Chapter 4: Separating Techniques Name: ______________________( Recommended reference: Heyworth, R. M., & R, B. J. (2013). All About Chemistry. Malaysia: Pearson Education South Asia Pte Ltd. Pages 13 - 29 ) Class: ______ Date: ____________ 4.1 A Pure Substance Learning Outcomes: Pupils are expected to (a) deduce from the given melting point and boiling point the identities of substances and their purity (b) explain that the measurement of purity in substances used in everyday life, e.g. foodstuffs and drugs, is important Comparing elements, compounds and mixtures Diagram Element ! Compound Pure ! Mixture ! ! ! ! 1 Diagram Element ! Compound Pure Mixture ! ! ! ! ! ! ! ! Pure Substances A pure substance is a single substance not mixed with anything else. For example, white sugar is a pure substance. The label on a packet of white sugar says `pure refined cane sugar'. This means no other substance is present. A crystal is a pure substance. White sugar is made up of crystals with the same shape. This shows that it is made up of the same single substance: it is pure. In nature, very few substances are found pure. Most Granite rock. You know it is a mixture because it is substances are mixtures. A mixture contains two or more substances. Sea water is a mixture. It contains water, salt and other dissolved gases. Air is a mixture of oxygen, nitrogen and other gases. made up of different black and white crystals. The coin shows the relative sizes of the granite crystals and the coin 2 Some solid mixtures are made up of different crystals. Most rocks are mixtures. Granite is commonly used for buildings and roads. Mixtures can easily be separated into pure substances. The process is called purification. This is done by physical methods such as filtration, crystallisation or distillation. A pure substance has definite properties. It has a fixed melting point and boiling point. Pure water, for example, melts at 0°C and boils at 100°C. All samples of pure water melt and boil at these temperatures. A mixture does not have definite properties. It does not have fixed melting and boiling points. An example of a mixture is coconut oil which is used for cooking. It melts over a range of temperatures. It starts melting at 14 °C and completes melting at 22°C. Petrol fuel for motorcars is also a mixture. It has a boiling point range of 35°C to 75°C. Several methods of purification are discussed in greater detail. The separation of mixtures into pure substances is important. Chemists need pure substances to study their properties. Pure substances are used in industry to make useful products such as food and drugs. Impurities in food and drugs can be dangerous because they can poison people. An example is a substance called tryptophan which some people consume as a health food. Some years ago, a cheap but impure quantity of tryptophan was made and sold by a chemical company for health food. Unfortunately the impurities caused irreversible damage to peoples' nervous systems and as a result several thousand healthy people became so sick that they were forced to live out the rest of their lives in wheelchairs. Very pure silicon crystals are needed to make the `chips' used in calculators, computers, watches and compact disc players. The great accuracy of a quartz watch is due to a little quartz crystal which vibrates at exactly 32 768 times per second. This controls the time. 3 4.2 Separation Techniques Pupils are expected to (c) describe methods of separations and purification for the components of the following types of mixtures: 1. solid-solid 2. solid-liquid 3. liquid-liquid (miscible and immiscible) (d) explain how the following methods are used to separate constituents of a mixture based on the properties of the constituents: 1. filtration 2. evaporation to dryness 3. separating funnel 4. crystallization 5. distillation & fractional distillation 6. chromatography 7. magnetic attraction 8. sublimation (e) describe paper chromatography and interpret chromatograms by comparison with 'known' samples (f) recognise chromatography as a technique for identification of substances and determining purity (g) recognise that crystallisation is a means of obtaining pure substances or removing impurities (h) recognise by that crystallisation often results from solutions which have exceeded their saturation points (i) state some useful applications of crystallisation e.g. production of silicon wafers for microchips (j) describe the applications of the various separation techniques in everyday life and industries (k) suggest suitable purification techniques, given information about the substances involved Advanced Topics (a) describe the techniques involved in obtaining pure water from sea water in desalination plants (e.g. distillation and reverse osmosis) (b) describe and explain the process reverse osmosis and how desalination plants work (c) able to perform calculations of Rf value using a chromatogram (d) explain the need to use locating agents in the chromatography of colourless compounds 4 Comparing Separation Techniques Techniques filtration evaporation to dryness separating funnel What do you separate Insoluble solid from liquid Soluble solid from liquid Immiscible liquid Properties of constituents involved Size of particles Thermal stability of solid Density Principle of separation Pore size of filter paper prevents large particles from passing through Liquid evaporates away, leaving behind the solid. Denser fluid sinks, less dense fluid floats. The Denser fluid can be drained first. When solubility of the salt drops as the mixture cools, the salt that cannot be dissolved will crystallise out. The liquid with the lower boiling point will be distilled out earlier. The more soluble constituent will travel further than the less soluble constituent on the chromatography paper The ferromagnetic material will be attracted to the magnet Some solids undergo sublimation instead of melting, hence it