C2 - Chemical Resources (OCR Gateway Science B)

The Earth has a crust, a mantle and a core:

• The crust is Earth's thin outer layer of solid rock (its average depth is 20km).
• The lithosphere includes the crust and upper part of the mantle, and is made up of a jigsaw of 'tectonic plates'. The lithosphere is relatively cold and rigid, and is over 100km thick in places.
• The mantle is the solid section between the crust and the core. Near the crust it's very rigid. As you go deeper into the mantle the temperature increases - here it becomes less rigid and can flow very slowly (it behaves like it's semi-liquid - like slush)
• The core is just over half the Earth's radius. The inner core is solid, while the outer core is liquid.
• Radioactive decay creates a lot of the heat inside the Earth. This heat creates convection currents in the mantle, which causes the plates of the lithosphere to move.

The Earth's surface is made up of large plates of rock:

• Tectonic plates are like big rocky rafts that float on the mantle (they're less dense than the mantle).
• The plates move very slowly - at a speed of about 2.5cm per year.

• Volcanoes and earthquakes often occur wherethe plates meet. It's the movement of the plates against each other that causes them.

Seismic waves can tell us what's below the crust:

• It's difficult to study the inner structure of the Earth - you can't get at it directly, because the crust is too thick to drill through.
• Scientists use seismic waves (shock waves) to study the Earth's structure. These are produced by earthquakes. Seismic waves can also be produced by setting off a big man-made explosion at the Earth's surface.
• By measuring the time that it takes for these wves to travel through the Earth and where they are detected, scientists can draw conclusions about the structure of the Earth.

There are two types of seismic wave that can travel through the Earth - P-waves and S-waves. P-waves travel through solids and liquids. S-waves can only travel through solids:

• S-waves can travel through the mantle, showing that it's solid.
• But S-waves are not detected in the core's shadow, showing that it's liquid.
• P-waves travel faster through the middle of the core, showing that it's solid.

Observations about the Earth hadn't been explained:

• For years, fossils of very similar plants and animals had been found on opposite sides of the Atlantic Ocean. Most people thought this was because the continents had been linked by 'land bridges', which had sunk or been covered by water as the Earth cooled. But not everyone was convinced.
• Other things about the Earth puzzled people too - like why the coastlines of Africa and South Africa fit together and why there are fossils of sea creatures in the Alps.

Explaining these observations needed a leap of imagination:

• In 1914, Alfred Wegener hypothesised that Africa and South America had previously been one continent which had then split. He started to look for more exidence to back it up.
• eg: there were matching layers in the rocks on different continents, and similar earthworms living in both South America and South Africa.
• Wegener's theory of 'continental drift' supposed that about 300 million years ago there had been just one 'supercontinental' - which he called Pangaea. According to wegener, Pangaea broke into smaller chunks and these chunks (our modern-day continents) are still slowly 'drifting' apart. This idea is the basis behind the modern theory of plate tectonics.

The theory wasn't accepted at first - for a variety of reasons:

• Wegener's theory explained things that couldn't be explained by the 'land bridge' theory (Eg: the formation of mountains - which Wegener said happened as continents smashed into each other). But it was a big change, and the reaction from other scientist was hostile.
• The main problem was that Wegener's explanation of how the 'drifting' happened wasn't convincing (and the movement wasn't detechtable). Wegner claimed the continents' movement could be caused by tidal forces and the Earth's rotation - but other geologists showed that this was impossible.

Eventually, the evidence became overwhelming:

• In the 1960s, scientists investigated the Mid-Atlantic ridge, which runs the whole length of the Atlantic.
• They found evidence that magma (molten rock) rises up through the sea floor, solidifies and forms underwater mountains that are roughly symmetrical either side of the ridge. The evidence suggested that the sea floor was spreading at about 10cm per year.

• Even better evidence that the continents are moving apart came from the magnetic orientation of the rocks. As the liquid magma erupts out of the gap, iron particles in the rocks tend to allign themselves with the Earth's magnetic field - and as it cools they are set in position. Now then... every half million years or so the Earth's magnetic field swaps direction - and the rock on either side of the ridge has bands of alternate magnetic polarity, symmetrical about the ridge.
• This was convincing evidence that new sea floor was being created, and continents were moving apart.
• All the evidence collected by other scientists supported Wegners theory - so it was gradually accepted.

