Limestone is a rock of which was once the bones and shells of sea animals. These died and collected as sediments at the bottom of seas, oceans and lakes.
Chemical name: Calcium carbonate
Chemical formula: CaCo3
Alternative names: Chalk, marble
Limestone can be used to prepare a number of new materials. For this to happen limestone must undergo endothermic decomposition.
Thermal decomposition is used on a large scale in industry to manufacture chemicals.
When limestone undergoes thermal decomposition it produces Quicklime (CaO)
Quicklime is produced in refractory-lined kilns. In most kilns, limestone is fed in at the top of the shaft and quicklime is drawn from the bottom.
Air is blown in at the base and burns with fuels part of the way up the kiln. The products of combustion of the fuel is mostly carbon dioxide gas which escapes through vents at the top of the shaft.
Various fuels are used e.g. coil, oil, natural gas and coke depending on the local fuel costs. The temperature created by these fuels reaches over 1000?C in the middle of the kiln.
Most modern kilns produce up to 300 tonnes/day of quicklime. Most quicklime is sold as a granular product but there is a growing market for the finely crushed solid.
Limestone is one of the world’s most common minerals. Because it is cheap and readily available it forms an important part of many other minerals.
Once limestone has been converted to quicklime, the quicklime is used to form slaked lime.
Slaked lime (calcium hydroxide) is made by adding a little water to quicklime.
* Put in lakes to neutralize acids in the water
* Put on soil to neutralize acid soil.
All rocks are made up of different minerals. In other words, minerals are the chemical constituents of rocks although you can’t easily tell what their chemical composition is from their names.
Sedimentary rocks are made of sediment formed when all other types of rock wear down (erosion) or break down because of the effects of weather (weathering). The particles of rock are transported by water or wind or ice to a low point on the earth (a sea or a valley). Gradually, over millions of years, they are converted to rock.
The particles of rock form layers of sediment. Gradually more and more layers form on top of each other over thousands of years. The weight of the layers on top causes the bottom layers to be pressed together or compressed so the particles fuse together. This is called CEMENTATION.
Over millions of years the layers of rock build up into what are called strata. Particles can be rubbed of sedimentary rock because it is the softest type of rock.
Different sized particles form different kinds of rock. Very fine particles like mud form mudstone. Larger grains like sand form sandstone. Gravel and pebbles form conglomerates.
Sedimentary rocks like Shale can be formed under the sea also. The shells from millions of tiny marine animals can build up on the sea floor. These shells are made from calcium carbonate. When these shells become slowly compressed together they form the rock limestone. When acid is added to this rock, it fizzes.
As sediments collects, the bodies of plants or animals fall into them. The bodies become buried and gradually turn to rock as there is no oxygen gas to allow them to rot. Over millions of years these bodies turn into fossils.
Fossils are only found in sedimentary rocks.
When liquid rock is beneath the surface of the earth it is called magma, but when it comes out of the ground it is called lava. When they cool, they become solid and form the rocks we call igneous rocks.
Magma and lava are made up of compounds called silicates. They are made from the elements silicon and oxygen, as well as other elements like aluminium and magnesium.
As the liquid rock cools it forms crystals. The size of the crystals depends on how quickly it cools. Granite is formed when magma cools slowly under the ground so it has large interlocking crystals. This is called an intrusive igneous rock.
When lava is erupted from a volcano it cools rapidly in the air so it has small interlocking crystals. This is called an extrusive igneous rock. An example of this type of rock is basalt.
The structure of existing rocks can be changed by contact with magma or great pressure from underground. When this happens, the rock is said to have metamorphosed. (This just means ‘changed’.)
Any kind of rock can metamorphose. Clay and Shale (which are sedimentary rocks) can be changed by pressure until they become slate.
This allows the slate to be split into layers easily. This is one reason why slate can be used for roofing material. If slate is then heated even more and put under more pressure it forms a rock with bands of interlocking crystals. This rock is called Schist.