can be separated as gas when heated. crystallization Soluble solid from liquid Solubility changes with temperature Different boiling points Different solubility of constituent in solvent Ferromagnetism Solid that undergoes sublimation instead of melting distillation & fractional distillation chromatography Miscible liquids Small quantities of solids dissolved in solvent Ferromagnetic solids from solids Solids from solid magnetic attraction sublimation 5 Filtration Filtration is the usual method of separating a solid from a liquid. A simple way of doing this is shown below. The mixture is poured through a filter which is usually made of paper. The filter paper has tiny holes through which the particles of the liquid are able to pass. The particles of the solid are large. They cannot pass through the holes and are trapped by the filter paper. The solid collected in the filter paper is called the residue. The liquid passing through the filter is called the filtrate. When a mixture of sand and water is poured into the filter paper, the solid sand is trapped in the filter paper (the residue) and the water passes through the paper (the filtrate). The filtrate is often a solution of, for example, a solid dissolved in water. This solution can be separated by crystallisation. Filtration is one of the steps in making drinking water. All solid impurities are first removed by filtration. The water is then treated to kill any harmful micro-organisms. Do you know? Some special filters can filter biological cells. The antibiotic penicillin is made from a yeast. The penicillin is separated from the yeast by filtration. While the small penicillin molecules pass through the filter the big yeast cells are trap. 6 Crystallisation A solid dissolves in a liquid to produce a solution . The solid that dissolves is called a solute and the liquid is called a solvent . When the solution is heated, most of the solvent is evaporated off. The hot solution is then allowed to cool. The solution is now said to be saturated and as it cools, the dissolved solid (solute) appears as pure crystals. This process is called crystallisation . The impurities remain dissolved in the solution. The main steps in purifying a solid by crystallisation are shown in the diagrams. Impure solids often contain impurities which are insoluble in the solvent. These insoluble impurities are removed by filtering the mixture before evaporating the solvent. Crystallisation vs. Evaporation to dryness In evaporation to dryness, any soluble impurities will be deposited along with the required solid, hence it cannot separate a pure substance from its impure form. e.g. when we attempt to obtain pure copper(II) sulfate salt from an impure mixture containing copper(II) sulfate and trace amount of another soluble salt, both salts will be deposited upon evaporation to dryness. Furthermore, evaporation to dryness is not suitable for all solutes. For example, sugar decomposes to give water and carbon while hydrated copper(II) sulfate crystals (blue) decompose to form anhydrous copper(II) sulfate powder (white) Crystallisation is similar to evaporation to dryness, except that we do not heat until all the solvent is evaporated. Instead, we stop heating when a saturated solution is obtained. Applications of crystallisation Crystallisation from solution is the most common method used by chemists to purify solids. Pure sugar is obtained this way. Some solids can be purified by melting them. The hot liquid is then cooled slowly. Crystals form when the liquid freezes. The first crystals to form are very pure. These are collected before the rest of the substance freezes completely. This is crystallisation without a solvent. This method is used to make the very pure 7 silicon used in computer chips. A pure silicon crystal is obtained by freezing molten silicon at 1410oC. Magnetic Attraction Magnetic attraction is used to separate magnetic substances from nonmagnetic ones in a mixture (e.g. mixture of iron and sulfur). Electromagnets are used to remove scrap steel and iron at the junkyard. In hospitals, magnets are often used to remove iron splinters from a patient’s eyes. Distillation Distillation is used to purify liquids. In this process, the liquid is changed into a gas by boiling. The gas is pure as other substances are left behind. This gas is then cooled. It condenses to a pure liquid which is called the distillate. Distillation can be used to obtain a pure solvent from a solution of a solute. An example of a simple distillation is the distillation of sea water to obtain pure water. Below shows the experimental set-up of obtaining pure water from sea water by distillation. The seawater is boiled in the flask and the steam produced is cooled by the condenser. The pure liquid is then collected in the beaker and the salt remains in the flask. The thermometer shows a temperature of 100°C during the distillation. This is the boiling point of water. Sea water is a solution of solid salts in water. The salts, like other solids dissolved in water, cannot be distilled and remain in the flask. 