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S279 Our Dynamic Planet: Earth and Life Are you ready for S279? Contents 1 Introduction 2 2 Suggested prior study 2 3 Mathematical skills 3 3.1 Self-assessed test for numeracy 3.1.1 3 Suggested further reading 4 4 Key concepts covered in prerequisite courses and developed in S279 5 4.1 Basic physical concepts 5 4.2 Chemical concepts 5 4.2.1 Atoms, molecules and compounds 5 4.2.2 Relative masses of molecules and compounds 6 4.2.3 Chemical bonding, valency and ions 6 4.2.4 Oxidation and reduction 7 4.2.5 Chemical equations 8 4.2.6 Suggested further reading 8 4.3 Biological concepts 8 4.3.1 Photosynthesis and respiration 8 4.3.2 Cells and cell division 8 4.3.3 Evolution 9 4.3.4 Suggested further reading 9 4.4 Geological concepts 9 4.4.1 Plate tectonics 4.4.2 Chemical weathering 11 4.4.3 Geological time 11 4.4.4 Suggested further reading 13 4.5 Other important concepts 9 13 4.5.1 Biogeochemical cycles 13 4.5.2 Feedback mechanisms 14 4.5.3 Suggested further reading 15 Answers to self-assessment questions Copyright © 2010 The Open University 16 WEB 02254 5 4.1 1 Introduction If you are intending to study S279, you will want to ensure that you have the necessary background knowledge and skills to be able to enjoy the course fully and to give yourself the best possible chance of completing it successfully. Please read through these notes carefully and work through the accompanying self-assessment questions. This will be a useful exercise for all prospective S279 students, including those who have already studied other Open University science courses and/or have completed the recommended S279 preparatory courses (see Section 2). Working through this booklet should serve as a reminder of some of the facts, skills and conceptual knowledge you will be expected to have prior to studying S279. If you are coming to S279 without having studied a Level 1 science course at The Open University, then it is essential that you establish whether or not your background and experience will give you a sound platform from which to tackle the work. If you find that you can answer most of the questions in these notes, then it is likely that you are well prepared to take on S279. If, however, you find that you have substantial difficulties with more than about five questions, then it may be worthwhile taking a course that would prepare you for S279 instead (see Section 2). If you are still not quite sure whether S279 is the course for you, please seek further help and advice from the Student Registration and Enquiries Service: email: [email protected] tel: +44 (0)870 333 4340 S279 Our Dynamic Planet: Earth and Life takes a holistic view of the Earth, and requires a basic level of understanding of biology, chemistry and some physical processes, in addition to geology (the Earth sciences), to appreciate how the various components of the ‘Earth System’, i.e. the atmosphere, hydrosphere, biosphere and geosphere, are linked. It also requires a thorough understanding of basic maths skills and the ability to apply this understanding. The course as a whole is attempting to address the issue of the changing conditions of the Earth through time – and should encourage you to think about how human activity is influencing these changes. 2 Suggested prior study Recommend OU courses: S104 Exploring Science; S276 Geology; S151 Maths for Science; S283 Planetary science and the search for life. Before embarking on S279 you should have studied and passed the Level 1 science course, S104 Exploring Science (or one of its predecessors S103 or S102), as S279 assumes that you have an understanding of basic scientific skills equivalent at least to this level. S279 also requires a basic understanding of mathematics, including algebra, powers of ten, significant figures and the use and interpretation of graphs, comparable to the Level 1 course S151 Maths for Science. S279 forms one of two core Earth sciences courses at Level 2 and will build on the general geological concepts developed in the other core course, S276 Geology (and its predecessor S260). As such, we recommend that you study S276 before, 2 rather than at the same time as, S279. S283 is also very useful preparation and should be studied before S279. If you completed your study of science several years ago, we recommend that you revise the key concepts listed in Section 3 before you begin the course. Your Regional Centre can provide details of where to find reference copies of S104 Exploring Science, or you can buy selected materials from: Open University Worldwide, The Berrill Building, Walton Hall, Milton Keynes MK7 6AA. 3 Mathematical skills The amount of maths in S279 is significant, and essential. Throughout the course, graphs are commonly used to show the relationship between two variables. Angles are measured in degrees and the sine, cosine and tangent of angles are also used at various points throughout the course. Several algebraic equations are used, and you are asked to manipulate a few, relatively simple equations. You should be able to put numerical values into algebraic equations to obtain the required answers, calculate areas and volumes, and express quantities as ratios, fractions and percentages. You also need to be familiar with positive and negative powers of ten and quoting numerical answers to the correct number of decimal places and significant figures. You do not have to memorise equations or the values of physical constants, as, if needed, these will be given in the examination paper. The following self-assessment questions outline some of the key mathematical concepts needed for S279. 