Properties and Structure of Matter Textbook - Chapter 2 PDF

» MODULE ONE PROPERTIES AND STRUCTURE OF MATTER 2 Properties of matter 3 Atomic structure 4 Periodicity 5 Chemical bonding 6 Intermolecular forces and allotropy Alamy Stock Photo/Victor Josan 18 ...

» MODULE ONE PROPERTIES AND STRUCTURE OF MATTER 2 Properties of matter 3 Atomic structure 4 Periodicity 5 Chemical bonding 6 Intermolecular forces and allotropy Alamy Stock Photo/Victor Josan 18  2 Properties of matter OUTCOMES INQUIRY Students: QUESTION explore homogeneous mixtures and heterogeneous mixtures through practical investigations: – using separation techniques based on physical properties (ACSCH026) How do the properties – calculating percentage composition by weight of component elements and/or compounds (ACSCH007) of substances help us to classify and separate investigate the nomenclature of inorganic substances using International Union of Pure and Applied Chemistry (IUPAC) naming conventions* them? classify the elements based on their properties and position in the periodic table through their: – physical properties – chemical properties. ICT *This item is treated in sections 5.4 and 5.9 on pages 92 and 102. Chemistry Stage 6 Syllabus © NSW Educational Standards Authority for and on behalf of the Crown in right of the State of New South Wales, 2017 Alamy Stock Photo/BSIP SA 9780170408929 19  We live in a world of materials – substances or matter or ‘stuff ’. We breathe air, we drink and bathe in water, we dig substances out of the earth, we grow crops and animals. These useful materials that surround us and the things we make from them are very different from one another. Some of them are hard and coloured (rocks, coal, iron ore, rubies); others are brown and green and soft (plants). Some are liquids (water, crude oil); others are invisible (air). They have very different characteristics or properties. Studying the properties of substances is one part of chemistry. Another is studying the structure of substances – working out what they are made of and how the various bits are joined and held together. A third part is studying the way some substances undergo permanent change – called chemical reactions – when they are mixed. Most substances in the world are present as mixtures, such as sea water, air, crude oil, soils, mineral ores and foods. We can separate mixtures into pure substances (Figure 2.1). Then we can study the properties of those pure substances and work out their structures. We can then use their structures to explain their very different properties. Mixtures are generally separated into pure substances by using the different physical properties of the components in the mixtures. Pure substances fall into two major categories: elements and compounds (section 1.2). There are millions of compounds in the world but only about 90 naturally occurring elements. We shall look at the properties of elements using the periodic table, a chart in which elements are arranged in ways that highlight similarities and trends in properties. Roland Smith FIGURE 2.1 Salt can be separated from sea water using evaporation. In Vietnam this is done manually, as shown, while in Australia machines are generally used. 2.1 Physical properties In section 1.5 we discussed the differences between physical change and chemical change. This gives rise to the terms ‘physical properties’ and ‘chemical properties’. The physical properties of a substance are its characteristics that we can observe or measure without changing it into a different substance. Melting point and boiling point are two physical properties. Others include colour, size of particles, density, solubility in particular solvents, hardness, electrical and thermal conductivity, viscosity and surface tension. The chemical properties of a substance are those associated with the chemical changes (or chemical reactions) it undergoes when it is mixed with other substances or heated or exposed to light. They are properties that involve the change of the substance into a different substance. Typical chemical properties are reaction with oxygen and reaction with acids or bases. How easily a substance decomposes into another substance when heated is also a chemical property. Differences in physical properties are often used to separate mixtures. In addition, physical properties are often used to identify the pure substances that are obtained. Some physical properties are useful for separating mixtures, while others are more useful for identifying pure substances. 20 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  Homogeneous and heterogeneous As explained in section 1.1, homogeneous substances have uniform composition throughout. Examples are pure water, sugar, aluminium foil, petrol and apple juice. Heterogeneous substances have non- WS uniform composition, meaning that we can recognise small pieces of the material that are different from Worksheets Homogeneous Homework and other pieces or parts of the mixture that have different properties from other parts. Examples are fruit heterogeneous mixtures cake, concrete, wood, beef and orange juice. Colour Some substances have quite distinct colours. Examples are deep brown for liquid bromine, reddish brown for solid copper, pale blue for copper sulfate solution, yellow for solid sulfur, red for solid iron(III) oxide and deep purple for potassium permanganate crystals. However, we should be cautious about identifying substances solely on their colour. This is because two substances often have very similar colours and so can be mistaken for each other. For example, iodine and potassium permanganate are both purple, copper chloride and nickel sulfate are both green. Nevertheless, colour can be a good first clue. Colour is not very useful for separating mixtures. Magnetism Shutterstock.com/dvande Some substances are strongly magnetic, such as iron and steel and some alloys such as alnico, a solid of iron, aluminium, nickel and cobalt. Magnetic substances can easily be separated from non-magnetic ones. For example, we can easily separate a mixture of iron filings and sulfur powder by dipping a magnet into the mixture. Magnetism is used at waste recycling depots to separate cans and other steel products from other materials. Scrap metal merchants use it also (Figure 2.2). Particle size Particle size is of little use in characterising substances because it is quite easy to change by simply grinding a coarse solid into a finer one. Particle size can be used to separate solids of different sizes in mixtures by using a sieve (in a process called sieving). A sieve is a device that allows particles smaller than a particular size to pass through while holding back larger particles. A sieve may be a shaped sheet of metal with uniform holes FIGURE 2.2 Magnetism is widely used to sort ferrous (magnetic) metals from other metals. punched through it (like a kitchen colander) or a woven mesh or gauze (of metal or plastic) held in a metal or plastic frame (as in a common kitchen sieve). 