6. Earth Materials

11.1 Why is limestone such a useful material? Limestone is a commonly occurring rock which can be used not only for building but also for making many other useful materials including lime, cement and glass. Limestone, which is mainly calcium carbonate, can be quarried and used as a building material. Powdered limestone can be used to neutralise acidity in lakes and soils. When limestone is heated in a kiln it breaks down into quicklime (calcium oxide) and carbon dioxide. This type of reaction is called thermal decomposition (other carbonates behave in a similar way). Quicklime reacts with water to produce slaked lime (calcium hydroxide) which is used to reduce the acidity of soil. Cement is produced by roasting powdered limestone with powdered clay in a rotary kiln. When cement is mixed with water, sand and crushed rock, a slow chemical reaction produces a hard, stone-like building material called concrete. Glass is made by heating a mixture of limestone, sand and soda (sodium carbonate). 11.2 How can so many useful products be made from crude oil? Crude oil is found inside the Earths crust. Many useful products can be obtained from crude oil by separating the many different substances it contains and by using some of these in chemical reactions to make new substances, for example plastics. Crude oil is obtained from the Earths crust. It was formed from the remains of organisms which lived millions of years ago. It is a fossil fuel. The fossil fuels coal, oil and natural gas have resulted from the action of heat and pressure over millions of years, in the absence of oxygen, on material from animals and plants (organic material) which has been covered by layers of sedimentary rock. Crude oil is a mixture of a very large number of compounds. A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged. This makes it possible to separate the substances in a mixture by physical methods including distillation. Most of the compounds in crude oil consist of molecules made up of hydrogen and carbon atoms only (hydrocarbons). The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by evaporating the oil and allowing it to condense at a number of different temperatures. This process is fractional distillation. The hydrocarbon molecules in crude oil vary in size. The larger the molecules (the greater the number of carbon atoms) in a hydrocarbon: - the higher its boiling point; - the less volatile it is; - the less easily it flows (the more viscous it is); - the less easily it ignites (the less flammable it is). This limits the usefulness of hydrocarbons with large molecules as fuels. Large hydrocarbon molecules can be broken down (cracked) to produce smaller, more useful molecules. This process involves heating the hydrocarbons to vaporise them and passing the vapours over a hot catalyst. A thermal decomposition reaction then occurs. Some of the products of cracking are useful as fuels. Most fuels contain carbon and/or hydrogen and may also contain some sulphur. The gases released into the atmosphere when a fuel burns may include: - carbon dioxide; - water (vapour), which is an oxide of hydrogen; - sulphur dioxide. Other products of cracking can be used to make plastics (polymers) such as poly(ethene) and poly(propene). Poly(ethene) is used for making plastic bags and bottles. Poly(propene) is used for making crates and ropes. Most plastics, including poly(ethene) and poly(propene), are not broken down by microorganisms. They are not biodegradable. This can lead to problems with waste disposal. ! Candidates should be able, when provided with appropriate information, to evaluate the impact on the environment of burning hydrocarbon fuels and of plastic waste disposal. Higher Tier Carbon atoms form the spine of hydrocarbon molecules. When the carbon atoms are joined by single covalent carbon carbon bonds (when the hydrocarbons are saturated) they are known as alkanes. Candidates should be able to represent and to interpret saturated hydrocarbon molecules in diagramatic form. Other hydrocarbons have carbon carbon double covalent bonds (they are unsaturated) and are known as alkenes. A simple laboratory test for an unsaturated hydrocarbon is to use bromine water. The yellow-brown bromine water becomes colourless as the bromine reacts with the hydrocarbon. Candidates should be able to represent and to interpret unsaturated hydrocarbon molecules using diagrams. These unsaturated hydrocarbons are reactive and so are useful for making many other substances including polymers. Polymers have very large molecules, and are formed when many small molecules, of substances called monomers, join together. This process is called polymerisation. When unsaturated monomers join together to form a polymer with no other substance being produced in the reaction, the process is called addition polymerisation. Plastics are polymers and are made by polymerisation. For example, poly(ethene) (often called polythene) is made by polymerising the simplest alkene, ethene. Candidates should be able: - to interpret diagrammatic representations of addition polymerisation; - to represent the formation of a simple addition polymer in a diagramatic form. 11.3 How was the Earths atmosphere formed? The Earths atmosphere has been much the same for millions of years. Before that it was very different from what it is today. For 200 million years the proportions of different gases in the atmosphere have been much the same as they are today: - about four-fifths (80%) nitrogen; - about one-fifth (20%) oxygen; - small proportions of various other gases, including carbon dioxide, water vapour and noble gases. During the first billion years of the Earths existence there was intense volcanic activity. This activity released the gases which then formed the early atmosphere and water vapour which condensed to form the oceans. During this period