3.1 Self-assessed test for numeracy Question 1 Express the following numbers in scientific notation: (a) 100 000 000 (b) 400 000 000 000 (c) 3500 (d) 95 × 105 (e) 0.51 × 103 (f) 0.00025 (g) 1 1 000 000 (h) 0.0035 Question 2 Complete the following calculations and express your answers (a) using the appropriate units and number of decimal places: (i) 0.43 m + 1.217 m (ii) 8.1 kg – 3.82 kg (b) using the appropriate units and number of significant figures: 3 (i) 2.373 m × 3.6 m (ii) 6342 kg 2.42 m 3 Question 3 Suppose x = 2z 2 and a = 4zr. a Combine these two equations to give an equation for x that does not involve a. Question 4 Use your calculator to work out the following, expressing your answers to two significant figures: (a) sin 60° (e) sin 45° (b) cos 60° (f) tan 10° (c) cos 30° (g) cos 10° (d) tan 45° Question 5 Study Figure 1 and then answer the following questions. lava flow/litres per second 200 150 100 50 0 00.00 04.00 08.00 12.00 16.00 20.00 24.00 time/hours Figure 1 The volume of a lava flow flowing past a fixed point per second during an eruption. (a) What was the flow of the lava at 12:00 hours? (b) What was the maximum flow of the lava and at what time did it occur? (c) Describe in words how the lava flow has changed over the period plotted on the graph. 3.1.1 Suggested further reading S151 Maths for Science. Northedge, A., Thomas, J., Lane, A. and Peasgood, A (1997) The Sciences Good Study Guide, Open University Worldwide. (Especially Chapters 3–5) 4 4 Key concepts covered in prerequisite courses and developed in S279 This section outlines the main scientific concepts that you should be familiar with before beginning S279. These are all introduced in the Level 1 science course and developed further in S279. (All references are to Books in S104 Exploring Science and not to S279. Where appropriate, references are also made to specific Level 2 Open University courses.) 4.1 Basic physical concepts Force and energy are physics concepts that you should be familiar with and that will be developed further in S279. Force: S104 Exploring Science, Books 3 and 7 (also S207 The Physical World, Book 2) Newton’s second law states that the magnitude of an unbalanced force (F) on an object is equal to the object’s mass (m) multiplied by its acceleration (a): F = ma where F is measured in newtons (N) and 1 N = 1 kg m s−2. Newton’s law of gravitation states that two bodies of mass m1 and m2 separated by a distance (d) between their centres attract each other with a gravitational force (Fg) proportional to the product of their masses (m1m2) and inversely proportional 1 to the square of the separation distance ( 2 ). r Fg = Gm1m2 r2 where G is the gravitational constant equal to 6.67 × 10–11 N m2 kg−2. Energy: S104 Exploring Science, Book 3 (also S207 The Physical World, Book 2) The law of conservation of energy states that the total amount of energy in an isolated system is constant, as energy can neither be created nor destroyed. Question 6 The mass of the Earth is 5.98 × 1024 kg and the mass of the Moon is 7.35 × 1022 kg. On average, they are 384 500 km apart. Calculate the gravitational force on the Earth due to the Moon. What is the force on the Moon due to the Earth? 4.2 Chemical concepts An understanding of chemical concepts underpins many of the processes discussed in S279. 4.2.1 Atoms, molecules and compounds You should be familiar with the idea that all ‘matter’ is composed of atoms. Every atom contains a nucleus at its centre which contains protons and neutrons. The nucleus is surrounded by particles known as electrons. Protons are 5 positively charged, neutrons have no charge and electrons are negatively charged. Protons and neutrons each have approximately the same mass, whereas the mass of an electron is negligible. The atom of any element is characterised by the number of protons in its nucleus – called the atomic number, Z. Carbon for example has six protons, whereas nitrogen has seven and oxygen has eight. Different atoms of the same element, which differ only in the number of neutrons they contain, are known as isotopes of that element, and have different atomic masses. The mass of any atom is expressed by the relative atomic mass, which compares the mass of that atom to the atomic mass of the isotope of carbon that has six neutrons. On this scale, the relative atomic mass of 12C is defined as 12.000. A particular isotope of an element can be specified