C3- Chemicals in our lives

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Looking at rocks

Geologists study rocks to see how the Earths surface has changed. They look at how rocks form, how they change, and when changes happen.

Geological changes happen by slow movements of tectonic plates. Plates can move by sliding past each other, colliding or pulling apart.

Plate collisions cause mountain ranges which erode over time.

Geologists can explain most of the History of the Earth by the processes they can observe today.

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Making Britain

Over millions of years, Britain has moved across the Earths surface.

600 million years ago, England and Wales were separated from Scotland by ocean, and both were near the South pole.

Gradually, different continents drifted and crashed together to form Pangea, a supercontinent.

Britain is made from rocks from different ancient continents. Originally, Britain was nearer the equator with a warmer climate. Different climates existed in Britain, from tropical swamps to ice ages.

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Magnetism

As volcanic lava solidifies, igneous rocks are formed.

Magnetic materials in the lava line up along the Earths magnetic field.

The Earths magnetic field changes over time.

Geologists can date rocks and track the slow movement of continents using changes in magnetic patterns linked to radioactive decay.

This evidence supports plate tectonic theory.

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Limestone, coal and salt

Rocks are raw materials found buried in the Earths crust. Coal, salt and limestone are three important raw materials.

200 years ago the industrial revolution started in North-West England. Chemical industries built up near to raw materials and transport links.

Limestone formed when Britain was covered by sea:

  • Shellfish died forming sediments on the sea bed.
  • Sediments compacted and hardened to form limestone, a sedimentary rock.
  • Tectonic plate movements pushed the rock to the surface.
  • Gradually the rocks above were eroded away until the limestone was exposed.

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Coal and salt

Coal formed in wet swampy conditions when plants like trees and ferns died and became buried. This excluded oxygen, slowing down decay.

Salt formed while cheshire was covered by a shallow sea:

  • Rivers brought dissolved salts into the sea.
  • Climate warming evaporated the water, leaving salt that mixed with sand blown in by the wind.
  • Rock salt formed and was buried by other sediments.

Geologists have found evidence for limestone, coal and salt formation.

  • Coal contains fossils of the plant that formed it.
  • Limestone contains shell fragments from sea creatures.
  • Rock salt contains different shaped water-eroded and wind-eroded grains.
  • Ripple marks in rocks indicate water flow from rivers or waves in the sea.

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Extracting salt

Salt is used in the food industry, as a source of chemicals or to treat icy roads in winter.

Salt can be obtained from: collecting and evaporating sea water or mining underground deposits of rock salt.

Salt is sodium chloride (NaCl) and has many industrial uses.

Rock salt is spread on icy roads because:

  • The rock is insoluble, but the sand in the rock salt gives grip.
  • It shows up so people know when roads have been gritted.
  • The salt in the solution lowers the freezing point, preventing ice from forming as easily.

Only one rock salt mine exists in Britain (cheshire) it mines a million tonnes a year.

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Extracting salt

Salt is used in the food industry, as a source of chemicals or to treat icy roads in winter.

Salt can be obtained from: collecting and evaporating sea water or mining underground deposits of rock salt.

Salt is sodium chloride (NaCl) and has many industrial uses.

Rock salt is spread on icy roads because:

  • The rock is insoluble, but the sand in the rock salt gives grip.
  • It shows up so people know when roads have been gritted.
  • The salt in the solution lowers the freezing point, preventing ice from forming as easily.

Only one rock salt mine exists in Britain (cheshire) it mines a million tonnes a year.

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Extracting salt

  • If more salt is needed, it is usually imported.
  • Extracting salt from sea water is only economical in hot climates.
  • Purer salt can be obtained by solution mining, which is mainly automatic.
  • Mining rock salt and solution mining can cause subsidence. About half of the rock salt cannot be mined, as it is left in place for support.
  • Mining can allow water in mines, which may let salt leach out into water supplies, contaminating them.
  • Evaporating salt from sea water takes up large areas and spreads salt into the local environment, damaging habitats.

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Health risks of salt

  • Salt is used in food as both flavouring and a preservative.
  • A higher salt level prevents bacteria growth.
  • Too much salt is bad for your health.
  • Many people are worried about salt intake, which can lead to high blood pressure, heart failure and strokes. So salt is classified as a hazard.
  • A risk is the chance of getting ill and the consequences if you did. Risk can be estimated by measuring salt intake.
  • Food labels show the amount of salt contained in the product.
  • Knowing the risk allows you to make decisions.

The government department of health (DH) and the department of the environment, food and rural affairs (Defra) are responsible for carrying out risk assessment for chemicals in food and advising the public about how food affects health.

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Alkalis

Alkalis make indicators change colour. Litmus turns blue in alkalis and red in acids.

Alkalis neutralise acids to make salts. This is called neutralisation.

Acid + Alkali = Salt + Water

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Using alkalis

Alkalis are used for; dying clothes, neutralising acidic soil, making soap and making glass.

Stale urine and ash from burnt wood were used in the past as sources of alkalis.

Due to increased industrialisation, by 1900s demand for alkalis outstripped the supply.

  • In the past one major use of soap was for cleaning wool. Soap was made by mixing the ashes from burnt wood with animal fat and boiling it.
  • In coastal areas, seaweed or seaweed ash could be used to neutralise acidic soils.
  • The first alkali to be manufactured was lime (calcium oxide). This was done by heating limestone (calcium carbonate) in a lime kiln, using coal as fuel.
  • Lime is used for neutralising acidic soils, making glass when heated with sand and removing impurities when iron is made.
  • Before modern dyes, clothes were coloured using dyes from plants and animals.
  • Alum is a mordant that 'sticks' dye to a fabric. It is purified by reacting it with ammonia contained in stale urine.
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Making alkalis

In 1787 the Frenchman Nicholas Leblanc discovered how to maufacture an alkali.