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 21  In home kitchens we often use a sieve to separate lumps from Science Photo Library/Monty Rakusen/CULTURA a powdery substance such as flour. Similarly, in the laboratory and in industry sieves are used to separate small particles from large ones (Figure 2.3). At quarries fine sand needed for making concrete and mortar is separated from the coarser material (sold as gravel) by sieving. Melting and boiling points The melting point of a solid is the lowest temperature at which the solid changes to a liquid. Solids can be melted at higher FIGURE 2.3 Industrial sieves are used to separate materials temperatures ( for example by dropping an ice cube into a glass according to particle size. of water at room temperature), but it is the lowest temperature at which melting occurs that is called the melting point ( for ice it is 0°C). A pure substance has a definite melting point at standard pressure. If a solid is a pure substance, its melting point is quite ‘sharp’. All of a sample of the solid melts at the one temperature. If the solid is a mixture (an impure substance), then it melts over a range of temperatures. An impure substance starts to melt at a lower temperature than does the pure substance. For example, solder, which is typically made of 60% tin and 40% lead, melts at 190°C, whereas the melting points of tin and lead are 232°C and 327°C respectively. Hence for solids, melting point can be used as a test of purity. If it is sharp and if it does not increase Standard pressure is 100.0 kilopascals after submitting the solid to a further purification process, then the substance is pure. The value of the (kPa). It is quite melting point can be used as one piece of evidence for identifying the substance. close to standard atmospheric The reverse process of converting a liquid to a solid is called freezing. The freezing point of a liquid is pressure, which is the highest temperature at which the liquid can be converted to a solid. The freezing point of the liquid 101.3 kPa. is the same temperature as the melting point of the solid. The boiling point of a liquid is the lowest temperature at which the liquid boils (changes from a liquid to a gas or vapour, with bubbling) at the stated pressure (usually standard atmospheric pressure). Pure substances boil at fixed temperatures (at standard pressure), so that the value of the boiling point can be used as one step towards identifying a liquid substance. Boiling points do vary with atmospheric pressure, so the term normal boiling point is used to refer to the boiling point at a pressure of 100.0 kPa. However, the ‘normal’ is often omitted when the meaning is clear. Mixtures boil over a range of temperatures. The presence of an impurity in a liquid may raise the boiling point (if the impurity is non-volatile, such as salt in sea water) or lower it (if the impurity is more volatile than the main substance, such as alcohol in water). Volatile means ‘easily converted to a vapour’ or that evaporation occurs quite rapidly; non-volatile means ‘not easily converted to a vapour’ or that evaporation is quite slow. Ordinary alcohol (ethanol) and ethyl acetate (nail polish remover) are volatile; cooking oil and ethylene glycol (motor car antifreeze) are non-volatile. Solids are non-volatile. Substances with low boiling points, less than about 100°C, are volatile; those with high boiling points, much greater than 100°C, are non-volatile. Boiling point can be used as a test of purity. If a liquid has a sharp boiling point that does not change after further attempts to purify it, then the liquid is a pure substance. The value of the boiling point can be used to help identify the substance. The term vapour is often used as a synonym for the same physical state as gas although it does have a slightly different meaning. A vapour is a gas that is easily liquefied or condensed; it is a gas that is close to its boiling point. We tend to talk about water vapour and oxygen gas in the atmosphere, because water is easily condensed out of air, while oxygen is much more difficult to liquefy. Melting and boiling points of some common substances are given in Table 2.1. 22 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  TABLE 2.1 Melting points, boiling points (at 100 kPa) and densities (at 25ºC) of some common substances MELTING BOILING DENSITY MELTING BOILING DENSITY SUBSTANCE POINT (°C) POINT (°C) (g mL−1) SUBSTANCE POINT (°C) POINT (°C) (g mL−1) Acetic acid 16.7 118 1.04 Hydrogen −259 −253 Aluminium 660 2450 2.7 Lead 327 1740 11.4 Argon −189 −186 Mercury −39 357 13.6 Carbon (graphite) 3730 4830 2.3 Nitrogen −210 −196 Chloroform −64 62 1.48 Oxygen −219 −183 Copper 1083 2600 9.0 Phosphorus 44 280 1.8 Ethanol −114 78 0.79 Sodium 98 892 0.97 Ethyl acetate −84 77 0.90 Sulfur 114 444 2.0 Ethylene glycol −16 198 1.11 Water 0.0 100.0 1.00 Hexane −95 69 0.66 Zinc 420 610 7.1 Density Density is a useful property for identifying a substance. Density is defined as mass per unit volume. mass density = ... (2.1) volume Common units for density are grams per millilitre (g/mL or g mL−1) or kilograms per cubic metre (kg m−3). Densities of some common substances are shown in Table 2.1. Density of a liquid can be determined by measuring the mass of a known volume of the liquid. For a geometrically shaped piece of a solid, density can be determined by measuring the dimensions of the solid, calculating its volume, then measuring its mass. For an irregularly shaped solid, its volume can be measured by displacement of water. Because the particles are generally packed more closely together in solids than in liquids (see Figure 1.7), we find that solids have greater densities than the same substances as liquids. (The exception is water and ice: this is why icebergs float.) The physical properties of a substance are its characteristics that we can observe or measure KEY CONCEPTS without changing it into a different substance. The chemical properties of a substance are those associated with the chemical changes (or chemical reactions) the substance undergoes when it is mixed with other substances or heated or exposed to light. Particles of different sizes can be separated by using a sieve. The melting point of a solid is the lowest temperature at which the solid changes to a liquid. The freezing point of a liquid is the highest temperature at which the liquid can be converted to a solid. The boiling point of a liquid is the lowest temperature at which the liquid boils (converts to a gas with visible bubbles forming) at the stated pressure (usually standard pressure, 100.0 kPa). Volatile means ‘easily converted to a vapour’ or that evaporation occurs quite rapidly; non-volatile means ‘not easily converted to a vapour’ or that evaporation is quite slow. A vapour is a gas that is easily liquefied or condensed; it is a gas that is close to its boiling point. Density is defined as mass per unit volume. mass density = volume 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 23  CHECK YOUR UNDERSTANDING 1 a Explain the difference between a physical property and a chemical property. b Give two examples of each type of property. 