Limestone is turned into marble when the high temperatures underground break up the shells and they then reform as smooth, hard crystals.
Scientists think ha the earth has been in existence for about 4500 million years. In the beginning, the surface was liquid, but gradually it cooled enough for a solid crust to form. (About 3700 million years ago) The earth has had an atmosphere of gasses since it was formed but the types and amounts of each gas has changed.
The earth’s crust is much thicker under the continents than under the oceans. Continental crust is about 40km thick compared to 10km for the crust under the oceans. Below the crust is a semi-liquid area called the mantle. The centre of the earth is called the core and it is made of the metals Iron and Nickel.
Because different parts of the earth’s crust are moving slowly in different directions, huge stresses and strains are set up. These can cause the rocks to become tilted, folded (bent) or faulted (split). They may even be turned upside down.
Sedimentary rocks are softer than other types of rocks and fold more easily when compressed (squashed). If this happens on a large scale, then Fold mountains like the Alps or Himalayas are formed. A downward dip or fold is called a Syncline and an arch is called an Anticline.
Brittle rocks fracture (fault) under stress.
The earths crust is not a continuous layer but is made up of many separate units called Tectonic Plates.
The plates move by convection currents in the molten rock of the Mantle. This moves the plates very slowly at the rate of a few centimetres a year. Over millions of years the continents have separated to reach their present positions.
When an oceanic plate moves below a continental plate, the rock gets very hot and melts to form Magma. This then reaches the surface in Volcanoes.
Also, where the oceanic plate moves downwards, a very deep area of sea floor is formed. These types of areas are called Ocean Trenches.
Formation of coal and oil:
In the carboniferous Era, which lasted 65 million years and ended 280 million years ago:
The British Isles was on the equator.
The land was tropical forest.
Coal was made from dead animals and plants.
The plants did not rot because they got covered in mud so no oxygen could get to them.
The coal seams were formed by weight of new deposits squashed the tree layers into thin seams.
Coal was formed 280 million years ago.
When oil is pumped from the ground it is as a thick, black, foul smelling substance called Crude Oil. It is a mixture of chemicals called hydrocarbons.
A mixture is just a mix of chemicals.
The many hydrocarbons in crude oil may be separated into individual chemicals called Fractions by a process called Fractional Distillation.
Crude oil is a mixture of Hydrocarbons. They can be separated by putting the oil into a fractioning column. The crude oil is heated on its way into the column so that the oil is evaporated. This turns the liquid into a gas. The vapour then moves into the column. The chemicals with the least/most? molecules (more carbon atoms) move to the top e.g. petrol.
As the molecules become heavier, their boiling points increase. Once at the correct temperature the vapour condenses to a liquid and is run off.
Crude oil contains most fuel and bitumen yet we do not need it all. We need lots of petrol (which contains 5-10 carbon atoms) but we have too much fuel oil etc. (over 70 carbon atoms), so what we do is this. We break up large useless molecules to make small, useful molecules like petrol by a process called cracking.
Paraffin is a general name for a group of long hydrocarbons used as fuels. If mixed with pieces of porous porcelain as a catalyst, it can be made to undergo thermal decomposition or cracking.
Porous means lots of holes, which gives a large surface area.
Thermal decomposition means breaking something down with heat.
Fractional distillation of petroleum (crude oil) produces different products because petroleum is a mixture of hydrocarbons. The compound in each fraction have similar boiling points. The fractions with higher boiling points tend to contain molecules with more carbon atoms. These bigger molecules are long chained and have larger molecular masses. When these are burnt they produce sooty flames. They are very viscous (syrup-like). As a result there is high demand for smaller molecules and less demand for these massive molecules. The oil industry tries to balance the problems of supply and demand by converting the less saleable fractions into saleable fractions. The main way in which this is done is by the process known as cracking. The reactions are usually carried out at high temperature using a catalyst.