Volcanoes are formed by molten rock:

• Volcanoes occur when molten rock (magma) from the mantle emerges through the Earth's crust.
• Magma rises up (through the crust) and 'boils over' where it can - sometimes quite violently if the pressure is released suddenly. (When the molten rock is below the surface of the Earth it's called magma - but when it erupts from a volcano it's called lava.)

Oceanic and continental crust colliding causes volcanoes:

• The crust at the ocean floor is denser than the crust below the continents.
• When two tectonic plates collide, a dense oceanic plate will be forced underneath a less dense continental plate. This is called subdunction.
• Oceanic crust also tends to be cooler at the esges of a tectonic plate - so the edges sink easily, pulling the oceanic plate down.
• As the oceanic crust is forced down it melts and starts to rise. If this molten rock finds its way to the surface, volcanoes form.

Volcanic activity forms igneous rock:

• Igneous rock is made when any sort of molten rock cools down and solidifies. Lots of rocks on the surface on the Earth were formed this way.
• The type of igneous rock (and the behaviour of the volcano) depends on how quickly the magma cools and the composition of the magma.
• Some volcanoes produce magma that forms iron-rich basalt. The lava from the eruption is runny, and the eruption is fairly safe. (As safe as you can be with very hot molten rock).
• But if the magma is silica-rich rhyolite, the eruption is explosive. It produces thick lava which can be violently blown out of the top of volcano.

Geologists try to predict volcanic eruptions:

• Geologists study volcanoes to try to find out if there are signs that a volcanic eruption might happen soon - things like magma movement below the ground near to a volcano.
• Being able to spot these kinds of clues means that scientists can predict eruptions with much greater accuracy than they could in the past.
• It's tricky though - volcanoes are very unpredicatable. Most likely, scientists will only be able to say that an eruption's more likely than normal - not that it's certain.

There are three steps in the formation of sedimentary rock:

• Sedimentary rocks are formed from layers of sediment laid down in lakes or seas.
• Over millions of years, the layers get buried down under more layers and the weight pressing down squeezes out the water.
• Fluids flowing through the pores deposit natural mineral cement.

Limestone is a sedimentary rock fromed from seashells:

• Limestone is mostly formed from seashells. It's mostly calcium carbonate and grey/white in colour. The original shells are mostly crushed, but there can still be quite a few fossilised shells remaining.
• When limestone is heated it thermally decomposes to make calcium oxide and carbon dioxide:
• calcium carbonate [CaCO2 (s)] -> calcium oxide [CaO (s)] + carbon dioxide [Co2 (g)]

Metamorphic rocks are formed from other rocks:

• Metamorphic rocks are formed by the action of heat and pressure on sedimentary (or even igneous) rocks over long periods of time.

• The mineral structure and texture may be different, but the chemical composition is often the same.
• So long as the rocks don't actually melt, they're classed as metamorphic. If they melt and turn into magma, they're gone (though they eventually resurface as igneous rocks).

Marble is a metamorphic rock formed from limestone:

• Marble is another form of calcium carbonate.
• Very high temperatures and pressure break down the limestone and it reforms as small crystals.
• This gives marble a more even texture and makes it much harder.

Igneous rocks are formed from fresh magma:

• Igneous rocks are formed when magma cools.
• They contain various different minerals in randomly arranged interlocking crystals - this makes them very hard.
• Granite is a very hard igneous rock (even harder than marble). It's ideal for steps and buildings.

Aluminium and iron are extracted from ores in rocks:

Rocks are usually a mixture of minerals. Ores are minerals we can get useful materials from. Aluminium and iron are construction materials that can be extracted from their ores.

Glass is made by melting limestone, sand and soda:

• Just heat up limestone (calcium carbonate) with sand (silicon dioxide) and soda (sodium carbonate) until it melts.
• When the mixture cools it comes out as glass. It's as easy as that.