2.1 2 Explain the difference between: a melting and freezing. b boiling and evaporating. c boiling point and normal boiling point. d gas and vapour. 3 Define density. What are the common units for it? 4 Explain how would you decide whether a a solid, b a liquid was a pure substance. 5 Melting and boiling points for some substances are shown in the table. MELTING BOILING MELTING BOILING SUBSTANCE POINT (ºC) POINT (ºC) SUBSTANCE POINT (ºC) POINT (ºC) Bromine −7 58 Magnesium 650 1110 Carbon disulfide −111 46 Neon −249 −246 Carbon 91 190 Phosphorus −91 74 tetrabromide trichloride Gallium 30 2400 Sulfur 114 444 Hydrogen −2 158 Sulfur dioxide −73 −10 peroxide Lead bromide 373 914 Sulfuric acid 10 330 a Identify which of these substances would be: i   solids ii liquids at room temperature (20°C). b Which of these substances would not be solids at −60°C? 6 Calculate the density of: a a block of aluminium measuring 0.50 cm × 1.20 cm × 0.80 cm, which has a mass of 1.30 g. b 5.0 mL chloroform, which has a mass of 7.40 g. c a cylinder of zinc, 3.0 mm in diameter and 3.0 cm long, which has a mass of 1.506 g. 7 Using density values from Table 2.1, calculate the mass of: a 5.0 mL ethanol. b a cube of copper with a side length of 0.80 cm. c 25.0 mL hexane. 8 Using density values from Table 2.1, determine the volumes of the following substances that would have the following masses. a 10 g chloroform b 25 g ethylene glycol 24 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  Using physical properties 2.2 to separate mixtures The physical properties described earlier can be used to separate mixtures. Separation using filtration We saw in the previous section that solids of different sizes could be separated with a sieve. Another separation method based on particle size is filtration. Mixtures of solids and liquids are commonly separated by filtration as shown in Figure 2.4. The liquid or solution passes through the filter paper while the suspended solid of much larger particle size remains on the top of the filter paper. The liquid or solution that passes Beaker through the filter paper is called a filtrate. Sand can be separated from sea water in this way. Filter paper Separation based on solubility Filter funnel Two solids can be separated if one is soluble in a particular solvent and the Conical flask other is not. A mixture of salt and sand can be separated by adding water to the mixture and stirring to dissolve the salt, then filtering off the insoluble sand. Filtrate Separations based on boiling points Methods for separating mixtures based on boiling points depend upon whether FIGURE 2.4 Filtration in the laboratory we want to collect just one component of the mixture or more than one. Separating a dissolved solid from a solution When a solid is dissolved in a liquid, the solid and liquid can be separated by vaporising the liquid. This is because the liquid has a much lower boiling point than the solid. This can be done either by boiling the solution (as seen by bubbles of vapour forming) or by allowing it to evaporate (see section 1.3). Evaporation is a slower process than boiling. Chemists frequently use the expression evaporation to dryness, which refers to heating a solution in an evaporating basin to remove all the solvent. It is illustrated in Figure 2.5. Evaporating to dryness is a common way of obtaining a solid from a solution, for example obtaining salt from sea water. Boiling or evaporating the liquid is effective if it is the solid that we want (e.g. getting salt from sea water). However, if it is the liquid that we want (e.g. getting fresh water from sea water), or if we want both the solid and the liquid, then we have to use distillation. FIGURE 2.5 Wire gauze Evaporating a solution Evaporating basin to dryness Tripod stand Bunsen burner 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 25  Distillation Distillation is the process in which a solution or mixture of Thermometer liquids is boiled, with the vapour formed being condensed back Still head Water out to a liquid in a different part of the apparatus and so separated from the mixture. Spherical flask The arrangement shown in Figure 2.6 is commonly used Condenser for distillation. The mixture or impure liquid, for example sea water, is placed in the flask and heated to boiling. The liquid changes to vapour, rises up the neck of the flask and diffuses down the side arm and into the water-cooled Heating mantle condenser. The vapour is cooled and condensed back to a liquid, which is collected in the beaker. The liquid collected Water in during distillation is called a distillate. The non-volatile Collection beaker solid remains in the flask. Distillate If the impurities are non-volatile (as is the case with sea water), pure liquid is collected (pure water from sea FIGURE 2.6 Simple distillation apparatus water). Similarly, if some coloured oil-based paint is distilled, colourless turpentine (turps) would be obtained since the oils The meaning and pigments of the paint are non-volatile compared with the turpentine. of ‘volatile’ was A mixture of two or more liquids can be separated by a simple distillation if the boiling points of explained on page 22. the liquids are sufficiently different ( for example by 40 or 50°C). For example, distillation of a mixture of water and ethylene glycol (motor car antifreeze) produces pure water as the distillate and leaves the ethylene glycol behind in the distillation flask. This happens because the boiling points are so different: 100°C for water and 198°C for ethylene glycol. If we distil a mixture of two liquids (such as ordinary alcohol and water) that do not have greatly different boiling points (78°C and 100°C), then the distillate is not a pure substance. Instead it is a mixture. Generally the distillate is richer in the lower boiling Thermometer point component. A mixture of 15% ethanol (ordinary alcohol) Still head with 85% water, when distilled, produces a distillate