million tonnes of rock salt is dug out from its caverns each year.

Salt also has industrial uses. It is called sodium chloride, and chlorine and other chemicals can be obtained from it for use in many different chemical industries. Salt for the chemical industry is usually obtained from underground deposits of rock salt. Water is pumped down into the salt deposits. The salt dissolves to form a concentrated solution called brine. The brine is then pumped to the surface and piped out to where it is needed. The process is almost all automatic. 


Solution mining is more convenient and cheaper than digging out rock salt, and the factory using the salt can draw as much salt as it needs from the underground source.

Solution mining is where water is pumped at high pressure into the rock salt and the salt solution is then pushed to the surface.

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Problems with salt extraction

Salt mining and extraction can have serious effects on the environment at ground level. In Cheshire, the rock salt is 200 metres below the surface. Since the early 19th century a huge amount of salt has been removed. This left large holes underground, which caused the ground to sink. Sometimes whole streets of houses sank into the Earth.

Now about half of the salt is left behind to support the roof of the mine. Subsidence could allow water into the salt deposits, which could then make wells become salty and undrinkable. Plants die if soil becomes contaminated with salt.

In tropical areas, vast areas of sea shore are used for solar evaportation of sea water. The ponds in which the sea water lies become so salty that no animals or plants can survive.

Extracting too much salt from below the surface can have disastrous results. 

Most people think that some foods taste better with a little salt added, such as potatoes and bread. But there are some foods that have a surprising amount of salt added to them, such as baked beans or tinned soups. Many foods have salt added to prevent bacteria and fungi from growing. Bacon and fish were preserved by coating them in salt. In the past, having salted food available during the winter stopped people from starving. Today we are more worried about health conditions that may be related to eating too much salt.

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Hazards and risks

Scientists know that eating a lot of salt raises blood pressure, which can lead to serious health conditions such as strokes. Salt is therefore a HAZARD. The chance of becoming ill because of a salty diet combined with the severity of the outcome is known as the RISK. We can estimate the risk by looking at data from a large group of people over a long time. 

In Britain, men eat on average about 11g of salt per day in their food, for women it is about 8g/day. About one person in 1000 will die of a stroke each year. People who reduce their intake of salt to less than 6g a day reduce their chance of having a stroke to about 1/2000, so low salt means low risk. Knowing the risk allows you to make decisions. One cup of salty vegetable soup won't kill you but a lifetime of eating a diet of high-salt food gives you a higher risk of dying from a stroke.

The Government department for Health and the department for Environment, Food and Rural Affairs help people to make decisions about their health and reduce their risk of death caused by salty diets. They examine foods for their salt content and calculate the risk that each food carries. This data may be published in leaflets and websites, or food manufacturers may be encouraged to put the information on the packaging. Government departments also run publicity campaigns to alert people to the risks of salt in diets and other dangers. Some people think that Governments should make food manufacturers reduce the amount of salt "hidden" in processed foods, but others think that will limit their freedom to eat what they want.

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ALKALIS are chemials that make INDICATORS change colour - for instance, litmus turns blue in an alkali but red in an acid.

Alkalis react with acids to form new substances called salts. The acid loses its acidity and becomes neutral - neither acidic nor alkaline. This is called NEUTRALISATION.

In the past, most people did not know what an alkali was but they knew they could be useful and where to find them. "Alkali" is an Arabic word for the ashes of burnt plants, one of the most useful Alkalis. Another useful alkali is formed in urine, which used to be collected from people's homes. To be an alkali - a substance must dissolve in water and neutralise acids. 

The alkali that forms when urine is left to stand for some time is called AMMONIA and gives it its familiar disgusting smell. Collecting urine became an important job for the dyers. 

Ammonia solution reacts with alum to form aluminium hydroxide. When cloth is dipped in the alum followed by the ammonia and then in a dye, the aluminium hydroxide binds the dye molecules to the fibres in the cloth. The dye becomes "fast" and is not washed out of the cloth.

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Using Alkalis

MAKING SOAP: While soap may be useful for washing your skin, its main use in the past was for cleaning wool before it was spun and woven into cloth. Soap was made by first mixing the ashes of burnt wood or other plants with animal fat and water, boiling up the mixture and then adding salt. The soap formed a layer on the surface that could be scooped off. Vegetable oils could be used instead of animal fats.