Bricks are made from clay:

• Clay is a mineral formed from weatered and decomposed rock. It's soft when it's dug up out of the group, which makes it easy to mould into bricks.
• But it can be hardened by firing at very high temperatures. This makes it ideal as a building material - bricks can withstand the weight of lots more bricks on top of them.

Limestone and clay are heated to make cement:

• Clay contains aluminium and silicates.
• Powdered clay and powdered limestone are roasted in a rotating kiln to make a complex micture of calcium and aluminium silicates, called cement.
• When cement is mixed with water a slow chemical reaction takes place. This causes the cement to gradually set hard.
• Cement can be mixed with sand, aggregate (gravel) and water to make concrete.
• Concrete is a very quick and cheap way of construction buildings, however it is unattractive.
• Reinforced concrete is a 'composite material - it's a combination of concrete and a solid steel support (like steel rods). It's a better construction material than ordinary concrete because it combines the hardness of concrete with the flexibility and the strength of steel.

Extracting rocks can cause environmental damage:

• Quarrying uses up land and destroys habitats. It costs money to make quarry sites look nice agaim
• Transporting rock can cause noise and pollution.
• The quarrying process itself produces dust and noise - they use dynamite to blast rocks out.
• Disused sites can be dangerous. Every year people drown in former quarries that are now lakes. Disused mines have been known to collapse - causing subsidence.

Electrolysis is used to obtain very pure copper:

• Electrolysis means 'splitting up with electricity' - in this case passing a current through a piece of impure copper splits the pure copper off from the nasty impurities.
• The copper is immersed in a liquid (called the electrolyte) which conducts electricity. Electrolytes are usually free ions dissolved in water. Copper(II) sulfate solution is the electrolyte used in purifying copper - it contains Cu2+ ions.

The electrical supply acts like an electron pump. This is what happens:

• It pulls electrons off copper atoms at the anode, causing them to go into solution as Cu2+ ions.
• It then offers electrons at the cathode to nearby Cu2+ ions to turn them back into copper atoms.
• The impurities are dropped at the anode as a sludge, whilst pure copper atoms bond to the cathode.
• During electrolysis, copper dissolves away from the anode and is deposited at the cathode. So the anode loses mass and the cathode gains mass. (The elecolysis process is often allowed to go on for weeks and the cathode can be 20 times bigger at the end of it.)

Recycling copper saves money and resources:

• It's cheaper to recycle copper than it is to mine and extract new copper from its ore.
• And recycling copper uses only 15% of the energy that'd be used to mine and extract the same amount.
• But it can be hard to convince people that it's worth the effort to sort and recycle their metal waste. Even then you have to sort out the copper from all the other waste metal - which takes time and energy.

An alloy is a mixture of a metal and other elements:

• Alloys can be a mixture of two or more different metals (like brass or bronze)
• They can also be a mixture of a metal and a non-metal (like steel)
• Alloys often have properties that are different from the metals they are made from - and these new properties often make the alloy more useful than the pure metal.

Steel is an alloy of iron and carbon;

• Steel is harder than iron.
• Steel is also stronger than iron, as long as the amount of carbon does not get large than about 1%.
• Iron on its own will rust (corrode) fairly quickly, but steel is much less likely to rust. A small amount of carbon makes a big difference.
• A lot of things are made from steel - girders, bridges, engine parts, cutlery, washing machines, saucepans, ships, drill bits, cars etc. 

Brass, bronze, solder and amalgam are also alloys:

• Brass is an alloy of copper and zinc. Most of the properties of brass are just a mixture of those of the copper and zing, although brass is harder than either of them. Brass is used for making brass musical instruments (trumpets, trombones, french horns etc.). It's also used for fixtures and fittings such as screws, springs, doorknobs etc.
• Bronze is an alloy of copper and tin. Unlike pure materials it doesn't have a definite melting point, but gradually solidifies as it cools down. This is pretty useful if you want to solder things together.
• An amalgam is an alloy containing mercury. A large-scale use of one kind of amalgam is in dentistry, for filling teeth.

Some alloys are smart:

• Nitinol is the name given to a family of alloys of nickel and titanium that have shape memory.
• This means they 'remember' their original shape and go back to it even after being bent and twisted.
• This has increased the number of uses for alloys. You can get glasses with Nitinol frames - these can be bent and even sat on and they still go back into their original shape.