of about Water out 40% ethanol and 60% water. This is how brandy is made from wine, or whisky from fermented grain solution. Therefore, if a mixture of a volatile liquid with a much less Condenser volatile liquid is distilled, the distillate is pure volatile liquid. If a mixture of two liquids of comparable volatility (similar boiling points) is distilled, the distillate is generally richer in the more volatile liquid (the one with the lower boiling point). Fractionating column Despite what has just been said, it is possible to use Water in distillation to separate liquids with similar boiling points using Collection a process called fractional distillation. Spherical flask beaker Distillate Fractional distillation To separate liquids by distillation when their boiling points are fairly close together we have to use many distillation steps. This can be done in specially designed equipment and the process Heating mantle is called fractional distillation. During this process a mixture of liquids is separated by being put through many successive distillations (vaporisations and condensations) in one piece of FIGURE 2.7 Fractionating column in the laboratory (commonly equipment. A typical laboratory fractionating column is shown about 40–50 cm high) in Figure 2.7. 26 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  These repeated vaporisations and condensations up the column effectively give many separate distillations (typically 10 to 100). This means that eventually a pure sample of the more volatile substance in the original mixture emerges from the top of the column. If a mixture of the two petrol- like liquids benzene and cyclohexene (boiling points 80°C and 83°C respectively) is fractionally distilled, the final distillate is pure benzene. If ethanol–water mixtures such as wines are fractionally distilled the final distillate is 98% ethanol (with 2% water) compared with 40% ethanol from a single distillation. Fractional distillation is widely used in industry. Examples of this are: ◗◗ separation of crude oil into various commercial products (petrol, diesel, heating oil) ◗◗ separation of ethanol ( for use as a fuel additive) from fermented solutions of sugar molasses or grain mashes ◗◗ production of liquid nitrogen and argon gas from liquefied air. Separations based on density Mixtures of solids and liquids can be separated by filtration (see page 25). Sometimes they can be separated on the basis of density, because in a mixture the denser particles or liquid will settle to the bottom of the container. This is particularly true if a solid is present as coarse particles (such as coarse sand in water). Sedimentation is the process in which solids settle to the bottom of a container. Decanting or decantation iStock.com/ejwhite is the process of carefully pouring off the liquid or suspension and leaving a solid undisturbed at the bottom of the container. Pouring tea off tea leaves is decantation. Panning for gold (Figure 2.8) is an example of sedimentation and decantation. River sand with fine particles of gold dispersed through it is swirled with water in a shallow basin or pan. The dense gold particles settle at the bottom of the pan while the less dense sand particles remain suspended and are decanted off. The process is repeated until just the dense gold particles remain in the pan. FIGURE 2.8 Panning for gold – separation based on density Separating immiscible liquids Two liquids are said to be immiscible if, when they are mixed, they do not form a homogeneous liquid, but instead stay as drops of one liquid dispersed through the WS other liquid. If left standing for some time, immiscible liquids separate into two distinct layers, one on Worksheets Properties Homework of top of the other. This is because the liquids have different densities – the more dense one settles to the matter key bottom of the container. Examples are water and kerosene, water and cooking oil. concepts Liquids that mix to form a homogeneous liquid are said to be miscible. Alternatively, we say that one liquid dissolves in the other. WS Immiscible liquids are generally separated using a separating funnel. This pear-shaped piece of Worksheets Physical Homework apparatus, shown in Figure 2.9, tapers to a narrow tube just above the stopcock. This shape allows us to separation run out the bottom liquid without getting it contaminated with any of the top liquid. A mixture of petrol techniques and water can be separated in this way. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 27  FIGURE 2.9 Two immiscible liquids a in a b a separating funnel, Separating funnel b just before all of the ‘heavier’ (more Less dense liquid dense) liquid has been run out More dense liquid Stopcock INVESTIGATION 2.1 Separating salt, oil and water You will be given a sample containing oil and sea water. Your task is to separate the mixture into its components: salt, oil and water. AIM To recover each of the components of a mixture of salt, oil and water. MATERIALS Identify the pieces of equipment that you will need to conduct this investigation. RISK ASSESSMENT Construct a table similar to the one shown. Identify specific risks involved in the investigation and ways that you will manage the risks to avoid injuries or damage to equipment. Ask your teacher to check your risk assessment before you proceed. ! WHAT ARE THE RISKS IN DOING THIS INVESTIGATION? HOW CAN YOU MANAGE THESE RISKS TO STAY SAFE? RISK ASSESSMENT Oil should not be poured down the sink. Glassware may get hot during this investigation. METHOD 1 Review the methods of separation and determine the sequence of methods required to conduct this investigation. 2 Set up the equipment for this investigation. 3 Draw a diagram or take a photo of your equipment. 4 Annotate your photo or diagram to indicate any safety precautions needed to ensure the investigation is safe. 5 Show your teacher your annotated photo or diagram and get their approval before you commence your investigation. 28 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  RESULTS INVESTIGATION 9.1 of your method in an appropriate way so that they are easy to analyse. Present your observations for each part ANALYSIS OF RESULTS 1 Relate the method(s) used to the properties of the components of the mixture. 2 Explain why it was not possible to recover all of each component of the mixture. 