8 Fractional Distillation Miscible liquids are completely soluble in each other. This means two liquids mix to form one liquid. Fractional distillation can be used to separate a mixture of miscible liquids. This is done using a fractionating column. A fractionating column separates liquids in order of boiling points. The liquid with the lowest boiling point is distilled first. The liquid with the highest boiling point is distilled last. Ethanol is an important solvent. It can be made by fermenting cane sugar. A mixture of ethanol and water is obtained from the fermentation. Ethanol is then obtained from this mixture by fractional distillation. The experiment below shows how this can be done in the laboratory. The boiling points of ethanol and water are 78°C and 100°C respectively. In the experimental set-up (top right) the vapour from the boiling mixture of ethanol and water in the flask contains a larger percentage of the lower boiling point ethanol. As the vapour moves up the fractionating column, it repeatedly condenses and boils inside the column. Each time the mixture boils, the percentage of the lower boiling point ethanol increases. By the time the vapour reaches the top of the fractionating column, it has become almost pure ethanol. This vapour then passes into the condenser where it is cooled, and condenses into liquid ethanol. The thermometer shows a constant temperature of 78° C when the ethanol is being distilled. The water, which has a higher boiling point, remains in the flask until almost all the ethanol has distilled. (See graph on top left) 9 In this experiment, a complete separation is not effected. The distillate contains a very large percentage of ethanol and a little water. Fractional distillation is used to obtain pure oxygen and pure nitrogen from air. The air is first cooled to about 200 ° C . At this temperature, air is a liquid. The liquid air is then distilled by allowing the liquid to warm up. Nitrogen has a lower boiling point than oxygen, so it distils first. Then the oxygen (with a higher boiling point) distils. This produces nitrogen and oxygen separately. Fractional distillation is also used in the petroleum industry. Petroleum is a mixture of liquids called hydrocarbons. Fractional distillation is used to separate the hydrocarbons into, for example, lower boiling point petrol and higher boiling point kerosene. Large amounts of liquid chemicals are manufactured in the petroleum industry, and fractional distillation is commonly used to separate the chemicals into pure liquids. Separating Funnel Many liquids do not mix together. An example is petrol and water. In a mixture of petrol and water, the lighter liquid (petrol) forms a separate layer on top of the heavier liquid (water). A separating funnel can be used to separate such mixtures that contain liquids which are not miscible . The tap is opened so that the lower water layer runs out first. The tap is closed as the last drop of water leaves the funnel. The tap is then opened again to run the petrol into another beaker. Separating petrol and water with a separating funnel 10 Chromatography Chromatography is a method of separating and identifying mixtures. There are several types of chromatography, one of which is paper chromatography. An example of the use of paper chromatography is in the separation of the dyes in black ink. This is shown in the experimental set-up below. Paper chromatography of black ink A i m : To separate and identify the dyes in black ink 1 Draw a pencil line on a piece of chromatography paper (this paper is like filter paper). 2 Place a drop of black ink on the pencil line. Also place drops of coloured dyes (red, orange, blue, green, etc.) on the pencil line. These dyes are the ones you think might be in the black ink. 3 Roll the paper into a cylinder and secure it. Then place the paper in a beaker with a suitable solvent. 4 The solvent will travel up the paper. When the solvent reaches the top of the paper, take the paper out of the beaker and allow it to dry. A suitable solvent for this separation is a mixture of butanol (an alcohol), ethanoic acid and a little water. The solvent chosen depends on the type of substances being separated. In the above experiment, the solvent travels up the paper. The dyes on the pencil line dissolve in the solvent and travel up the paper at different speeds. Hence, the dyes are separated. The result is called a chromatogram. A typical example is shown on the next page. . Chromatogram of dyes in black ink Identical dyes travel the same distance up the paper. The unknown dyes in the black ink can be identified by comparing them with the known dyes. The result of the experiment shows that - black ink is made up of four different dyes; - three of the dyes are the known red, blue and green dyes; - there is no orange dye in the black ink; - black ink contains one dye that is different from the four known dyes. 11 Advanced: Chromatography can also be used to separate and identify colourless substances. The chromatogram is sprayed with a locating agent to show where the substances are on the paper. The locating agent is a chemical that reacts with the substances to produce a coloured product. An example of separating colourless substances is in identifying the sugars in a fruit juice. The same method is used as for the black ink. A drop of fruit juice is used in place of the black ink. A different solvent is used. At the end, the chromatogram is sprayed with silver nitrate solution followed by dilute sodium hydroxide. These chemicals make the sugars appear as brown spots. A typical result is shown. The result of the experiment shows that fruit juice contains three different sugars;- two of the sugars are sucrose and glucose; the fruit juice does n o t contain maltose; the fruit juice contains one other sugar different from the three known sugars. Paper chromatography can also be carried out with the solvent running down the paper. The arrangement of the apparatus is shown on the left. This descending method (shown on the left) of paper chromatography works better for longer pieces of paper as the solvent does not have to move against gravity, and thus flows more quickly. This means that the solutes which are separated can travel further and thus the separation between the spots is greater. 