by the mass number, A, which is the relative atomic mass rounded to the nearest whole number; it gives the total number of protons and neutrons in an atom. Question 7 The element carbon has an atomic number of 6. Carbon has three isotopes: 12C, 13C and 14C. (a) How many neutrons do the isotopes 13C and 14C contain? (b) How many electrons does a (neutral) atom of 14C contain? Atoms may combine with other atoms. Some common elements exist as diatomic molecules rather than simple atoms, e.g. the gases oxygen (O2), hydrogen (H2), nitrogen (N2) and chlorine (C12). Atoms may also combine with atoms of different elements to form compounds, e.g. calcium carbonate (CaCO3). In each case, the chemical formula indicates the relative numbers of the different atoms combining together. Question 8 (a) How many atoms of oxygen are there in a molecule of oxygen? (b) What are the relative numbers of the three different atoms calcium (Ca), carbon (C), oxygen (O) in the compound calcium carbonate (CaCO3)? 4.2.2 Relative masses of molecules and compounds The relative mass of molecules and compounds is the sum of the relative atomic masses of all the atoms in the chemical formula. Question 9 The relative atomic masses of some common elements are as follows: hydrogen (H) = 1.01, carbon (C) = 12.0, oxygen (O) = 16.0, calcium (Ca) = 40.1. What are the relative masses of: (a) water (H2O)? (b) carbon dioxide (CO2)? (c) calcium carbonate (CaCO3)? 4.2.3 Chemical bonding, valency and ions The electrons surrounding the nucleus of an atom occupy energy levels, or shells, each of which is capable of holding a different number of electrons. The relationship between the electrons and the electron shells is referred to as the electronic configuration of an atom. The most stable electronic configurations are 6 those of the noble gases (e.g. helium, neon and argon), which take part in few chemical reactions. Atoms chemically bond with other atoms in order to achieve the stable electronic configuration of a noble gas. This can be achieved by either: (i) transferring electrons (ionic bonding) to form positive or negative ions, or (ii) sharing pairs of electrons (covalent bonding) to form molecules. Calcium carbonate is an example of an ionic compound (it consists of positively charged calcium ions and negatively charged carbonate ions) whereas the atoms in the gases hydrogen (H2) and carbon dioxide (CO2) are covalently bonded. Metals such as sodium (Na), calcium (Ca), magnesium (Mg) and iron (Fe) may form ionic bonds with other atoms, and themselves become positively charged ions. In simple terms the valency of an atom is the number of bonds that an atom can form within a molecule or compound. When bonding is ionic, valency can be considered as the number of electrons gained or lost in the transfer. When bonding is covalent, it is equal to the number of covalent bonds that an atom forms with other atoms. Question 10 In ionic compounds, sodium forms Na+ ions and calcium forms Ca2+ ions. (a) What are the valencies of sodium and calcium? (b) When calcium reacts with chlorine, calcium chloride (CaC12) is formed, comprising calcium ions and chloride ions (i.e. charged chlorine (C1) atoms). What is the charge on each chloride ion, and what is the valency of chlorine in CaC12? 4.2.4 Oxidation and reduction Oxidation is said to occur when the proportion of oxygen in a compound increases, or the proportion of hydrogen decreases. Conversely, reduction occurs when the proportion of hydrogen in a compound increases, or the proportion of oxygen decreases. Any chemical reaction leading to oxidation of one substance must be accompanied by the reduction of another substance, and vice versa. The term oxidation is also used more generally to include any reaction in which an atom loses electrons. For example, the change of an iron(II) ion, Fe2+, to an iron(III) ion, Fe3+, is an oxidation (and the reverse process is a reduction). A reaction which causes iron to share more of its electrons to form bonds can also be considered as an oxidation reaction. Question 11 (a) When oxygen forms ionic compounds with iron, negatively charged ions are produced, O2−. What is the valency of oxygen? (b) What is the charge on the iron ion in the following iron oxide compounds? (i) FeO (ii) Fe2O3? (Remember that in each case the compound must be electrically neutral.) (c) When FeO is converted into Fe2O3 is it reduced or oxidised? 