3 Discuss whether the investigation could have been carried out more effectively using another method. CONCLUSION Summarise your findings about the effectiveness of this investigation in separating the mixture into its separate components. Summary of methods of separation Table 2.2 shows the methods discussed here for separating mixtures and includes the physical properties these methods depend upon. Because purification of a substance is removal of unwanted substances (impurities), these separation methods are also methods of purifying substances. TABLE 2.2 Separation methods and the physical properties they depend upon SEPARATION METHOD PHYSICAL PROPERTY USED IN THE SEPARATION Sieving Particle size Filtration One substance a solid, the other a liquid or solution; particle size Vaporisation (evaporation or boiling) Liquid has a much lower boiling point than the solid Distillation Big difference in boiling points Fractional distillation Significant but small difference in boiling points Sedimentation and decantation Density Using a separating funnel Components are immiscible liquids; different densities Adding a solvent, then filtering One substance is soluble in the chosen solvent, while the others are insoluble Filtration is a method of separating an undissolved solid from a liquid or solution. The solution KEY CONCEPTS or liquid that passes through a filter paper is called the filtrate. Evaporate to dryness means to heat a solution in an evaporating basin to drive off all the solvent. Distillation is the process in which a solution or mixture of liquids is boiled, with the vapour formed being condensed back to a liquid in a different part of the apparatus and so separated from the mixture. The liquid collected from a distillation is called a distillate. Fractional distillation is a process in which a mixture of liquids is separated by being put through many successive distillations (vaporisations and condensations) in one piece of equipment. Sedimentation is the process in which solids settle to the bottom of a container. Decanting or decantation is the process of carefully pouring off the liquid and leaving the solid undisturbed at the bottom of the container. Two liquids are immiscible if, when they are mixed, they do not form a homogeneous liquid, Liquids that mix to form a homogeneous liquid are miscible. A separating funnel can be used to separate immiscible liquids. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 29  CHECK YOUR UNDERSTANDING 1 Distinguish between a: a solution and suspension. 2.2 b solute and solvent. c miscible liquid and immiscible liquid. 2 Explain with an example the meaning of ‘sedimentation’ and ‘decantation’. 3 Outline how the following processes can be used to separate mixtures: sieving, filtration, evaporation to dryness, distillation, fractional distillation, use of a separating funnel. Draw diagrams of the experimental set-up. 4 A student inadvertently mixed the laboratory supplies of aluminium oxide and lead pellets. Although small, the lead pellets were larger than the fine crystals of aluminium oxide. Aluminium oxide is much less dense than lead. Both are insoluble in water. Suggest a way of separating these two substances. 5 A school’s supply of crystalline magnesium sulfate became contaminated with some barium sulfate. Draw a flow chart for the method you would use to separate these two substances, ending up with pure dry crystals of each substance. Magnesium sulfate is soluble in water; barium sulfate is not. 6 Suppose you were given a solution of sucrose (sugar) in water. How would you separate this mixture if your aim was to obtain: a pure sucrose? b pure water? 7 Use Table 2.1 on page 23 to answer the following questions. a If a mixture of alcohol (ethanol) and glycerol (boiling point 290°C) was distilled, what would be the composition of the distillate? Explain why. b Would distillation of a solution of acetic acid in water, for example vinegar, produce a distillate more concentrated or less concentrated (in acetic acid) than the starting solution? Give your reason. c If a mixture of chloroform and ethanol was distilled, the distillate would be richer (than the starting material) in which substance? d How would you obtain a pure sample of one liquid from the mixture in part c? e If you distilled a mixture of hexane and heptane (boiling point 98°C), what would the distillate be? 2.3 Gravimetric analysis Sometimes it is important to know the quantitative composition of a mixture; that is, to know what mass of each substance is present in a given quantity of the mixture (Figure 2.10), or to know the percentage composition of the mixture. Determining the quantities (masses) of substances present in a sample is called gravimetric analysis – analysis by mass. Reasons for wanting to know the percentage composition of a mixture include: ◗◗ to decide whether a newly discovered mineral deposit contains a sufficiently high percentage of the required compound to make its extraction from that deposit economically viable Gravimetric analysis ◗◗ to decide whether a particular commercial mixture has the same percentage composition as a similar can also be used to determine the mixture being marketed by a rival company. percentage of each element in a To carry out a complete gravimetric analysis on a sample we need to determine the mass of each compound – see component present in the mixture. This often involves using separation techniques discussed in this section 7.9. chapter. 30 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  Roland Smith FIGURE 2.10 Many commercial products WS such as these show Worksheets their composition, Percentage Homework either in percentage composition or mass forms, on their labels. WS Worksheets Gravimetric Homework analysis WORKED EXAMPLE 2.1 A team of geologists discovered a new mineral in a remote desert location; it was a mixture of barium sulfate and magnesium sulfate. Its composition was determined as follows. They first ground up a 3.61 g sample with water; magnesium sulfate dissolves, barium sulfate does not. The barium sulfate was filtered off, dried and its mass determined to be 1.52 g. They evaporated the filtrate to dryness to recover the magnesium sulfate, and determined its mass to be 2.08 g. Calculate the percentage composition of the sample. ANSWER LOGIC Total mass of the two sulfates = 1.52 + 2.08 Check that the sample = 3.60 g contains only barium and magnesium sulfates. This is within experimental error of 3.61 g (less than 0.3% discrepancy), so the sample contains no other substance. 