Iron and steel corrode much more than aluminium:

• Iron corrodes easily. In other words, it rusts. 
• Rusting only happens when the iron's in contact with both oxygen (from the air) and water.
• The chemical reaction that takes place when iron corrodes in an oxidation reaction. The iron gains oxygen to form iron (III) oxide. Water then becomes loosely bonded to the iron (III) oxide and the result is hydrated iron (III) oxide - which we call rust.
• iron + oxygen + water -> hydrated iron (III) oxide
• Unfortunately, rust is a soft crumbly solid that soon flakes off to leave more iron available to rust again. And if the water's salty or acidic, rusting will take place a lot quicker. Cars in coastal places rust a lot because they get covered in salty sea-spray. Cars in dry deserty places hardly rust at all.
• Aluminium doesn't corrode when it's wet. This is a bit odd because because aluminium is more reactive than iron. What happens is that the aluminium reacts very quickly with oxygen in the air to form aluminium oxide. A nice protective layer of aluminium oxide sticks firmly to the aluminium below and stops any further reaction taking place (the oxide isn't crumbly and flaky like rust so it won't fall off).

Car bodies: aluminium or steel?:

Aluminium has two big advantages over steel:

• It has a much lower density, so the car body of an aluminium car will be lighter  than the same car made of steel. This gives the aluminium car much better fuel economu which saves fuel resources.
• A car body made with aluminium corrodes less and so it'll have a longer lifetime.

But aluminium has a massive disadvantage. It costs a lot more than iron or steel that's why car manufacturers tend to build cars out of steel instead.

You need various materials to build different bits of a car:

• Steel is strong and it can be hammered into sheets and welded together - good for bodywork.
• Aluminium is strong and has a low density - it's used for parts of the engine, to reduce weight.
• Glass is transparent - cars need windscreens and windows.

• Plastics are light and hardwearing, so they're used as internal coverings for doors, dashboards etc. They're also electrical insulators, used for covering electrical wires.
• Fibres (natural and synthetic) and hard-wearing, so they're used to cover the seats and floor.

Recycling cars is important:

• As with all recycling, the idea is to save natural resources, save money and reduce landfill use.
• At the moment a lot of the metal from a scrap car is recycled, though most of the other materials (eg: plastics, rubber etc.) go into landfill. But European laws are now in place saying that 85% of the materials in a car (rising to 95% of a car by 2015) must be recyclable.
• The biggest problem with recycling all the non-metal bits of a car is that they have to be seperated before they can be recycled. Sorting out different types of plastic is a pain in the neck.

The pH scale and universal indicator:

• The lower the numbers, the more acidic it is and the higher the numbers, the more alkali it is.

An indicator is a dye that changes colour:

• The dye in an indicator changes colour depending on the pH of a substance.
• Universal indicator is a combination of dyes. It changes colour gradually as the pH changes.
• It's very useful for estimating for estimating the pH of a solution. You just add a drop of the indicator to your solution, then compare the colour it turns to a colour chart.
• Some indicators change colour suddenly at a particular pH, eg: phenolphthalein changes suddenly from colourless to pink as the pH rises above 8.

Acids and bases neutralise each other:

• An ACID is a substance with a pH of less than 7. Acids form H+ ions in water. The pH of an acid is determined by the concentration of the H+ ions.
• A BASE is a substance with a pH of greater than 7. An ALKALI is a base that is soluble in water. Alkalis form OH- ions in water.

• The reaction between acids and bases is called neutralisation: ACID + BASE -> SALT + WATER
• Neutralisation can also be seen in terms of H+ and OH- ions like this: H+ + OH- = H2O
• When an acid neutralises a base (or vice versa) the products are neutral, eg: they have a pH of 7.