NEUTRALISING SOIL: Farmers have long known that mixing an alkali with acidic soils would improve their crops. Seaweed or seaweed ash is a source of alkali that could be used in coastal areas. A manufactured alkali that has been commonly used for a long time is lime, calcium oxide, obtained by heating limestone, calcium carbonate, in a lime burner. The lime can also be mixed with sand and water to make mortar, and also used as the whitewash painted onto houses.

MAKING GLASS: Lime and seaweed ashes were also mixed with sand to make glass. Until the 18th century, glass was a valuable substance and few houses had glass windows. 

DYEING CLOTH: Dyes obtained from plants have long been used to colour cloth. Dyers found that an alkali mixed with a mineral called alum made the dye stick to the cloth better. The alkali they needed was found in urine. 

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Reacting alkalis

In the 1700s, the demand for manufactured goods increased in Britain and elsewhere as the population grew and became better off. Larger and larger factories were built to produce iron, textiles, glass and pottery in a greater quantity than had previously been possible. There was more demand for limestone for the ironworks and for alkalis for the textile, glass and pottery industries. 

In Britain, trees were needed for ships and other buildings so manufacturers looked for other sources of plants to be turned into ash. Huge amounts of seaweed from the coasts of Scotland were burned and shiploads of alkali from the ash of Canadian trees were carried to British ports. By the early 19th century, even these supplies of ash could not keep up the demand for alkalis, particularly from the soapmakers - another source of alkali was needed.

An alkaline solution is one with a pH greater than 7. It turns pH indicator blue or violet. Two groups of substances form alkalis when dissolved in water. One of these groups is the SOLUBLE hydroxides, such as sodium hydroxide. The other is the soluble carbonates, such as potassium carbonate. Hydroxides and carbonates that are not solluble, such as calcium carbonate are not alkalis but are called BASES. Bases react with acids in a similar way to alkalis, but do not affect indicators.

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Nicolas Leblanc's idea

Nicholas Leblanc was a Frenchman who worked out a method of manufacturing alkali in 1878. An irishman called James Muspratt took up Leblanc's ideas and settled in Liverpool where most of the raw materials needed, including salt, were available. A problem was that salt was too expensive, because of a "salt tax". Luckily for Muspratt, the salt tax endd in 1823. Soon Muspratt's factories in North-West England and others in Glasgow were producing all the alkali that was needed. Their method used the "Leblanc process" - reacting salt with sulfuric acid and then heating it with coal and limestone.

The Leblanc process produced sodium carbonate, or "soda", but the process also gave off a large amount of hydrogen chlorine gas and produced huge amounts of solid waste called "galligu". To get rid of the hydrogen chloride, Muspratt built very tall chimneys to try to spread the acidic gas in the air - but most of it sank to the ground and killed plants and wildlife for miles around causing scenes of desolation. The galligu was a serious pollutant because it gave off a stinking and poisonour "bad egg" fumes of hydrogen sulfide.

Part of this pollution problem was solved by William Gossage. He invented a tower in which the hydrogen chloride dissolved in water to make hydrochloric acid. In 1862, the Alkali Acts forced alkali manufacturers to use for Gossage's method to stop the pollutant being released. By then it had been discovered that the hydrochloric acid was itself a useful product. It could be used to make chlorine, which was widely used as bleach.

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Making chlorine

Soon after chlorine was discovered in the 1770s it was found to be a bleach. Textile manufacturers had to bleach their cloth before dyeing it. The alkali industry would get a good price for chlorine if they could find a way of producing it from hydrochloric acid, the industry's by-product. The best method of getting chlorine was to react hydrochloric acid with manganese dioxide.

Hydrochloric acid + Mangnese dioxide ---> Chlorine + Manganese Chloride + Water

In this reaction, the hydrogen is taken away from the hydrogen chloride to form water. We say the hydrogen chloride has been oxidised because the hydrogen has joined with oxygen. Taking hydrogen from a compound is an oxidation reaction.

Chlorine is an element and is quite different from hydrogen chloride. It's a green gas, while hydrogen chloride is colourless. Chlorine is less soluble in water than hydrogen chloride. Chlorine is a powerful bleach; hydrogen chloride is not. 

