07 Mixtures And Solutions 2009

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MIXTURES AND SOLUTIONS SYSTEMS When something is too complex to understand, we divide it in different parts and study them individually. Any part of the Universe on which we focus our attention to study from a scientific point of view is called a system. When we study how a system is formed and “peep” inside it deeper and deeper we are analysing the system. Analysing systems we find that sometimes they are very simple and others they are quite complex and require further divisions. The hydrosphere is a system (quite a complex one). To study it, maybe some scientists (ecologists) will study the relations among the different communities. But some others will just study in detail the behaviour of one species in the ecosystem. Chemists will centre their attention just on the sea water trying to find its composition PURE SUBSTANCES AND MIXTURES From a chemical point of view, any system is made of particles. If all the particles in a system are equal, we call it a pure substance. Being all the particles the same, a pure substance has very definite characteristics or (as it has been studied in 1st year) intensive properties (density, melting point, colour, etc.).

PURE SUBSTANCE: A SYSTEM IN WHICH ALL THE PARTICLES ARE EQUAL.

Pure substances can be ELEMENTS IF ALL ATOMS FORMING EACH PARTICLE BELONG TO THE SAME CLASS COMPOUNDS IF PARTICLES ARE FORMED BY ATOMS OF DIFFERENT ELEMENTS

If particles in a system are not all equal, then the system is called a mixture. Mixtures do not show always the same properties because these will depend on the proportion in which the different particles are found in it. A mixture generally shows some properties of each of the components. A MIXTURE IS A SYSTEM FORMED BY TWO OR MORE SUBSTANCES THAT ARE NOT CHEMICALLY UNITED.

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How many pure substances can be “picked” out from the mixture to the right side of paper? Can you find any elements in it? The differences between pure substances and mixtures are shown in the chart below:

MIXTURES PURE SUBSTANCES A mixture has no fixed proportion between A pure substance has a constant composition its components, it is not fixed. with fixed ratios of elements. A mixture can be physically separated intoA compound can be decomposed (broken) pure substances by physical methods by drastic chemical methods Mixtures may exhibit a changing set ofPhysical properties such as boiling point or physical properties. For example, mixturesmelting point of pure substances are of alcohol and water boil over a range ofinvariant. For example, pure water boils at temperatures. 100 degrees C Just about everything that you can think of is probably a mixture. Even the purest of materials still contain other substances as impurities. Although it is almost physically impossible to isolate absolutely pure substances, a substance is said to be pure if no impurities can be detected using the best available techniques. Can you detect how many brown impurities are mingled with the green capsules of the drawing to the right?

HOW DO WE KNOW A SUBSTANCE IS PURE? There are some criteria used to assess purity: • A pure substance as stated above (see chart) has a set of sharp invariant physical properties (melting point, boiling point, etc). Mixtures do not have a melting or boiling point: they have melting and boiling ranges (start at one temperature finish at a different one) • A pure substance shows a single spot in different chromatography experiments (to be explained later). An impure substance will show more than one spot. REVISING HOMOGENEOUS AND HETEROGENEOUS SYSTEMS Last year, you have studied what the properties of a material are: colour, hardness, density, melting and boiling points, mass, weight, volume.

3 Intensive Properties Intensive properties are those properties such as density, boiling point, melting point etc. that do not depend on the amount of substance considered. Weight, volume, mass, are not intensive but extensive properties. In this course we will be interested in the intensive properties of systems. Phase A phase is any observable part of a material or system that has its own set of intensive properties. A mixture of sand and sodium chloride has two phases, one of them having the properties of salt, the other one those of sand. In a chocolate chip cookie the dough and the chips have different properties. A spoonful of salt stirred in water forms just one phase because we cannot see regions with different properties. Heterogeneous Systems These are made up of more than one phase that can be separated mechanically. Homogeneous Systems Show just one phase. Component A component is any of the pure substances in a mixture. A salt – water mixture is homogeneous because it has one phase but it has two components. Freezing water has two phases (solid and liquid) but just one component. Some systems seem to be homogeneous to the naked eye but show as heterogeneous when seen through a microscope. Milk and mayonnaise belong to this class of systems. SEPARATING THE PHASES OF A HETEROGENEOUS MIXTURE As probably you have forgotten what was taught in 7th grade it will be necessary to revise the methods used to separate the phases of a system. These separations will lead to (pure) substances or to solutions (both homogeneous systems). Further work will allow us to purify the substances and fractionate the solutions into components to be purified later. COARSE MIXTURES In the case of a coarse mixture (stones and water) you can just pick up the pieces of the solid with tongs or pour the liquid out. In the case of two liquid layers, you can suck out one of the layers or drain it through a tap in a special apparatus called separatory funnel (see below). In the “Granola” to the right you can see and separate most of its components using tweezers (or your fingers)