Metal oxides and metal hydroxides are bases:

• Some metal oxides and metal hydroxides dissolve in water. These soluble compounds are alkali.
• Even bases that won't dissolve in water will still react with acids.
• So, all metal oxides and metal hydroxides react with acids to form a salt and water.

acid+metal oxide->salt water

acid+metal hydroxide->salt+water

• hydrochloric acid + copper oxide -> copper chloride + water
• 2HCl + CuO -> CuCl2 + H2O
• sulfuric acid + potassium hydroxide -> potassium sulfate + water
• H2SO4 + 2KOH -> K2SO4 + 2H2O
• nitric acid + sodium hydroxide -> sodium nitrate + water
• HNO3 + NaOH -> NaNO3 + H2O
• phosphoric acid + sodium hydroxide -> sodium phosphate + water
• H3PO4 + 3NaOH -> Na3PO4 + 3H2O

Acids and carbonates produce carbon dioxide:

acid+carbonate->salt+water+carbon dioxide

• hydrochloric acid + sodium carbonate -> sodium chloride + water + carbon dioxide
• 2HCl + Na2CO3 -> 2NaCl + H2O + CO2
• sulfuric acid + calcium carbonate -> calcium sulfate + water + carbon dioxide
• H2SO3 + CaCO3 -> CaSO4 + H2O +CO2
• phosphoric aicd + sodium hydroxide -> sodium phosphate + water + carbon dioxide
• 2H3PO4 + 3Na2CO3 -> 2Na3PO4 + 3H2O + 3CO2

Acids and ammonia produce ammonium salts:

acid+ammonia->ammonium salt

• hydrochloric acid + ammonia -> ammonium chloride
• HCl + NH3 -> NH4Cl
• sulfuric acid + ammonia -> ammonium sulfate
• H2SO4 + 2NH3 -> (NH4)2SO4
• nitric acid + ammonia -> ammonium nitrate
• HNO3 + NH3 -> NH4NO3

Fertilisers provide plants with the essential elements for growth:

• The three main essential elements in fertilisers are nitrogen, potassium and phosphorus. If plants don't get enough of these elements, their growth and life processes are affected.
• These elements may be missing from the soil if they've been used up by previous crops.
• Fertilisers replace these missing elements or provide more of them. This helps to increase the crop yield, as the crops can grow faster and bigger. For example: fertilisers add more nitrogen to plant proteins, which makes the plants grow faster.
• The fertiliser must first dissolve in water before it can be taken in by the crop roots.

Ammonia can be neutralised with acids to produce fertilises:

• If you neutralise nitric acid with ammonia you get ammonium nitrate. It's an especially good fertiliser because it has nitrogen from two sources, the ammonia and the nitric acid.
• AMMONIUM SULFATE can also be used as a fertiliser. You make it by neutralising sulfuric acid with ammonia.
• AMMONIUM PHOSPHATE is a fertiliser made by neutralising phosphoric acid with ammonia.
• PHOSPHATE NITRATE is also a fertiliser - it can be made by neutralising nitric acid with potassium hydroxide.

Fertilisers are really useful - but they can cause big problems:

The population of the world is rising rapidly. Fertilisers increase crop yield, so the more feriliser we make, the more crops we can grow, and the more people we can feed. But if we use too many fertilisers we risk polluting our water supplies and causing eutrophication.

Fertilisers damage lakes and rivers - eutrophication:

• When fertiliser is put on fields some of it inevitably runs off and finds its way into rivers and streams.
• The level of nitrates and phosphates in the river water increases.
• Algae living in the river water use the nutrients to multiply rapidly, creating an algal bloom (a carpet of algae near the surface of the river). This blocks off the light to the river plants below. The plants cannot photosynthesise, so they have no food and they die.
• Aerobic bacteria feed on the dead plants and start to multiply. As the bacteria multiply they use up all the oxygen in the water. As a result pretty much everything in the river dies (including fish and insects).
• This process is called EUTROPHICATION

Preparing ammonium nitrate in the lab:

You can make most fertilisers using this titration method - just choose the rght acid (nitric, sulfuric or phosphoric) and alkali (ammonia or potassium hydroxide) to get the salt you want. You'll need ammonia and nitric acid to make ammonium nitrate.