Compounds have different properties from those of the elements they contain.

OXIDATION can be defined as the gain of oxygen or as the loss of hydrogen. REDUCTION is the opposite.

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Chlorine in water

More and more people lived in crowded cities in the 19th century. Sewage drained into rivers, which were also used to supply drinking water. Many died from diseases such as cholera and typhoid. Building sewers and sewage treatment works improved health but water supplies still often contained dangerous microorganisms. In 1908, Jersey City in the USA became the first city to add chlorine to its water supply all the time. Chlorine is a disinfectant because it kills microorganisms. Many cities in the US, Britain and Europe soon did the same. There was a great improvement in public health in the developed countries but water treatment remains a problem in less developed countries.

The effect on public health of disinfesting water with chlorine shows the number of deaths due to typhoid and the years. It is a negative correlation. Most cities in the USA began chlorinating water between 1908 and 1930. There appears to be a correlation between the CHLORINATION of water and reducing deaths from typhoid. 

There were other improvements to living conditions during this period and some groups of people were vaccinated against typhoid. These factors also contributed to the falling death rate.

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Benefits and risks of chlorination

Some people have always disapproved of the addition of chlorine to water. Chlorine is a toxic gas - as well as killing microorganisms it can affect human health if too much is present in water. Chlorine can react with organic material (chemicals from plants or animals) in the water supply. A number of compounds, called disinfectant by-products, are formed that are tocix or carcinogenic. When this information was published, many people became worried about their health, and in some places local leaders stopped chlorine being added to water supplies.

People have no choice in their supply of water. Because the stories about chlorine were new and unfamiliar, people perceived the risk as being higher than more familiar causes of death such as car accidents. 

Governments took advice on the problem and decided that the risk from DBPs could be kept low if the amount of chlorine and other materials in water was controlled carefully. They thought that the benefits of the disinfecting properties of chlorine outweigh the dangers.

Alternatives to using chlorine to disinfect drinking water include ozone and ultraviolet light - but chlorine remains the cheapest and most effective and is used by most water companies.

The level of risk a chemical has depends on how much harm it can cause and how much is present.

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Electrolysis of brine

ELECTROLYSIS is a process in which electricity is passed through a molten salt or a solution of a salt. It causes chemical changes to take place. Brine is a concentrated solution of sodium chloride (salt). When electricity is passed through brine, three products are formed : CHLORINE, HYDROGEN AND SODIUM HYDROXIDE. 

All three products have important uses so there is no waste. 

Uses of chlorine:

PVC: 36% Paper treatment: 5%

Other compounds: 15% Water treatment: 5%

Solvents: 6% Disinfectants/cleaners: 14%

Other polymers: 20%

In the electrolysis solutions, the water is involved in the chemical change that takes place. Hydrogen, chlorine and sodium hydroxide are the products of electrolysing sodium chloride solution in the membrane cell.

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Brine electrolysis

The MEMBRANE CELL is one of the methods used to electrolyse brine in industry. Salt and water are the only raw materials. A great deal of electrical energy is needed to make the chemical changes happen on a large scale and is the main running cost of the process. 

Electricity enters the solution through two ELECTRODES. Chlorine gas is given off and collected at the ANODE, the positive (+) electrode. Hydrogen gas is given off and collected at the CATHODE, the negative (-) electrode. Sodium hydroxide is left in the solution. Water is added to the membrane cell and the sodium hydroxide solution is piped off.

The electrolysis of brine is one of the most widely used chemical processes and produces millions of tonnes of chlorine, sodium hydroxide and hydrogen each year. The industry has come a long way since 3/4 of the raw materials used in the Leblanc process ended up as highly polluting waste. In the membrane cell, all the products have important uses - nevertheless, any loss of the products can harm the environment. Chlorine is toxic and corrosive, sodium hydroxide is a powerful alkali and hydrogen is flammable. Even a loss of salt could damage freshwater sources. Until the membrane cell became the most commonly used process, the electrolysis of sodium chloride was often done using a liquid mercury cathode. Up to 200g of mercury were lost for every tonne of chlorine produced. As a result, steps were taken to limit the loss of mercury. By 2008, the loss of mercury was reduced to 1g per tonne of chlorine. By 2020, all mercury cathode processes will have been replaced by membrane cells.