7 4.2.5 Chemical equations Reactions between various elements and compounds are written as chemical equations showing the reactants on the left-hand side and the products on the right-hand side. The reactants and products are linked by an arrow indicating the direction of the reaction. As atoms are neither created nor destroyed during chemical reactions, the number of atoms of each type present on one side of the equation must be the same as the number of each type on the other side, i.e. the equation must be balanced. Question 12 During the process of photosynthesis green plants use atmospheric carbon dioxide (CO2) and water (H2O) to produce glucose (C6H12O6). Balance the following overall chemical equation for this reaction, so that you have the correct numbers of molecules of the reactants. CO2 + H2O → C6H12O6 + 6O2 4.2.6 Suggested further reading S104 Exploring Science: � Book 4 ‘The right chemistry’ (basic concepts in chemistry, the Periodic Table, chemical bonding and chemical reactions) � Book 8 ‘Quarks to quasars’ (atomic structure) 4.3 Biological concepts A key biological concept that recurs in S279 is the relationship between photosynthesis and respiration. In addition, you will need to understand the basic structures of different types of cells, how cell division takes place and the basic principles of evolutionary theory. Although all these concepts are re-taught in S279, you will find the course less demanding if you already have some previous knowledge of them. 4.3.1 Photosynthesis and respiration Photosynthesis is the basis of almost all of the primary production on the Earth. Using the energy derived from sunlight, green plants manufacture carbohydrates (glucose) from the raw materials of water and carbon dioxide by the reaction shown in the answer to Question 6. There is a related reaction called respiration in which oxygen is used to break down carbohydrate, producing carbon dioxide and water again. Question 13 (a) Write down the overall chemical equation for respiration. (b) Explain which of the reactants in your equation is oxidised during the reaction, and which is reduced. 4.3.2 Cells and cell division At the broadest level organisms are classified into three domains: Archaea, Bacteria and Eukarya. All Archaea and Bacteria have prokaryotic cells and most are unicellular. Eukarya include plants, animals and fungi; they have eukaryotic cells and are mostly multicellular. There are fundamental structural differences between prokaryotic and eukaryotic cells, and between the features of plant and animal cells. 8 Question 14 The items in the following list describe features of cells. For each, state in which of the following three types of cell it occurs: a prokaryotic cell, a eukaryotic animal cell and/or a eukaryotic plant cell. (a) contains a nucleus; (b) possesses cell walls; (c) contains organelles; (d) DNA is contained within the nucleus; (e) contains chloroplasts; (f) DNA is free within the cytoplasm. Cell division in eukaryotes takes place either by mitosis or meiosis. Question 15 Which type of cell division takes place during growth? 4.3.3 Evolution Over the course of geological time, plants and animals have evolved; in other words they have changed both in their morphology and their genetic make-up, resulting in new species. The basis for evolutionary change is the operation of natural selection upon heritable variation in organisms. Such variation is underwritten by the genes of the organism. These are composed of long strands of the complex organic molecule, deoxyribonucleic acid (DNA). Variation may be brought about by recombination, the genetic ‘shuffling’ that takes place as the result of sexual reproduction, and by mutation, involving spontaneous changes in the composition of the DNA. Eventually, the changes may be so significant that the DNA of related organisms becomes incompatible (i.e. they can no longer interbreed) and thus new species arise. Question 16 Is recombination a feature of mitosis or meiosis? 4.3.4 Suggested further reading S104 Exploring Science: � Book 5 ‘Life’ (photosynthesis, mitosis, meiosis, genetics and evolution) � Book 6 ‘Exploring Earth’s history’ (evolution, the fossil record) 4.4 Geological concepts The three most important concepts with which you should be familiar are plate tectonics, chemical weathering and geological time. These concepts are re-taught in S279, but again you will find the course less demanding if you already have some previous knowledge of them. 4.4.1 Plate tectonics The outer layer of the Earth, consisting of the crust and the top-most part of the mantle, is known as the lithosphere. The lithosphere is divided into a number of rigid plates which are in constant motion relative to each other across the underlying asthenosphere. Plates may consist of oceanic lithosphere alone or 9 oceanic and continental lithosphere. Plate tectonics is the name given to the processes involving the movement and deformation of lithospheric plates. Question 17 During plate movement the following situations may arise: (i) divergence of two oceanic plates; (ii) convergence of two oceanic plates; (iii) convergence of an oceanic plate and a continental plate; (iv) convergence of two continental plates. (a) In which situation(s) is one plate subducted into the mantle beneath the other? (b) In which situation(s) does an ocean ridge form and sea-floor spreading take place? (c) Which situation(s) are associated with volcanic activity? (d) In which situation(s) does a continental mountain range develop? (e) In which situation(s) is an island arc produced? Question 18 (a) Use the following definitions to describe the mode of