1.52 Percentage of the sample that Percentage of sample that is barium sulfate = × 100 3.61 is barium sulfate is the mass of barium sulfate divided by the = 42.1% total mass, all multiplied by 100. Percentage of sample that is magnesium sulfate = 100 − 42.1 Percentage of magnesium = 57.9% sulfate will be 100 minus the percentage of barium sulfate. The sample consists of 42.1% barium sulfate and 57.9% magnesium For both percentages, check sulfate, with accuracies of about ±0.3%. that the number of significant figures matches that in the original data. If you are unsure of what ‘experimental error’ and ‘significant figures’ mean, see Appendix 1, ‘Experimental error and significant figures’, page 409 or for greater detail see chapter 17, ‘Working scientifically and depth studies, page 387. TRY THIS YOURSELF A certain mineral was a mixture of silver sulfide and zinc sulfide. Upon analysis it was found that 5.32 g of the mineral contained 1.76 g silver sulfide and 3.54 g zinc sulfide. Calculate the percentage composition of this mineral. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 31  INVESTIGATION 2.2 Determining the percentage composition of a mixture You will be given a sample containing salt, sand and iron filings. Your task is to determine the percentage composition of its components (salt, sand and iron filings) using gravimetric analysis. AIM To determine the percentage of each of the components present in a mixture of salt, sand and iron filings. MATERIALS Identify the pieces of equipment that you will need to conduct this investigation. RISK ASSESSMENT Construct a table similar to the one shown. Identify specific risks involved in the investigation and ways that you will manage the risks to avoid injuries or damage to equipment. Ask your teacher to check your risk assessment before you proceed. ! RISK WHAT ARE THE RISKS IN DOING THIS INVESTIGATION? HOW CAN YOU MANAGE THESE RISKS TO STAY SAFE? ASSESSMENT Glassware may get hot during this investigation. METHOD 1 Review the methods of separation and determine the sequence of methods required to conduct this investigation. 2 Set up the equipment for this investigation. 3 Draw a diagram or take a photo of your equipment. 4 Annotate your photo or diagram to indicate any safety precautions needed to ensure the investigation is safe. 5 Show your teacher your annotated photo or diagram and get their approval before you commence your investigation. 6 Write your method for this investigation. RESULTS 1 Identify the measurements that you will need to take during this investigation. 2 Present your results in an appropriate way so that they are easy to analyse. ANALYSIS OF RESULTS 1 Calculate the percentage of each component of the mixture. 2 Relate the method(s) used to the properties of the components of the mixture. 3 Discuss limitations associated with your methodologies and how these may impact on the accuracy of your results. CONCLUSION 1 Compare your values with the values for the mixture provided by your teacher. 2 Account for any differences between your values and the values provided by your teacher. 32 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  KEY Determining the quantities (masses) of substances present in a sample is called gravimetric CONCEPTS analysis – analysis by mass. The percentage composition of a mixture gives the percentage of each pure substance (element or compound) present in the mixture. CHECK YOUR 1 Define the term ‘gravimetric analysis’. UNDERSTANDING 2 The suitability of water for irrigating crops or for animals to drink depends upon the amount of dissolved solids present. To assess the suitability of water from a particular bore, a farmer took 500 g of the water, 2.3 evaporated it to dryness, then determined the mass of solids remaining to be 3.63 g. Calculate the percentage of dissolved solids in this water. 3 Upon analysis a 3.67 g sample of a certain NPK (nitrogen, phosphorus, potassium) fertiliser was found to contain 1.79 g urea and 0.81 g ammonium phosphate, with the rest being potassium chloride. Calculate the percentage composition of this fertiliser. 4 The product of a certain industrial process was a mixture of three alcohols: ethanol (ordinary alcohol), ethylene glycol (motor car antifreeze) and glycerol (used in sweets and cosmetics). The boiling points are 78°C, 198°C and 290°C respectively. 18.33 g of this mixture was carefully distilled. 3.79 g of distillate was collected while the thermometer in the apparatus registered 78°C. The temperature increased to 198°C and 8.64 g of a second distillate was collected. When the temperature was greater than 200°C the distillation was stopped. Assuming that what was left in the distillation flask was glycerol, calculate the percentage composition of the original mixture. 2.4 A survey of the elements Having seen how to separate pure substances from mixtures, let us now survey the simplest of pure substances, the elements. Most elements are solids at room temperature. Investigation 2.3 will allow you to identify those that are liquids or gases. Occurrence of the elements Most of the elements occur in nature as compounds. This is because over the billions of years since planet Earth was formed the elements reacted chemically with one another. The insoluble compounds stayed as part of the solid Earth; the soluble ones slowly dissolved in water and ended up in lakes and oceans. Hence the elements that form predominantly soluble compounds such as sodium, potassium, calcium and magnesium are commonly found in rivers, lakes and oceans. The minerals or ores from which commonly used elements such as aluminium, iron and copper are extracted are insoluble compounds. The most common elements present on Earth as free elements are oxygen and nitrogen, which make up 21% and 78% by mass of the dry atmosphere. Other free elements are argon (about 1% of the atmosphere), and the other so-called noble gases, helium, neon, krypton, xenon and radon, which are present in the atmosphere in trace amounts. Other elements that occur as uncombined elements are the precious metals gold and platinum. Sulfur, copper and silver are sometimes found as uncombined elements, although they are more commonly found as compounds. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 33  INVESTIGATION 2.3 Classifying elements as solids, liquids or gases AIM WS To use secondary data sources to classify all of the elements as solids, liquids or gases. Worksheets MATERIALS Blank periodic Homework table Blank periodic table worksheet Information and Secondary data source containing several properties of the elements in a table communication technology METHOD capability 1 Use the table of data to identify relevant information to determine which elements are: a solids Weblink Chemical b liquids elements c gases at 25°C. RESULTS 1 Use the blank periodic table to colour the: a solids green. b liquids blue. c gases red. ANALYSIS OF RESULTS 1 What data were required before you could determine whether an element was a solid, liquid or gas? 2 Outline any trends in the data. 3 Identify the data that were most useful to determine which elements were solids, liquids and gases. 