4 DISPERSE SYSTEMS In disperse systems or dispersions, components is finely divided and evenly distributed in the second one. For practical reasons, disperse systems are further classified. • Suspensions are systems in which a finely divided solid is distributed all through a liquid. • Emulsions show a finely divide liquid distributed all through a second one (instead of specks there are droplets moving around). Very well known emulsions are salad dressing, milk and mayonnaise (previously mentioned). • Smokes are solid in gas dispersions • Mists are liquid in gas (a cloud). • Foams are the opposite of mists: a gas is dispersed in a liquid (or a solid) We will name the methods used to separate mixtures of the first two cases SEPARATING SUSPENSIONS Suspensions are separated by means of filtration (or filtering) using a funnel and filter paper. The solid residue remains in the filter and the filtrate is received in a beaker. The diagrams show both simple filtration and vacuum filtration apparatuses. In case the suspension sediments (solid goes to the bottom) the liquid can be eventually poured off the system.

Centrifuging is also very frequently used for this purpose: the figure to the right shows a laboratory centrifuge. In biological chemistry this is the method that is regularly used.

SEPARATING EMULSIONS Emulsions are frequently quite a predicament: They cannot be filtered so they are allowed to decant (settle in two separate layers) and both liquids are separated by siphoning or using a separatory funnel as mentioned before.

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Destroying mists and smokes is usually not a problem you will find in the lab, but have enormous relevance in industrial pollution control. Electric fields are used to make them collapse in the factories’ chimneys to avoid damaging of the atmosphere. THE TWO-SOLIDS CASE The general method to be applied in these cases is differential dissolution also called extraction. It consists in adding to the mixture an appropriate solvent that will dissolve one of the solids but not the other. This will change the two solids problem into a liquid solid problem: filtration will then be adequate. That’s how you can cope with the problem of separating salt from sugar: add some ethanol (spirits) and sugar will dissolve. Now filter to separate the salt that remains as a residue and recover sugar from the solution. Other methods are of limited interest but have many industrial applications (magnetic separation, sieving, levigation, sublimation, etc.). HOMOGENEOUS SYSTEMS Homogeneous systems can be either pure substances or solutions. Solutions are those homogeneous systems that can be separated by adequate methods into more than one pure substance. The act of separating the solution into its components is called fractionation. Fractionating a solution will result in two or more different pure (or relatively pure) substances. Solutions can be liquid, solid or gaseous. Air is a gaseous solution: nitrogen, oxygen, argon, water vapour and carbon dioxide are mixed but the system is homogeneous. We can fractionate air into its components and in fact this is the way we gat to these gases as pure substances for industrial, scientific or medical uses. Alloys are solid solutions. We prepare alloys melting a metal and mixing it with other elements (another metal, carbon, phosphorus, etc.). Once the melt is homogeneous it is allowed to cool down and solidify. Steels (iron with some carbon and different metals added) are prepared this way. Bronze is made out of copper dissolving some tin and maybe other metals in it in a similar fashion. Liquid solutions are frequently used and prepared. Sea water is a liquid solution. So are whisky and plasma. When “Tang” is prepared, the sugar, the colorant, the preserving