formation and texture of igneous, metamorphic and sedimentary rocks. (Select two definitions for each rock type.) (i) The rocks have a granular or fragmental texture. (ii) The rocks form from a molten state, either within the Earth or at its surface. (iii) The rocks form by the deposition of sediment and fossil remains. (iv) The texture of these rocks is often aligned or foliated. (v) The rocks form in the solid state, when another rock is subjected to changing pressures and/or temperatures. (vi) The rocks have a crystalline, interlocking texture. (b) Place the following rock types in the correct columns in Table 1: basalt quartzite chalk schist gneiss sandstone phyllite gabbro diorite breccia shale limestone siltstone granite migmatite slate rhyolite greywacke marble peridotite 10 Table 1 Examples of common igneous, metamorphic and sedimentary rocks. Igneous rocks Metamorphic rocks Sedimentary rocks 4.4.2 Chemical weathering This is the process whereby rocks at the Earth’s surface are chemically decomposed by reactions between the minerals they contain and slightly acidic surface waters. Question 19 Explain, with the aid of a simple chemical equation, the role of atmospheric carbon dioxide in the formation of slightly acidic surface waters. 4.4.3 Geological time Geologists routinely refer to enormous lengths of time with apparent nonchalance. The age of the Earth is calculated at around 4.6 billion years (4600 million years, or 4600 Ma) and Earth processes take place over time periods up to hundreds of Ma in length. Discussion of geological time generally takes place in two ways. First, there is the relative measurement of geological time, in which the age of one geological event or rock layer is considered as simply older or younger than another. Second, there is absolute measurement in which the actual age of an event or rock layer can be calculated to within a few Ma. Relative measurements (or dating): the relative timescale is complied as the geological column, which is divided into Eons, Eras, Periods, as shown in Table 2. The Tertiary and Quaternary Periods are further divided into Epochs. The age of an event is referred to by the Eon, Era, Period or Epoch within which it falls. 11 Table 2 The geological timescale*. Eon Era Period Age of lower boundary of Period or Era/Ma Cenozoic Quaternary 1.75 Tertiary 65 Cretaceous 142 Jurassic 205 Triassic 245–248 Permian 290 Carboniferous 354 Devonian 417 Silurian 443 Ordovician 495 Cambrian 540–545 PHANEROZOIC Mesozoic Paleozoic CRYPTOZOIC Proterozoic 2500 Archean 2800 Hadean 4600 *You may find slight differences in the geological timescale represented in other texts. The timescale shown in Table 2 is the one developed in S279. Question 20 (a) A lava flow within a succession of limestones in northern England is dated as approximately 340 Ma old. During which geological Period and Era was it erupted? (b) The appearance of anatomically modern humans has been dated as 150 000 years ago. During which geological Period and Era was this? Absolute measurements (or dating) allows the boundaries between various Eons, Eras and Periods to be assigned an age in Ma (see Table 2). The technique used is radiometric dating. The isotopes of some elements are not stable, and they breakdown (or decay) through the loss of particles at the subatomic level until they are in a stable configuration. The length of time it takes to halve the number of original atoms is defined as the half-life of the isotope. The proportion of the number of original atoms of the isotope to the number of atoms of the decay product in a naturally occurring material can be used to determine how many half-lives have elapsed and, from this, the age of the sample can be calculated. Question 21 The proportion of the carbon isotope 14C in the remains of an ancient wooden shelter is only one-quarter of that found in modern-day trees. The half-life of 14C is 5700 years, so what is the approximate age of the shelter? 12 4.4.4 Suggested further reading It would be helpful if you were familiar with the following concepts, which are developed extensively in S279. S104 Exploring Science: � Book 1 ‘Global warming’ (an introduction to the interactions between the atmosphere, biosphere, hydrosphere and lithosphere; climate change) � Book 2 ‘Earth and space’ (igneous, metamorphic and sedimentary rocks; plate tectonics) � Book 6 ‘Exploring Earth’s history’ (distribution of silicate minerals in crustal rocks; crystallisation processes; partial melting; weathering; geological time) S260 Geology: � igneous, metamorphic and sedimentary rocks: Books 1–4 � distribution of silicate minerals in crustal rocks: Books 2–3 � crystallisation processes: Book 3 � partial melting: Book 3 � palaeoenvironments: Book 4. Or S276 Geology: � igneous, metamorphic and sedimentary rocks: Books 1–3 � distribution of silicate minerals in crustal rocks: Books 2–3 � crystallisation processes: Book 2 � partial melting: Book 2 � palaeoenvironments: Book 3. Geological concepts are also covered in any good elementary geological textbook, such as: Rothery, D.A. (2003) Teach Yourself Geology, Hodder & Stoughton. Skinner, B.J. and Porter, S.C. (2004) The Dynamic Earth: an introduction to physical geology, 5th edition, Wiley, Chichester. 