4 Describe the conditions necessary for an element to be classified as a solid at 25ºC. 5 Describe the conditions necessary for an element to be classified as a liquid at 25ºC. 6 Describe the conditions necessary for an element to be classified as a gas at 25ºC. 34 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  INVESTIGATION 2.4 Classifying elements as metals or non-metals Chemists have found it convenient to classify the elements into metals and non-metals. Metals are elements that are lustrous (have a shiny appearance), are good conductors of heat and electricity, and are both malleable (able to be rolled into sheets) and ductile (able to be drawn into wires). We shall observe a selection of elements to see whether they are lustrous, then test them for hardness, malleability and electrical conductivity, to classify them as metals or non-metals. AIM To classify some commonly available elements as metals or non-metals. MATERIALS Commercial multimeter with leads and probes attached Hammer and brick Spatula Tissues/paper towel Steel wool and emery paper Mortar and pestle Sharp pointer or knife Strips or rods or lumps of solid elements such as aluminium, carbon (lumps of charcoal), copper, iodine (0.2 g), iron, lead, magnesium, sulfur and zinc in labelled bottles RISK ASSESSMENT Construct a table similar to the one shown. Identify any additional risks involved in the investigation and the ways that you will manage all the risks to avoid injuries or damage to equipment. Ask your teacher to check your risk assessment before you proceed. ! WHAT ARE THE RISKS IN DOING THIS INVESTIGATION? HOW CAN YOU MANAGE THESE RISKS TO STAY SAFE? RISK ASSESSMENT Iodine is very toxic. It is a lung irritant. Only use a small amount of iodine (0.2 g) and always use in a fume cupboard. Injury from sharp pointer or knife Contamination from handling samples METHOD 1 Draw up a table in your logbook with these column headings: appearance, hardness, malleability, conductivity, metal or non-metal. Have a separate row for each element you are examining. 2 Appearance: For elements provided as crystals or powders or unevenly shaped lumps record their appearance in the table. State their colour and use terms such as shiny, lustrous or dull. For the other elements provided as strips, sheets or rods clean the sample with emery paper or steel wool then record their appearances. 3 Hardness: Decide how easy it is to scratch the surface of each strip or rod with a sharp knife. The more difficult it is to scratch the surface the harder is the element. Record the hardness of the element as soft, moderately hard or very hard. For the other elements put a spatula full in a mortar and grind it gently with a pestle. If gentle grinding does not break up the crystals grind more vigorously. Based on how easy it is to break up the crystals, enter ‘soft’, ‘moderately hard’ or ‘very hard’ in the hardness column. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 35  4 Malleability: This is the ability of the material to be rolled out into sheets. Place each of the strips or rods on a brick on the floor in turn and hit it with a hammer, gently at first then with increasing force. Observe whether the element flattens or spreads out and how much force was required to do so. Enter very malleable, slightly malleable or not malleable in your table. 5 Conductivity: Set the multimeter to its resistance mode and choose a middle range on the dial. For the strips and rods and lump of charcoal press the pointed probes attached to the meter on to them about 1 cm apart and note the reading. Adjust the range to get a meaningful reading. Calculate the reciprocal of this reading (conductance = 1/resistance) and record the value in your table. Resistance is measured in ohms and so conductance in ohm−1. For the powders, immerse the probes 1 cm apart to a depth of about 1 cm in the sample bottle and note the meter reading. Wipe the probes thoroughly after each measurement. RESULTS 1 Enter all results in your table. 2 Examine your results and decide which elements are metals and which are non-metals. Record your conclusions in the appropriate column. ANALYSIS OF RESULTS 1 Explain how you reached your decisions. 2 Assess the usefulness of the various properties you observed (measured) for deciding whether an element is a metal or non-metal. 3 Outline any shortcomings of the equipment and methods you used and discuss their effects upon the reliability of your conclusions. 4 Suggest other properties that you could have used to help you classify these elements. 5 Check some data sources for each of the elements studied to identify whether they are metals or non-metals. CONCLUSION 1 Compare your results with the information from your literature search and discuss any discrepancies. 2 Discuss the validity of your results by referring to the limitations in available data, i.e. how many properties of elements were tested to reach these conclusions. 3 Outline any trends regarding the location of metals and non-metals in the periodic table. Metals and non-metals We saw in Investigation 2.4 that metals are elements that: ◗◗ are solids at room temperature ◗◗ have a shiny or lustrous appearance ◗◗ are good conductors of heat and electricity ◗◗ are malleable (able to be rolled into sheets) and ductile (able to be drawn into wires). Most other elements are called non-metals. The common properties of non-metals are that they: ◗◗ can be solids or gases at room temperature (except one liquid, bromine) ◗◗ generally have a dull non-lustrous appearance (except diamond, a form of carbon) ◗◗ are generally poor conductors of heat and electricity (although graphite, another form of carbon, is a moderately good conductor) ◗◗ are neither malleable nor ductile. 36 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  Melting and boiling points are not useful for distinguishing between metals and non-metals. On this basis, cobalt, gold, nickel, potassium, sodium, silver and tin are clearly metals. Argon, bromine, chlorine, hydrogen, nitrogen, oxygen and phosphorus are non-metals. There are, however, some elements that don’t fit these descriptions. Mercury has a shiny appearance and is a good conductor of electricity, but it is a liquid. We choose to classify mercury as a metal because, on balance, its properties are closer to those of the other metals than the non-metals. Carbon in the form of graphite is a fair conductor of electricity and is a solid. Despite this it is classified as a non-metal because, considering all its properties, it resembles the non-metals more closely than the metals. Figure 2.11 shows some common metals and non-metals. Roland Smith a b FIGURE 2.11 Some common elements. a Common metals (clockwise from bottom left): gold, aluminium, zinc, magnesium, copper, iron; b common non-metals (clockwise from bottom left): carbon, phosphorus, iodine, sulfur Some elements are difficult to classify as metals or non-metals because they have some properties of both classes. Boron, silicon, germanium, arsenic, antimony and tellurium do not fall clearly into either category. They are called semi-metals. This classification of the elements into metals and non-metals is useful because very frequently, as we shall find in later chapters, the metals have one property while the non-metals have a different one. 2.5 The periodic table Because there are so many elements (about 118), with such a wide variety of properties, chemists have devised a chart that organises the elements into groups with similar properties. It is called the periodic table. A typical periodic table is shown on the inside front cover of this book. In this table the elements are arranged so that those with similar properties fall into the same vertical column. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 37  Lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs) all have very similar properties; they fall into the extreme left-hand column of the table. Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba) have similar properties; they fall into another vertical column – the one second from the left. Fluorine (F), chlorine (Cl), bromine (Br) and iodine (I) have similar properties; they are also in the same vertical column, the one second from the right. The noble gases, helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe), are very unreactive and fall into the right- hand group of the table. The vertical columns are called groups. They are numbered from 1 to 18. The elements in groups 3 to 12 are called transition elements. The other elements (in groups 1, 2 and 13–18) are called main-group elements. Some groups of the periodic table have special names. ◗◗ Group 1 elements (Li, Na, K, Rb, Cs) are the alkali metals. ◗◗ Group 2 elements (Be, Mg, Ca, Sr, Ba) are the alkaline earth metals. ◗◗ Group 17 (F, Cl, Br, I) elements are the halogens. ◗◗ Group 18 elements are the noble gases. The horizontal rows are called periods and they are numbered from 1 to 7. ◗◗ Period 1 contains only two elements: hydrogen (H) and helium (He). ◗◗ Periods 2 and 3 each contain eight elements – lithium (Li) to neon (Ne) and sodium (Na) to argon (Ar), respectively. ◗◗ Periods 4 and upwards contain larger numbers of elements (because they contain transition elements). There is an older numbering system There is often a gradual change in properties as we go across any one period. While periods are useful for the groups. This numbers for seeing trends in properties, they are less useful than the groups for working out actual properties of the main groups particular elements. I to VIII, leaving the groups of A big advantage of the periodic table is that it allows us to deduce the properties of elements we may transition elements not be familiar with. For example, if we know nothing about rubidium (Rb), we note that it occurs in the unnumbered. same group of the table as sodium (Na). Hence the properties of rubidium and the compounds it forms are very similar to those of sodium. If we know that sodium reacts with water, then the table tells us that rubidium also reacts with water. Similarly, the table can be used to deduce that strontium will undergo the same reactions as magnesium and calcium, or that bromine and iodine will form the same types of compound as chlorine. These strong family relationships occur in the groups at either side of the table (groups 1, 2, 17, 18 in particular). In the middle of the table, such as in group 14, properties change quite significantly going down the group. Metals and non-metals in the periodic table On the periodic table the non-metals occur near the top and right of the table. Most of the other elements are metals. A diagonal band of semi-metals separates the metals from the non-metals. This division is shown in Figure 2.12. Figure 2.12 shows that in groups 13 to 16 metallic character increases from top to bottom of a group. 38 MODULE ONE » PROPERTIES AND STRUCTURE OF MATTER 9780170408929  H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ru Db FIGURE 2.12 Division of the elements into metals and non-metals. Elements to the left of the blue shaded zone are metals; elements to the right (in the pink shaded zone) are non-metals. Elements within the blue shaded zone have properties intermediate between metals and non-metals and are therefore hard to categorise; they are called semi-metals. Hydrogen is usually classed as a non-metal. Metals are elements that: KEY CONCEPTS – are solids at room temperature – have a shiny or lustrous appearance – are good conductors of heat and electricity – are malleable (able to be rolled into sheets) and ductile (able to be drawn into wires). Most other elements are called non-metals. The periodic table is a chart of the elements arranged so that those with similar properties fall into the same vertical column. The vertical columns are called groups. They are numbered from 1 to 18. The elements in groups 3 to 12 are called transition elements. The other elements (in groups 1, 2 and 13 to 18) are called main-group elements. The horizontal rows are called periods and they are numbered from 1 to 7. CHECK YOUR If you did not perform Investigation 2.3 you will need to use secondary sources to answer questions 1 and 2. UNDERSTANDING 1 Identify the only two elements that are liquids at room temperature. 2 Identify six elements that are gases at room temperature. 2.4 2.5 3 Define these terms. a Metal b Non-metal c Semi-metal 4 a Compare and contrast the physical properties of metals and non-metals. b Which two of the properties named in part a are the most useful for determining if an element is a metal or a non-metal? 5 Define the following terms in relation to the periodic table. a Group b Period c Transition elements 6 Where are non-metals located on the periodic table? 7 Outline the trend in metallic character a across a period, b down a group. 8 a Name five metals that are used in your home and state what each is used for. Identify the property that makes the metal particularly suitable for that use. b List four items of jewellery that you or your family own and that are made of predominantly pure elements. Name the elements. Identify the properties that make these elements particularly suitable for jewellery. 9780170408929 CHAPTER 2 » PROPERTIES OF MATTER 39  9 Which of the elements A, B, C, D, E in the table would you class as metals? Justify your decisions. RELATIVE ELECTRICAL ELEMENT MELTING POINT (°C) CONDUCTIVITY* Iron 1540 100 Weblink Chemical periodicity Sulfur 113