6 and the flavouring agents dissolve in water; but there is an additive in the mixture that doesn’t dissolve! Tang in fact is turbid, it is a solution but also it is a suspension. (By the way this substance is added on purpose to make it look like real orange juice that is not totally transparent). People frequently call solution what in fact is a suspension The importance of solutions stands on the fact that many chemical processes occur only (or go better and faster) in solution. On the other hand, manipulating solutions is much easier than working with solids. SOLUTIONS, SOLUTES AND SOLVENTS A SOLUTION is a mixture of two or more substances in a single phase. At least two substances must be mixed in order to have a solution. The substance in the smallest amount, the one that is not seen (unless it is coloured) is called the SOLUTE. The substance in the larger amount that surrounds the solute’s particles is called the SOLVENT. In many instances but not always water is the solvent. The gases, liquids, or solids dissolved in water are the solutes. HOW DOES A SOLUTE DISSOLVE? Dissolving is a physical change: it will leave both the solute’s particles and the solvent’s particles unchanged. The figure shows a blue solid’s particles (a solute) as they are captured and dispersed among the red ones (a solvent). In a solution the solvent’s particles are just surrounding the solute’s particles and separating them. This process is called solvation (hydration in the case of water as solvent). As we cannot see an individual particle but clusters of millions of particles the solute is not seen but nevertheless it is still there.

CONCENTRATION OF A SOLUTION The composition of a mixture can be stated in different ways. The amount of solute dissolved in a given mass or volume of either solvent or total solution is called its concentration. Concentration can be stated in different forms. We frequently state the concentration of a solution referred to 100 g of solvent or 100 g of solution. Percentage mass to mass (% m / m): grams of solute dissolved in 100 grams of solution. Percentage volume to volume (v / v): cm3 of a liquid solute (alcohol for instance) dissolved in 100 cm3 of solution.

7 Grams of solute in 100 cm3 of solvent: this is used mainly for solubility data. Grams per litre (g /dm3): grams of solute dissolved in one litre of solution A solution can be concentrated, if the solute is around 10 to 15 % or more of the total mixture, or diluted, in case that it is in a lesser proportion. To concentrate a solution we either evaporate the solvent or add more solute. To dilute a solution we add more solvent. Concentrated solutions are usually stocked in the lab or in a chemical factory, and they are diluted just before they are used. This is the kind of thing people do everyday with consumer products like fruit juice. Some concentrated solutions are used as "stock" solutions. Dilute solutions are typically used but the concentrated solutions require less storage space. SOLUBILITY In some cases (as water and alcohol) you can add more and more solute (alcohol) and it will always dissolve. What happens in this cases is that the solute becomes the solvent and vice versa. In this example the water molecules that originally surrounded the alcohol particles are now in a lesser proportion and they are in turn surrounded by the alcohol molecules! We call these two liquids MISCIBLE LIQUIDS. But this is not the general case. A definite amount of a solvent can usually dissolve a definite amount of a solute if we consider no changes in temperature. A solution is said to be SATURATED when it admits no more solute. It is in equilibrium (no macroscopic changes are detected) with an excess of solute. SOLUBILITY of a solute in a solvent at a given temperature can be defined as • •

the maximum amount of solute that can be dissolved in a fixed amount of solvent at a given temperature, or the concentration of its saturated solution.

The conventional reference for solubility is the number of grams of solute that can dissolve in 100 cm3 of solvent. This value is the amount of solute that will dissolve and form a saturated solution at the temperature listed. The solvent cannot dissolve more solute at that temperature. SOLUBILITY CURVES The solubility of any solute changes if the temperature is increased. As a general rule, all gases are less soluble and all solids are more soluble as temperature increases There are exceptions for the solids rule such as Ce2(SO4)3 (look at the graph below)

8 Solubility data are usually presented in a solubility chart or as solubility curves that show graphically how the solubility of a substance changes with temperature

A saturated KCl solution at 10oC will have 31 grams of KCl dissolved in 100 grams of water. If we add 40 grams of KCl to 100g of water in a flask 31 g of it will “disappear” (dissolve) and there will be 9 grams of undissolved KCl remaining as a solid. Raising the temperature of the mixture to 30oC will increase the amount of dissolved KCl to 37 grams and there will be only 3 grams of solid undissolved. The entire 40 grams can be dissolved if the temperature is raised above 40oC. Cooling the hot 40oC solution will reverse the process. When the temperature decreased to 20oC the solubility will eventually be decreased to 34 gram KCl. There is a time delay before the extra 6 grams of dissolved KCl crystallizes. This solution is "supersaturated" and is a temporary condition. The "extra" solute will crystallise out of solution when the randomly moving solute particles can form the crystal pattern of the solid. A "seed" crystal is sometimes needed to provide the surface for solute particles to crystallize on and establish equilibrium. FRACTIONATING A SOLUTION We will focus on the fractionation of liquid solutions. Gaseous and solid solutions are beyond the scope of this course. To separate the components of a solution we make use of the different physical properties of its components (solubility, boiling point). Liquid solutions form by far the bulk of solutions you have to deal with in common life and in the chemistry lab. They can have a solid or a liquid as the “hidden” component or solute. To separate the liquid component you may eventually evaporate the liquid. This is an easy method but can be only applied if the solute is a solid and the solvent cheap and non flammable (water). In the general case simple distillation is used. The solution is heated in a flask and water vapours are cooled down to condense and finally collected in a separate container. (See diagram)