• Set up your apparatus as in the diagram. Add a few drops of methyl orange indicator to the ammonia - it'll turn yellow.
• Slowly add the nitric acid from the burette into the ammonia, until the yellow colour just changes to red. Gently swirl the flask as you add the acid. Go especially slowly when you think the alkali's almost neutralised. Methyl orange is yellow in alkalis, but red in acids, so this colour change means that all the ammonia has been neutralised and you've got ammonium nitrate solution
• To get solid ammonium nitrate crystals, gently evaporate the solution until only a little bit is left. Leave it to crystallise.
• The ammonium nitrate crystals aren't pure - they've still got methyl orange in them. To get pure ammonium nitrate crystals, you need to note exactly how much nitric acid it took to neutralise the ammonia, and then repeat the titration using that volume of acid, but no indicator.

Percentage yield compares actual with predicted yield:

• The mass product that you end up with is called the yield of a reaction.
• You should realise that in practice you never get a 100% yield, as not all of the reactant will be converted into product. This means that the amount of product will be slightly less than you would expect if it worked absolutely perfectly.
• The more reactants you start with, the higher the actual yield will be - that's pretty obvious. But the percentage yield doesn't depend on the amount of reactants you start with - it's a percentage:
• Percentage yield is always somewhere between 0 and 100%
• 0% yield means that no reactants were converted into product, ie: no product at all was made.
• The predicted yield of a reaction is just the amount of product that you'd get if all the reactant was converted into product.

The Haber Process is a reversible reaction:

• N2 + 3H2 -- 2NH3
• The nitrogen is obtained easily from the air, which is 78% nitrogen (and 21% oxygen)
• The hydrogen comes from the cracking of oil fractions or natural gas.
• Because the reaction is reversible not all the nitrogen and hydrogen will covert to ammonia.
• The N2 and H2 which don't react are recycled and passed through again so none is wasted.
• PRESSURE: High (200 atmospheres), TEMPERATURE: 450*c CATALYST: Iron

Because the reaction is reversible, there's a compromise to be made:

• Higher pressures favour the forward reaction (producing ammonia from nitrogen and hydrogen), so the pressure is set at 200 atmospheres. This high pressure increases the percentage yield of ammonia.
• High temperatures favour the reverse reaction (where ammonia is broken down to give N2 and H2) - so high temperature decreases the percentage yield of ammonia.
• The trouble is, lower temperatures mean slow reaction rates. So manufacturers tend to use high temperatures anyway, to increase the reaction rate.

• 450*c is the optimum temperature - it gives a fast reaction rate and is a reasonable percentage yield. In other words, it's a compromise - better to wait 20 seconds for a 10% yield then to have to wait 60 seconds for a 20% yield.
• The unused H2 and N2 are recycled, so nothing is wasted.

The iron catalyst speeds up the reaction and keeps costs down:

• The iron catalysts makes the reacrtion go faster, which gets it to the equillibrium proportions more quickly. But remember, the catalyst doesn't affect the position of the equilibrium (ie: the % yield).
• Without the catalyst the temperature would have to be raised even further to get a quick enough reaction and that would reduce the % yield even further. So the catalyst is very important.

Production cost depends on several different factors:

There are 5 main things that affect the cost of making a new substance. It's these five factors that companies have to consider when deciding if and then how, to produce a chemical:

• PRICE OF ENERGY: a) industry needs to keep its energy bills as low as possible. b) if a reaction needs a high temperature, the running costs will be higher.
• COST OF RAW MATERIALS: a) this is kept to a minimum by recycling any materials that haven't reacted b) a good example of this is the Haber process. The % yield of the reaction is quite low (about 10%), but the unreacted N2 and H2 can be recycled to keep waste to a minimum.
• LABOUR COSTS (WAGES): a) everyone who works for a company has got to be paid. b) labour-intensive processes (ie: those that involve many people), can be very expensive. c) automatation cuts running costs by reducing the number of people involved. d) but companies have always got to weigh any savings they make on their wage bill against the initial cost and running costs of the machinery.
• PLANT COSTS (EQUIPMENT): a) the cost of equipment depends on the conditions it has to cope with. b) for example, it costs far more to make something withstand very high pressures than something which only needs to work at stmospheric pressure.

• RATE OF PRODUCTION: a) Generally speaking, the faster the reaction goes, the better it is in terms of reducing the time and costs of production. b) So rates of reaction are often increased by using catalysts. c) But the increase in production rate has to balance the cost of buying the catalyst in the first place and replacing any that gets losts.