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Industrial chemicals

Chemicals are made of elements. Elements cannot be destroyed and so remain in the environment for ever. Most elements react to form compounds. Some compounds are more dangerous than others. We must carry out risk assessments to decide how dangerous substances are. Dangerous compounds must be controlled. There is a risk in using every chemical but we need to know if the risk is high or low. Even eating salt can harm health, but a gram of sodium cyanide is much more dangerous than a gram of sodium chloride.

Chemicals spread if they are in the environment - they are carried by wind and water whether they are gases, liquids or solids. Sulfur dioxide from British power stations was carried by winds to Norway and fell there as acid rain.

Chemicals may be absorbed by plants and animals and passed up the food chain. Animals may not be able to excrete some chemicals, so they accumulate until they reach dangerous concentrations. Mercury released in alkali factories accumulated in fish and has been found in the bodies of the Inuit people in the Arctic.

To decide the level of risk of a particular chemical we need to know: how much of it is needed to cause harm, how much will be used, the chance of it escaping into the environment and who or what it may affect.

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PVC and plasticisers

30 years ago, European laws made the risk assessment of every new chemical compulsory, but there are thousands of substances that have been used, sometimes for centuries, for which we have very little data about their hazards. REACH will eventually include all chemicals that are used today, but testing, involving the use of millions of laboratory animals, will take many years. Many familiar substances are hazardous but people may be more suspicious of substances whose names are new. The risks with new chemicals may be thought to be higher because there is less data available and the risks haven't been discovered as much. There also may be long-term risks that scientists are unaware about. 

PVC is a polymer containing the elements carbon, hydrogen and chlorine. It is made from chlorine gas and a hydrocarbon obtained from crude oil. PVC is a plastic - many plastics soften when heated and can be moulded into shape. PVC is a stiff, tough material that has many uses in buildings. It is hardwearing and will last for many years. 

Small molecules called PLASTICISERS are added to PVC to make it softer and more flexible. Plasticised PVC is used to insulate wires and cables, and as sheeting. It is also used instead of leather in clothes and seat coverings.

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Safety of plasticisers

Plasticised PVC can behave a bit like a sponge with soap on it. When the sponge is wet, the soap washes out of the sponge - this is called LEACHING. The small plasticiser molecules trapped between the large PVC molecules are not held very tightly. When the PVC is squeezed or stretched or wetted the plasticiser molecules may leach out.

The chemicals used as plasticisers have been tested for safety. Some have been shown to harm the health of test animals, such as rats, when fed to the animals in large amounts. It is also thought that some of the plasticisers can harm fish when they are leached into rivers and the sea. For these reasons, the use of plasticised PVC in children's toys has been banned in Europe and the USA.

PVC has been used for over fifty years for many different purposes. At least 2/3 of the PVC produced is used by the building industry. There are environmental concerns about the manufacture of PVC. The process uses a lot of chlorine and produces toxic by-products. If allowed to escape, these chemicals would be harmful pollutants. PVC is flammable and gives off highly toxic gases when it burns. It is a known cause of death in fires in buildings.

There is a lot of dispute about the risks from the plasticisers that are mixed with 1/3 of all the PVC in use. Other materials are available that could replace PVC for most, if not all, of its uses.

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Life Cycle assessment

We use many different materials for many purposes. All materials have to be made, turned into a product, which is used and then disposed of. Each of these stages uses the Earth's resources and causes pollution that may harm us and other organisms. A LIFE CYCLE ASSESSMENT helps to add up all the effects that a product has on the environment. It helps us to decide whether the product benefits us and the planet, or if there is a high risk of harm to health and damage to plants and animals.

To produce a LCA for any product, its life must be broken down into stages.

1. Preparing the chemicals for raw materials found in plants, animals, rocks, the oceans or air.

2. Making the product from the chemicals - including transporting the chemicals and the finished product.

3. Using the product.

4. Disposing of the product and the materials in it when it is of no more use.

When all the answers to each stage have been combined, we have an overall picture of the costs and impact of the product on the Earth. Then alternative materials can be compared to see if they would have a lesser impact.

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