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In case the solute is a liquid it will also distil though at a different temperature. But there is a problem: as both substances evaporate at the moment one starts boiling it helps the other one to find its way to the gaseous phase into the atmosphere. Both liquids co-distil and the separation results by all means unsatisfactory. Several simple distillations will be needed to achieve a reasonable result. Luckily a device called the fractional distillation column allows accomplishing the same results in just one operation. In fractional distillation, the vapours condense and go back again to the flask. As they go down they “plunge” into the ascending vapours and as they mix, a second vaporisation process occurs and the new vapours are enriched in the most volatile component. This occurs once and again and finally the outgoing vapours are almost pure volatile component. The flask is enriched in the less volatile one and so both components get finally separated.

10 Petroleum fractions are separated this way although, being the process a non – stop distillation, in which crude oil is added continuously into the boiler, the fractions are collected at different points of the fractionating column CHROMATOGRAPHY Chromatography is the most powerful method for separating substances. It was first developed by Tswett to separate and study the coloured components of flowers. He used filter paper and different mixtures of solvents to achieve this separation. Nowadays, chromatography has turned into sophisticated method: coils of capillary tubes more than 300 m long, lined with special polymers (huge molecules) are used instead of paper as a support, and gases or pressurised liquids flow along them. In paper chromatography a sample is spotted on a standardised filter paper and this paper is placed inside a tank dipping in a small amount of a solvent. The solvent climbs up the paper because of capillary action and sweeps the sample up. After some time, if the sample is a mixture, the components will be separated because of their being swept at different rates. These “sweeping rates” are different because the affinity of different substances in the mixture to both the paper and the solvent vary between wide ranges. The coloured components are easily seen, but in case a colourless substance has to be located, it can be visualised by different methods (UV lamps, iodine vapours, specific reagents). The figure shows a chromatography tank with a chromatogram with three different mixtures. One mixture is missing the red components, a second one is missing the green component and the third one the blue. Notice that the individual colours are the same for the different mixtures (not two different greens or blues). Chromatography can be used to identify substances just as the melting or the boiling point. If an unknown sample and different standards are spotted one beside the other on a horizontal line and the solvent is allowed to climb. The standards will have climbed up different distances from the starting line and the unknown will lie on the same horizontal line as the standard it is equal to. Currently GC (gas chromatographs) and HPLC (High performance Liquid chromatographs are the cutting edge technology for the separation of substances in very complex mixtures. Study the scheme of the HPLC. It shows the essential features that both GC and HPLC have. QUESTIONS AND PROBLEMS 1- How many phases and how many components are there in each of the following systems: a- A spoonful of sugar in 1 cup of water when just added. b- The same system after three hours. c- Five iron nails under an aqueous salt solution. d- An emulsion of salty water and oil in kerosene.

11 2- Give some examples of suspensions, emulsions and solutions you can find at home. 3- How would you separate a mixture containing: a- water and oil c- water and clay b- water and salt d- water and alcohol e- fourteen amino acids 4- The table below shows the solubility of some gases in water at different temperatures (in g / 100 g water)

a. b. c. d.

What is the solubility in water at 40ºC of (i) ammonia (ii) oxygen Which of the gases are only slightly soluble in water? Do they become more or less soluble as temperature rises? Explain how raising the temperature will affect cold water fish.

5- The following figure shows a chromatography experiment with coloured inks. a- Which of the inks were mixtures? b- Which of the dyestuffs appear in more than one ink? 6- The following chart shows the solubility curves for 4 different salts. Plot them and answer the following questions: a- Which is the general trend for these curves? b- Which of the salts show “special” behaviours? c- Which is the less soluble salt at room temperature (20ºC)? d- How many g of each will dissolve in 250 g of water at 45ºC?