Optimum conditions are chosen to give the lowest cost:

• Optimum conditions are those that give the lowest production cost per kg of product - even if this means compromising on the speed of reaction or % yield. Learn the definition:
• OPTIMUM CONDITIONS are those that give the LOWEST PRODUCTION COST.
• However, the rate of reaction and percentage yield must both be high enough to make a sufficient amount of product each day.
• Don't forget, a low percentage yield is okay, as long as the starting materials can be recycled.

Salt is mined from underneath Chesire:

• In Britain salt is extracted from underground deposits left millions of years ago when acient seas evaporated. There are huge deposits of this rock salt under Cheshire.
• Rock salt is a mixture of salt and impurities. It's drilled, blasted and dug out and brought to the surface using machinery.
• It can also be mined by pumping hot water underground. The salt dissolves and the salt solution is forced to the surface by the pressure of the water - this is called solution mining.
• When the mining is finished, it's important to fill in the holes in the ground. If not, the land could collapse and slide into the holes - this is called subsidence.
• Rock salt can be used in its raw state on roads to stop ice forming, or the salt can be seperated out and used to preserve or enhance the flavour in food or for making chemicals. If salt's going to be used to make chemicals, usually the first thing they do is electrolyse it using the chor-alkali process.

Electrolysis of brine gives hydrogen, chlorine and NaOH:

Concentrated brine (sodium chloride solution) is electrolysed industrially. The electrodes are made of an inert material - so that they won't react with the electrolyte or the products of electrolysis.

There are three useful products:

• HYDROGEN GAS given off at the negative cathode
• CHLORINE GAS given off at the positive anode
• SODIUM HYDROXIDE (NaOH) formed from the ions left in the solution.
• These are collected and used to make all sorts of things.

The half equations - make sure the electrons balance:

The sodium chloride solution contains four different ions: Na+, OH-, Cl- and H+

• At the cathode, two hydrogen ions accept one electron each to become one hydrogen molecule: 2H+ + 2e- -> H2
• At the anodem two chlorine (Cl-) ions lose one electron each to become one chlorine molecule: 2Cl- - 2e- -> Cl2
• Oxidisation is the loss of electrons and reducation is the gain of electrons. So the reaction at the anode is an oxidation reaction, and the reaction at cathose is a reducation reaction.

There are three useful products:

• HYDROGEN GAS given off at the negative cathode
• CHLORINE GAS given off at the positive anode
• SODIUM HYDROXIDE (NaOH) formed from the ions left in the solution.
• These are collected and used to make all sorts of things.

The half equations - make sure the electrons balance:

The sodium chloride solution contains four different ions: Na+, OH-, Cl- and H+

• At the cathode, two hydrogen ions accept one electron each to become one hydrogen molecule: 2H+ + 2e- -> H2
• At the anodem two chlorine (Cl-) ions lose one electron each to become one chlorine molecule: 2Cl- - 2e- -> Cl2
• Oxidisation is the loss of electrons and reducation is the gain of electrons. So the reaction at the anode is an oxidation reaction, and the reaction at cathose is a reducation reaction.

The electrolysis of brine is done by the chlor-alkali industry:

• The products of the chlor-alkali processare used for all types of things.
• For example, the hydrogen gas is used to make ammonia (in the Haber process) and margarine.
• The chlorine is used to disinfect water, to make platics (eg: PVC) , solvents or hydrochloric acid.
• The sodium hydroxide is used to make soap, or can be reacted with chlorine to make household bleach.
• All of these uses of the products of the electrolysis of brine makes the chlor-alkali industry very important to the economy - lots of new products can be made and lots of jobs are created.

The electrolysis of brine is done by the chlor-alkali industry:

• The products of the chlor-alkali processare used for all types of things.
• For example, the hydrogen gas is used to make ammonia (in the Haber process) and margarine.
• The chlorine is used to disinfect water, to make platics (eg: PVC) , solvents or hydrochloric acid.
• The sodium hydroxide is used to make soap, or can be reacted with chlorine to make household bleach.
• All of these uses of the products of the electrolysis of brine makes the chlor-alkali industry very important to the economy - lots of new products can be made and lots of jobs are created.