4.5 Other important concepts Two other important concepts that recur in S279 are the concepts of biogeochemical cycles and feedback mechanisms. 4.5.1 Biogeochemical cycles The Earth’s system (atmosphere, hydrosphere, biosphere and lithosphere) operate to recycle the biologically important elements through chemical transformations of these elements as they move between organisms (bio-) and the rest of the Earth (geo-). The pathways that these elements take through the Earth’s systems are known as biogeochemical cycles. The cycle for an atom of a particular element comprises two main parts: reservoirs where the atom is stored for various lengths of time and transfers between the reservoirs. 13 residence time = mass of element in reservoir rate at which element enters (and/or leaves) reservoir In other words, residence time is the average time that an atom of an element spends in a reservoir. The most important biogeochemical cycle is the carbon cycle. Question 22 Figure 2 illustrates the main reservoirs (boxes) and transfer processes (arrows) of carbon in and on the Earth, and the timescales involved for the return of carbon to the atmosphere (as CO2). (a) What are the main reservoirs for carbon in decreasing order of size? (b) The processes by which atmospheric carbon dioxide is transferred to plants and animals on the land are not shown on Figure 2. What are they? (c) Which are by far the most long-term storage reservoirs for carbon? (d) What is the residence time for carbon in the atmosphere? (Assume an average rate of transfer of carbon from the atmosphere to the surface ocean to be 95 × 1012 kgC y−1.) atmospheric CO2 760 120 60 60 0.4 soil and detritus 1500 burial and lithification uplift and weathering, volcanism 90–100 90–100 plant biomass 560 preserved organic carbon (incl. fossil fuels) ~10000000 surface ocean 1000 36 biomass 2 mixing upwelling 37 deep ocean 4−5 38000 sinking detritus 33 0.4 0.6 0.05 burial and lithification carbonate rocks ~40000000 40 marine sediments ~3000 0.2 Figure 2 Summary diagram for the carbon cycle. The approximate mass of carbon in each reservoir is given in units of 1012 kgC (kilograms of carbon), and the transfers (or fluxes) in units of 1012 kgC y−1. Biomass is the total mass of organisms in a given area, expressed as the mass of carbohydrate in the dry material. 4.5.2 Feedback mechanisms The Earth’s systems are interlinked. Processes should not be treated in isolation as they affect other processes. When a change in one quantity causes changes to others, which lead to a further change in the original quantity, this is called feedback. If an increase in a quantity leads, eventually, to a further increase in this quantity (or a decrease leads to a further decrease), it is called positive 14 feedback. However, if an increase in a quantity leads, eventually, to decrease in this quantity (or a decrease leads to an increase), it is called negative feedback. Question 23 Consider the following situation: an increase in snow cover increases the amount of solar radiation reflected from the Earth, which in turn lowers the amount of energy from the Sun which is retained on the planet, which causes cooling. This may allow more snow to fall and to remain for longer, which in turn increases the reflectance, causing more cooling, which may allow more snow to fall, and so on. Is this an example of positive or negative feedback? 4.5.3 Suggested further reading S104 Exploring Science: � Book 1 ‘Global warming’ (water cycle, carbon cycle, global warming and feedbacks) 15 Answers to self-assessment questions Question 1 (a) 1.0 × 108 (b) 4.0 × 1011 (c) 3.5 × 103 (d) 9.5 × 106 (e) 5.1 × 102 (f) 2.5 × 10–4 (g) 1.0 × 10–6 (h) 3.5 × 10–3 Question 2 (a) You were asked to quote to your answer to the appropriate number of decimal places: (i) 0.43 m + 1.217 m = 1.647 m, which should be quoted as 1.65 m to two decimal places. Note: the number of significant figures in the result should be the same as the measurement with the fewest significant figures. (ii) 8.1 kg – 3.82 kg = 4.28 kg, which should be quoted as 4.3 kg to one decimal place. Note: the number of decimal places in the result should be the same as the measurement with the fewest decimal places. (b) You were asked to quote to your answer to the appropriate number of significant figures: (i) 2.373 m × 3.6 m = 8.5428 m2, which is 8.5 m2 to two significant figures. 