  • The Leblanc process made sodium carbonate by reacting salt and limestone, heated with coal.
  • It gave off large amounts of hydrogen chloride (an acidic harmful gas). It also produced heaps of solid waste called galligu, that slowly released hydrogen sulfide, a foul smelling toxic gas.
  • Later a process was invented to change the harmful hydrogen chloride into useful substances: chlorine used to bleach textiles prior to dying, hydrochloric acid which is a starting material for making other chemicals.

Chlorine can be made by reacting hydrochloric acid and manganese dioxide.

Oxidation converts hydrogen chloride to chlorine.

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

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Patterns of reactions

  • An alkali is a solution with a PH greater than 7. It turns PH indicator blue or violet.
  • Alkalis are soluble metal hydroxides and soluble metal carbonates. Some examples are:

Soluble hydroxides: Sodium hydroxide NaOH, Calcium Hydroxide Ca(OH)2, Potassium Hydroxide KOH

Soluble Carbonates: Sodium Carbonate Na2CO3, Potassium Carbonate K2CO3

  • Most metal hydroxides and metal carbonates are insoluble. They are not alkalis but are called bases. Bases react with acids in a similar way to alkalis, but do not affect indicators.
  • The general pattern of these reactions is:

Hydroxide + Acid = Salt + Water                                                    

Carbonate + Acid = Salt + Water + Carbon dioxide gas

For example: Sodium hydroxide + Sulfuric acid = Sodium sulfate + Water

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Chlorine in drinking water

  • In the 19th century many people died from dirty drinking water. Chlorine is now added to drinking water to kill microorganisms.
  • The introduction of chlorination made a major contribution to public health. It killed water-borne microorganisms that cause diseases such as cholera and typhoid.
  • A correlation exists between the start of water chlorination in the USA and a fall in the number of deaths from typhoid.
  • Chlorine is a toxic gas and can affect human health if too much is present in water.
  • Some people disapprove of adding chlorine to water supplies. People using mains water supplies have no choice about chlorination .
  • Chlorine can react with organic materials in water supplies, forming toxic or carcinogenic compounds called disinfectant by-products (DBPs).
  • In the UK the government has decided the risk from DBPs is very small , so the benefits outweigh the risks.

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Electrolysis of brine

  • Electrolysis breaks up compounds using an electric current.
  • The electrolysis of brine (sodium chloride solution) makes: chlorine gas, hydrogen gas and sodium hydroxide solution.
  • All three products have uses, so there is no waste.
  • Electrolysis causes a chemical change, making new products.
  • The anode is the positive electrode and the cathode is the negative electrode.
  • Large amounts of energy are needed for electrolysis so it is expencive.
  • The membrane cell method is one way to electrolyse brine continuously.

During brine electrolysis, chlorine forms at the anode and hydrogen at the cathode, industrial uses of these products are:

  • Chlorine for making plastics like PVC, in medicines and crop protection.
  • Hydrogen for making margarine, as rocket fuel and in fuel cells in vehicles.
  • Sodium hydroxide for paper recycling, industrial cleaners and refining aluminium.
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Electrolysis of brine

Brine electrolysis is one of the most vastly used industrial processes.

While the products have many uses, they can have an environmental impact:

  • Chlorine products e.g. from fridges and aerosols are linked to ozone depletionand have been banned.
  • Chlorine used in paper bleaching releases dangerous dioxins increasing the rick of cancer.
  • The mercury diaphragm method of electrolysis, which is still used releases mercury waste. This can enter the food chain and is a cumulative poison.
  • Plastics made using chlorine are non-biodegradable.

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Chemical safety

  • Chemicals contain elements. elements cannot be destroyed, so they remain in the environment forever.
  • A risk assessment is used to find out how dangerous substances are.
  • Chemicals in the form of solids, liquids and gases can spread out in the environment.
  • Some toxic chemicals persist in the environment, can be carried over large distances, and may accumulate in food chains, ending up in human tissues.
  • To decide the level of risk of a particular chemical we need to know: How much is needed to cause harm, how much will be used, the chance of it escapinginto the environment, who or what it may affect.
  • EU laws make risk assessments compulsory for new chemicals.
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PVC

  • PVC is a plastic containing carbon, hydrogen and chlorine.
  • Smaller particles called plasticisers are added to the PVC to make it softer.
  • Plasticised PVC is used to cover electric wires, for clothing and for seat covers.
  • Plasticiser molecules can leach ou of PVC and into the surroundings, where they may have harmful effects.
  • They have passed safety tests, however they may have unknown long term affects on fish, and have been shown to harm animals in large amounts.
  • Plasticised childrens toys have been banned in the USA and EU.
  • If PVC is burnt it gives of toxic gases including dioxins. If eaten these chemicals build up in fat and are thought to cause cancer.

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Life cycle assessment

A life cycle assessment (LCA) measures the energy used to make, use and dispose of a substance, and its environmental impact.

There are 4 main stages of an LCA which is sometimes known as a 'Cradle, Use, Grave' assessment.

At each stage we need to consider:

  1. How much natural resources are required?
  2. How much energy is needed or produced?
  3. How much water or air is used?
  4. How much is the environment affected?

When an LCA has been completed different products can be compared fairly.

To produce a fair and accurate LCA alot of data is required. Some aspects are hard to measure though.

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