We have seen that when large hydrocarbon molecules are broken open, or cracked, they produce smaller molecules like Ethene. It is possible to join lots of Ethene molecules together to form a type of plastic.
Simple building blocks like the paper clip (or Ethene) are called MONOMERS. When they are made into a long line of units we call it a POLYMER. The process which converts lots of Monomers to a Polymer is called PLYMERISATION.
Common polymers include plastics, fibres and rubber.
The elements present in Ethene monomers are hydrogen and carbon.
What happens to convert monomers to polymers? Double bond breaks open during polymerisation.
The name of the polymer made from Ethene is polyethene.
Polyethene is more commonly known as polythene.
Another type of plastic is called Polyvinyl Chloride – PVC for short, is strong, hard and less flexible than polythene. It is used to make pipes, guttering and electrical insulation.
Combustion is another word for burning. Burning is a chemical reaction in which materials react with heat. The equation for burning is:
Fuel + oxygen = CO2 + water + energy.
If there is not enough oxygen in the air, the fuel dies not burn fully and the poisonous gas Carbon Monoxide is formed. Several people are killed each year due to poisoning by carbon monoxide from badly ventilated gas fires.
Some fuels also contain Sulphur e.g. Coal. When this burns, it produces the gas sulphur dioxide.
Sulphur + oxygen = sulphur dioxide.
When this gas dissolves in rain, it produces acid rain.
Waste gases from the exhausts of motor vehicles are also released into the atmosphere. They are also responsible for making the rain acidic and they pollute the air.
Oxygen gas is:
* Colourless and odourless
* Molecules are diatomic
* Approx. same density as air.
* Essential for combustion and respiration
* Reacts with most metals and non-metals to form oxides.
Test for glowing oxygen = glowing splint re-lighting.
Oxygen is a by-product of photosynthesis.
Oxides can be acidic, basic (alkaline) or neutral.
The link between the terms alkali and base is that an alkali is a base which dissolves in water.
Non-metals form acidic oxides.
Metals form alkaline oxides.
Many metals and non-metals are found as oxides in the earth’s crust. They are called ores.
When elements react with oxygen it is called oxidation.
During respiration and combustion, oxygen is used up and carbon dioxide is released back in to he air. All of the air is not used up because plants use up the carbon dioxide and release oxygen when they make their food by photosynthesis or respiration? This process has been happening for thousand of years. However, in the last 200 years, the amount of carbon dioxide in the atmosphere has increased because:
* There are many people on earth.
* We have burned millions of tonnes of fuel
* We have cut down thousands of square miles of forest.
The carbon dioxide is acting like a ‘blanket’ around the earth and it is trapping the heat from the sun, which leads to a raising of the earth’s temperature. This is called the greenhouse effect.
Acid rain is made when acid gases dissolve in rain.
Carbon dioxide is made when carbon burns in the oxygen in the air. When coal is burned it releases the gas sulphur dioxide. This also makes acid rain because it dissolves in rain to form sulphuric acid.
Inside car engines, the temperature is hot enough to make nitrogen and oxygen join together to make nitrogen oxides. These dissolve in rain to make nitric acid.
When acid rain falls on the earth it has the following effects:
* Stonework on building is dissolved and crumbles away
* The water in rivers and lakes becomes too acidic so plants and animals die
You could reduce the acid level in rivers and lakes by putting limestone in it.
The gases in the atmosphere originally came from volcanoes.
The gases produced were carbon dioxide, hydrogen chloride, argon etc.
There is not hydrogen left in the atmosphere because it was so light it evaporated and escaped.
Distillation of oil:
1. Vaporisation – the thick oil is heated to from vapour/gas
2. Separation – lighter molecules move upwards and heavy molecules sink. Lighter molecules move upwards to the coolest region at the top. There is a temperature and density gradient.
3. Condensation – the vapour is cooled to form a liquid (or solid)