6342 kg (ii) = 2620.6612 kg m−3, which is 2.62 × 103 kg m−3 to three 2.42 m 3 significant figures. Question 3 Since a = 4zr (second equation), we can eliminate a from the first equation by substituting 4zr in its place: x= 2z 2 2z 2 z = = a 4zr 2r As z2 = z × z, one of these z terms will cancel the z on the bottom of the fraction: x= 2z × z 4zr x= z 2r So: 16 Question 4 (a) sin 60° = 0.87 (b) cos 60° = 0.50 (c) cos 30° = 0.87 (d) tan 45° = 1.0 (e) sin 45° = 0.71 (f) tan 10° = 0.18 (g) cos 10° = 0.98 Question 5 (a) At 12:00 hours, the lava flow was about 95 litres per second. (To find this flow, you first locate 12:00 hours on the horizontal axis, and then draw a vertical line from there up to the point where it intersects the curve. From this point you then draw a horizontal line to meet the vertical axis, and you read the appropriate flow from the scale. In this case, the line intersects the scale one division below 100. Since ten divisions correspond to 50 litres per second, one division corresponds to 5 litres per second, and so the flow is (100 – 5) litres per second, or 95 litres per second.) (b) The maximum flow is about 155 litres per second, and this occurs at approximately 08:00 hours. (The maximum flow corresponds to the peak of the curve; by drawing horizontal and vertical lines from the peak to the axes you can read off the flow and the time, respectively.) (c) The flow was slow and steady at 20 litres per second until 04:00 hours. It then increased very rapidly for about 2 hours. After this it increased more slowly for approximately 1 hour before increasing more rapidly once again for a further hour. At 08:00 hours, a maximum flow was reached. After this time, the flow then started to decrease; there was a small peak at about 11:00 hours, and the flow gradually decreased until it became fairly steady after 20:00 hours. Question 6 Fg = Gm1 m 2 r2 Substituting values: Fg = (6.67 ×10−11 N m 2 kg −2 ) × (5.98 ×1024 kg) × (7.35 ×1022 kg) (3.845 ×108 m) 2 Fg = 1.98 × 1020 N (to 3 significant figures). This is also equal to the magnitude of the force on the Moon due to the Earth. Question 7 The atomic number of 6 for carbon refers to the number of protons in the atom, whereas the mass number (written as a superscript preceding the chemical symbol) is the number of protons plus neutrons. (a) 13C therefore contains 13 – 6 = 7 neutrons. 14C contains 14 – 6 = 8 neutrons. 17 (b) In a (neutral) atom, the number of electrons equals the number of protons. There are 6 electrons in an atom of 14C (and all other isotopes of carbon). Question 8 (a) The subscript ‘2’ in O2 indicates that there are two oxygen atoms. (b) The symbols for carbon and calcium have no subscripts, so there is only one atom of each. However, the symbol for oxygen has the subscript ‘3’ (O3) so there are three atoms of oxygen indicated in the chemical formula. The relative numbers of atoms indicated by the chemical formula are therefore 1 calcium, 1 carbon, 3 oxygen. Question 9 Relative masses of molecules and compounds are calculated by summing the relative atomic masses of the elements according to the weighting indicated by the chemical formula. (a) A molecule of water contains two hydrogen atoms and one oxygen atom, so its relative mass is (2 × 1.01) + 16.0 = 18.02 (or 18.0 to 3 significant figures). (b) A molecule of carbon dioxide contains one atom of carbon and two atoms of oxygen so its relative mass is 12.0 + (2 × 16.0) = 44.0. (c) The compound calcium carbonate contains calcium, carbon, oxygen in the ratio 1:1:3 so its relative mass is 40.1 + 12.0 + (3 × 16.0) = 100.1 (or 100 to 3 significant figures). Question 10 (a) The sodium ion carries only a single positive charge (+), implying that only one electron has been transferred, therefore sodium has a valency of 1. The calcium ion carries two positive charges (2+), implying that two electrons have been transferred, therefore calcium has a valency of 2. (b) The charge on the calcium ion is 2+, so two electrons have been transferred to chlorine atoms to form chloride ions. Calcium chloride contains twice as many chloride ions as calcium ions so each chloride ion must have received one of the electrons. Therefore the charge on each chloride ion that is formed is a single negative charge (C1–), and so the valency of chlorine must be 1. Question 11 (a) The superscript ‘2−’ on the symbol for oxygen indicates that two electrons have been received, so the valency of oxygen is 2. (b) (i) In FeO, the ratio of Fe to O is 1:1, so the iron must be in its ferrous form, Fe2+, in order to balance the 2− charge on the oxygen. (ii) In Fe2 O3 the ratio of iron atoms to oxygen atoms is 2:3. The total charge contributed by the oxygen ions is 3 × (2−) = 6−. In order to maintain electrical neutrality the iron must be in its ferric form, Fe3+, i.e. 2 × (3+) = 6+. (c) It is oxidised. The ratio of Fe to O in FeO is 1:1 and in Fe2O3 is 1:1.5. Also Fe2+ has converted into Fe3+, with the loss of an electron. 