12 The chromatogram of the sugars present in fruit juice shows the presence of three different sugars. One of the sugars is different from the three known sugars (sucrose, glucose and maltose) on the chromatogram. This sugar can be identified from its Rf value. The Rf value for any substance on a chromatogram is equal to: Distance moved by the substance Distance moved by the solvent The distances are measured from the starting line. In the figure on the right, the Rf value of the substance in the `spot' equals x/y. The Rf value for the unknown sugar in the earlier chromatogram can be calculated as follows (shown below). The sugar is then identified from the R f values for different sugars that have already been calculated. The R f values depend on the solvent and the temperature. The R f values for some sugars in the solvent used to produce the chromatogram are given in the table (right). From the table, the unknown sugar is fructose. Chromatography can be used to separate and identify complicated substances such as dyes and drugs. It is used to identify artificial dyes in food. Only a few dyes are safe for food. Many are poisonous. Government laboratories use chromatography to check that only approved dyes are used in food. Pesticides are sprayed on green vegetables to kill insects while herbicides are used to destroy weeds. These chemicals can be harmful to people. That is why green vegetables must be tested to check that they only contain tiny amounts of these chemicals when they are sold in 13 markets. This testing is done by chromatography which can identify the chemicals present and their amounts. Paper chromatography is only one type of chromatography. Another important type is gas chromatography. In gas chromatography, a gas is used in place of a liquid to separate the substances being analysed. Gas chromatography is used to find out if athletes are using illegal drugs to improve their performance in competitions (as in the Olympic Games). The big advantage of chromatography is that it can be used to identify very tiny amounts of substances. It can detect less than 0.000 000 000 001 gram or 10-12 g of the substance. Testing the Purity of Substances Pure substances have fixed melting and boiling points. These temperatures are given in most books. So we can easily find out the melting point or boiling point of a pure substance. Melting and boiling points are changed by the presence of impurities. This fact can be used to find out if a substance is pure. Melting Point Impure substances are mixtures. Mixtures do not have a fixed melting point. An example of a mixture is coconut oil which is used for cooking. It melts over a range of temperature. It starts melting at 14°C and completes melting at 22°C. This fact can be used to find out if a substance is pure. If it is pure, then all of it should melt at the sam e temperature. An impure substance melts over a range of temperature. Also, impurities lower the melting point. Pure naphthalene melts at 80°C. If the naphthalene is impure, it melts below 80°C. Impure naphthalene may melt at 78°C or 76°C. The greater the percentage of impurity, the lower the melting point. Solids must be very pure to melt exactly at their true melting points. Hence we can find out if a substance is pure by measuring its melting point. Boiling Point We can find out if a liquid is pure by distilling it. If it is pure, all of it distils at the same temperature. This temperature is the boiling point. If a liquid is impure (that is, a mixture), it distils over a range of temperature. For example, petrol which is used as a fuel for motorcars is a mixture. It has a boiling range of 35°C to 75°C. Similarly, impure ethanol containing water as an impurity, might start distilling at about 78°C (the boiling point of pure ethanol), but the distillation may not be complete until the temperature reaches about 100°C. The boiling range for an impure substance depends on the actual impurities present and their percentage. 14 Chromatography Chromatography can be used to test the impurity of complicated substances and those that cannot easily be melted or distilled. It is a very sensitive test and shows up very tiny amounts of impurity. An example is testing the purity of a dye used in food. The figure of the right shows the result of a chromatography test. Dye X is pure because it forms one spot on the chromatogram. Dye Y is impure because it has separated into several spots on the chromatogram. The intense spot is pure dye Y; the three faint spots are impurities. Supplementary Notes for Reverse Osmosis Seawater desalination Desalination refers to any of several processes that remove some amount of salt and other minerals from saline water. Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation. The traditional industrial process used in these operations is vacuum distillation while the principal competing processes use membranes to desalinate, applying reverse osmosis technology. Vacuum distillation Singapore’s NeWater plants uses reverse osmosis technology The industrial method of distilling seawater involves a process known as vacuum distillation, whereby water is boiled at less than atmospheric pressure. The boiling of a liquid occurs when the vapour pressure equals the ambient pressure and vapour pressure increases with temperature. Hence, when the ambient pressure is reduced, a much lower temperature is required for the vapour pressure to be equal to the ambient pressure and for the water to boil. As the temperature required for the distillation process is lowered, energy can be saved. 15 Reverse Osmosis In an osmosis process, a selective membrane is used. There are very small holes in this membrane (known as pores) which only allow very small molecules like water molecules to pass through. Undesirable molecules, bacteria and virus cannot pass through the membrane and is filtered out. In a normal osmosis process, the solvent naturally moves from an area of low solute concentration (higher water potential, through a membrane, to an area of high solute concentration (lower water potential). In reverse osmosis, pressure is applied on one side of the membrane, forcing the water molecules to pass through the membrane, leaving behind the undesirable salt, virus and bacteria. Clean water is obtained from the other side of the membrane. References http://en.wikipedia.org/wiki/Desalination http://www.pub.gov.sg/water/Pages/DesalinatedWater.aspx 16