the Earths atmosphere was probably mainly carbon dioxide and there would have been little or no oxygen gas (like the atmospheres of Mars and Venus today). There would also have been water vapour, and small proportions of methane and ammonia. When plants evolved and successfully colonised most of the Earths surface: - the atmosphere gradually became more and more polluted with oxygen. This meant that, gradually, there were fewer habitats suitable for microorganisms which could not tolerate oxygen; - most of the carbon from the carbon dioxide in the air gradually became locked up in sedimentary rocks as carbonates and fossil fuels; - the methane and ammonia in the atmosphere reacted with the oxygen; Higher Tier Nitrogen gas was released into the air, partly from the reaction between oxygen and ammonia, but mainly from living organisms, including denitrifying bacteria; the oxygen in the atmosphere resulted in the development of an ozone layer. This filters out harmful ultraviolet radiation from the sun allowing the evolution of new living organisms. Carbonate rocks are sometimes moved deep into the Earth by geological activity. They may then release carbon dioxide back into the atmosphere via volcanoes. The release of carbon dioxide by burning the carbon locked up over hundreds of millions of years in fossil fuels increases the level of carbon dioxide in the atmosphere. Though the reaction between carbon dioxide and sea-water also increases, producing insoluble (mainly calcium) carbonates which are deposited as sediment and soluble hydrogencarbonates (mainly calcium and magnesium), this does not wholly absorb the additional carbon dioxide released into the atmosphere. 11.4 Why have all mountains on Earth not worn away by now? Though the land masses on Earth seem to us to be very fixed they are, in fact, slowly moving about. This movement causes parts of the Earths crust to rise and so form mountains. The Earth is nearly spherical and has a layered structure comprising: - a thin crust; - a mantle extending almost halfway to the Earths centre which has all the properties of a solid except that it can flow very slowly; - a core, with just over half of the Earths radius, made of nickel and iron, the outer part of which is liquid and the inner part of which is solid. The overall density of the Earth is much greater than the mean densities of the rocks which form the crust. This indicates that the interior of the Earth is made of material different from, and denser than, that of the crust. At the surface of the Earth younger sedimentary rocks usually lie on top of older rocks. Sediments contain evidence for how they were deposited (e.g. layers formed by discontinuous deposition, ripple marks formed by currents or waves). Sedimentary rock layers are often found tilted, folded, fractured (faulted) and sometimes even turned upside down. This shows that the Earths crust is unstable and has been subjected to very large forces. Large scale movements of the Earths crust can cause mountain ranges to form very slowly over millions of years. These replace older mountain ranges worn down by weathering and erosion. Metamorphic rocks are associated with the Earth movements (tectonic activity) which created present-day and ancient mountain belts. They are evidence of the high temperatures and pressure created by these mountain-building processes. The edges of land masses (continents) which are separated by thousands of kilometres of ocean (e.g. the east coast of South America and the west coast of Africa): - have shapes which fit quite closely; - have similar patterns of rocks and fossils. This suggests that they were once part of a single land mass which has split and been moved apart. The Earths lithosphere (the crust and the upper part of the mantle) is cracked into a number of large pieces (tectonic plates) which are constantly moving at relative speeds of a few centimetres per year as a result of convection currents within the Earths mantle driven by heat released by natural radioactive processes. Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates. ! Candidates should be able, when provided with information about the complex probable causes of earthquakes and volcanic eruptions and the difficulty of making measurements of many of the factors involved, to explain why scientists cannot yet accurately predict when they will occur. At one time it was believed that the major features of the earths surface were the result of the shrinking of the crust as the Earth cooled down following its formation. ! Candidates should be able, when provided with appropriate additional information, to explain why Wegeners theory of crustal movement (continental drift) was not generally accepted until more than 50 years after it was proposed. Higher Tier Tectonic plates: - may slide past each other. This is happening along the Californian coast giving rise to earthquakes; - may move towards each other. As this happens, a thinner, denser oceanic plate can be driven down (subducted) beneath a thicker granitic continental plate where it partially melts. The continental crust is compressed, causing folding and metamorphism. Earthquakes are produced and magma may rise through the continental crust to form volcanoes. This is happening along the western side of South America (the Andes); - may move away from each other. This causes fractures which are filled by magma producing new, basaltic, oceanic crust. This is known as sea floor spreading and is happening along oceanic ridges, including the mid-Atlantic ridge. The iron-rich minerals in the magma record the direction of the Earths magnetic field at the time when the rising magma solidified. Magnetic reversal patterns in oceanic crust occur in stripes parallel to oceanic ridges, matching the periodic reversals of the Earths magnetic field and so supporting the concept of sea floor spreading.