18 Question 12 The balanced equation is: 6CO2 (6 × C, 12 × O) + 6H2O → (12 × H, 6 × O) C6H12O6 (6 × C, 12 × H, 6 × O) + 6O2 (12 × O) Question 13 (a) The equation or respiration is the reverse of that shown in the answer to Question 12: C6H12O6 + 6O2 → 6CO2 + 6H2O (b) During respiration the carbohydrate is oxidised to carbon dioxide (by removal of hydrogen) and the oxygen is reduced to water (by addition of hydrogen). Note that this is effectively the reverse of photosynthesis, during which carbon dioxide is reduced to carbohydrate by the addition of hydrogen and water is oxidised to oxygen by the removal of hydrogen. Question 14 The items are features of the cells as follows: (a) A eukaryotic cell contains a nucleus (b) Prokaryotic cells and eukaryotic plant cells possess cell walls (c) Eukaryotic cells contain organelles (d) Eukaryotic cells have DNA contained within the nucleus (e) Eukaryotic plant cells contains chloroplasts (f) Prokaryotic cells have DNA free within the cytoplasm. Question 15 Mitosis is the type of cell division that takes place during growth. Mitosis produces diploid cells each with a set of chromosomes identical to that produced by the parent cell. Question 16 It must be a feature of meiosis. The progeny cells are genetically different to the parent cell, which indicates that genetic shuffling has taken place. (The progeny cells from mitosis are genetically identical to the parent cell (see Question 9).) Question 17 (a) (ii) and (iii). During oceanic/oceanic plate convergence either plate may be subducted as both have the same density. During oceanic/continental plate convergence, the oceanic plate is always the one to be subducted as it is denser than the continental plate. (b) (i). Where two oceanic plates diverge the hot, underlying asthenosphere rises up in the gap and an ocean ridge is formed. Igneous activity at the ridge helps to push the two plates apart by the process of sea-floor spreading. 19 (c) (i), (ii) and (iii). Melting the upper mantle beneath an ocean ridge, results in igneous activity, including submarine volcanic eruptions. Partial melting of a subducted plate occurs as it descends into the hotter mantle. The heat of the rising magma may partially melt the overlying plate as well. Magma reaching the surface gives rise to volcanic activity. (d) (iii) and (iv). In situation (iii) a volcanic mountain range forms along the continental margin, e.g. the South American Andes. In situation (iv) both plates are too buoyant (low density) for either to be subducted into the denser mantle and so convergence results in crumpling of the continental margins and the folding of the continental margin sediments to produce a mountain range, e.g. the Himalayas. (e) (ii). Volcanic activity above the plate that is being subducted leads to the formation of volcanic islands on the surface of the plate that is not subducted, e.g. the Philippine Islands or the Caribbean Islands. Question 18 (a) Igneous rocks: (ii) and (vi); metamorphic rocks: (v) and (iv); sedimentary rocks: (i) and (iii). (b) Check your answers with Table 1 (completed) overleaf. 20 Table 1 (Completed) Examples of common igneous, metamorphic and sedimentary rocks. Igneous rocks Metamorphic rocks Sedimentary rocks basalt quartzite chalk gabbro schist sandstone diorite gneiss breccia granite phyllite shale migmatite* migmatite* limestone rhyolite slate siltstone peridotite marble greywacke *Migmatite (meaning ‘mixed rock’) can be placed in either the igneous or metamorphic column, as it represents a transitional state, where a rock under high grades of metamorphism is just beginning to melt. Question 19 Carbon dioxide from the atmosphere dissolves in rain and river water. Hydrogen ions (H+) and bicarbonate ions (HCO3−) are produced. It is the H+ ions that make the water acidic. H2O + CO2 → H+ + HCO3− (Organic acids in soils increase the acidity of soil water.) Question 20 (a) The Carboniferous Period of the Paleozoic Era. (b) The Quaternary Period of the Cenozoic Era. Question 21 For the proportion of 14C to be reduced to one-quarter, the number of 14C atoms must have halved and then halved again. This means that two half-lives have elapsed, so the approximate age of the shelter is 2 × 5700 = 11 400 years. Question 22 (a) In order of decreasing size the reservoirs are: carbonate rocks, preserved organic carbon (including fossil fuels), the deep ocean, marine sediments, soil and detritus, surface ocean, atmospheric CO2, plant biomass, biomass (in ocean). (b) From Section 4.3, you know that initially CO2 is taken up by plants in the process of photosynthesis. Animals then consume the plants to take in carbon. (c) The most long-term storage reservoirs for carbon are the carbonate rocks and the preserved organic carbon. It takes hundreds of millions of years for the carbon in these reservoirs to be returned to the atmosphere. 21 (d) residence time = mass of element in reservoir rate at which element enters (and/or leaves) reservoir = 760 × 1012 kgC Mass of carbon in atmosphere Rate at which carbon is transferred from atmosphere to plant biomass = 120 × 1012 kgC y−1. Rate at which carbon is transferred from atmosphere to surface ocean = 95 × 1012 kgC y−1. Therefore, total rate at which carbon leaves the atmosphere = 215 × 1012 kgC y−1. Residence time of carbon in the atmosphere = 760 ×1012 kgC = 3.53 years 215 ×1012 kgC y −1 Question 23 It is an example of positive feedback because an increase in snowfall leads eventually to a further increase in snowfall. 22