A2 Mixtures and Their Separation PDF

# Mixtures and Their Separation ## A2 Objectives By the end of this topic you will be able to: - distinguish between pure substances and mixtures - explain the difference between an element, a compound and a mixture - give examples of elements, compounds and mixtures - explain the difference betw...

# Mixtures and Their Separation ## A2 Objectives By the end of this topic you will be able to: - distinguish between pure substances and mixtures - explain the difference between an element, a compound and a mixture - give examples of elements, compounds and mixtures - explain the difference between a homogeneous and a heterogeneous mixture. ## Mixtures and Their Separation Elements, compounds and mixtures form a part of our everyday lives. When we wrap our food in aluminium foil we are using an element. When we place salt on our food we are eating a compound. When we drink a cold soda we are drinking a mixture. It may be useful to know how to separate some of these mixtures into their component parts. An example of this is the purification of drinking water. ## A2.1 Elements, Compounds and Mixtures Matter can be classified into two main groups: pure substances and mixtures. ### Pure Substances Pure substances have the following general characteristics: - They have a fixed, constant composition. - Their properties are fixed and constant. - The component parts of a pure substance cannot be separated by physical means. ### Mixtures Mixtures have the following general characteristics: - They have a variable composition. - Their properties are variable since their components retain their own, individual properties. - The component parts of mixtures can be separated by physical means. Pure substances can be further classified into elements and compounds. Mixtures can be further classified into homogeneous mixtures and heterogeneous mixtures. The tree diagram in Figure 2.1.1 shows the breakdown of these groups. ``` matter pure substances elements compounds mixtures homogeneous heterogeneous mixtures mixtures ``` ### Pure Substances A pure substance is composed of only one type of material and has the following fixed properties: - a sharply defined, constant melting point or freezing point - a sharply defined, constant boiling point - a constant density. ## Elements An element is the simplest form of matter. It cannot be broken down into anything simpler by ordinary chemical or physical means. We say 'ordinary chemical means' to exclude nuclear reactions. The smallest particle in an element that has the same properties as the element is an atom. Each element is composed of only one kind of atom. ## Compounds Compounds are composed of more than one kind of atom. These atoms are combined together chemically, they are always present in the same proportions by mass and they cannot be separated by physical means. A compound can be represented by a chemical formula, which indicates the elements that the compound is made up of and the ratio in which they have combined, e.g. the chemical formula of water is H₂O. Examples of compounds are water, which is composed of hydrogen and oxygen in a ratio of 2:1, sodium chloride (NaCl), which is composed of sodium and chlorine in a ratio of 1:1 and methane (CH₄), which is composed of carbon and hydrogen in a ratio of 1: 4. The properties of a compound are fixed and are different from the properties of the individual elements that form the compound. For example, hydrogen and oxygen are both gases at room temperature, however, water is a liquid. ## Mixtures Mixtures are composed of two or more substances which are not chemically combined, therefore their components can be separated by physical means. Some of the physical methods for separating mixtures, which we will be investigating in Unit 2.4, are filtration, evaporation, crystallisation, distillation, fractional distillation and chromatography. In a mixture the component parts are not in a fixed ratio and they retain their own, individual physical properties. ### Homogeneous Mixtures A homogeneous mixture is one in which the properties and composition are uniform throughout the mixture. The component parts cannot be distinguished from each other. A solution is a homogeneous mixture. Examples of homogeneous mixtures are air, salt dissolved in water and metal alloys such as brass, a mixture of copper and zinc. ### Heterogeneous Mixtures A heterogeneous mixture is a non-uniform mixture, for example, a mixture in which the component parts are in different states. The component parts can be distinguished from each other, although not always with the naked eye. Suspensions and colloids are heterogeneous mixtures. Examples of heterogeneous mixtures are salt and sand, mayonnaise, and muddy water. ## Solutions, Suspensions and Colloids Solutions, suspensions and colloids form part of our everyday lives. For example, sea water is a solution, muddy water is a suspension and milk and fog are both colloids. ### Solutions A solution is a homogeneous mixture. The major component of a solution is known as the solvent and the minor component is known as the solute. Some solutions may contain more than one solute, e.g. sea water. The solute and solvent can be gases, liquids or solids. When a gas or a solid dissolves in a liquid, the liquid is always the solvent, e.g. in a mixture of salt in water, salt is the solute and water is the solvent. Table 2.2.1 gives examples of various types of solutions. **Table 2.2.1 Examples of different types of solutions.** | Solute | Solvent | Example | Components of the solution | |---|---|---|---| |Solid | Liquid | Sea water | Sodium chloride in water | | Gas | Liquid | Soda water | Carbon dioxide in water | | Solid | Solid | Brass | Zinc in copper | | Liquid | Liquid | White rum | Ethanol in water | | Gas | Gas | Air | Oxygen, water vapour, argon and carbon dioxide in nitrogen | ### Saturated Solutions A saturated solution is one that contains as much solute as can be dissolved at a particular temperature in the presence of undissolved solute. You will study saturated solutions in more detail in Unit 2.3. ### Suspensions A suspension is a heterogeneous mixture containing minute particles which are visible to the naked eye. If left undisturbed, the particles in a suspension eventually settle. The components of a suspension can be separated by filtration. #### Examples of Suspensions: - Dust in air is a suspension of a solid in a gas. - Powdered chalk in water is an example of a suspension of a solid in a liquid. - Muddy water is another example of a solid suspended in a liquid. ### Colloids A colloid is a heterogeneous mixture containing particles that are intermediate in size between those of a solution and those of a suspension. The particles in a colloid cannot be seen even with a microscope, and if left undisturbed they do not settle. The properties of a colloid are intermediate between those of a solution and those of a suspension. #### Examples of Colloids: - Smoke in air is a colloid of a solid in a gas, also known as a solid aerosol. - Fog and aerosol sprays in air are colloids of a liquid dispersed in a gas, also known as liquid aerosols. - Milk and mayonnaise are colloids of a liquid dispersed in a liquid, also known as emulsions. - Gelatine and jelly are colloids of a solid dispersed in a liquid, also known as gels. ## A2.3 Solubility The solubility of a solute is an indication of how much of the solute can dissolve in a fixed mass of solvent at a particular temperature. For example, we can find the solubility of sodium chloride in water by determining how much sodium chloride can dissolve in 10 cm³ of water at a particular temperature. When no more solute can be dissolved in the solvent, the solution reaches saturation point and we say the solution is saturated. If any more solute is added to the solvent, the solute will remain in solid form and will be mixed with the saturated solution. The solubility of a solute in water is usually measured as the mass of solute that will saturate 100 g of water. Each solute and solvent combination has a specific solubility at a given temperature. The solubility of a solute in a solvent is determined by the structure of the solute and solvent, and the temperature. ### The Effect of Temperature on Solubility For most solid solutes in water, solubility increases as temperature increases. This means that as the temperature increases, a greater mass of solute will saturate a fixed mass of water. A solution which is saturated at one temperature will not be saturated at a higher temperature, and if a solution which is saturated at one temperature is cooled, crystals of the solute will form since less of the solute can dissolve at a lower temperature. You learnt earlier in this unit that solubility in water is usually measured as the mass of solute which will saturate 100 g of water. The unit for solubility is g per 100 g water. However, it is not practical to weigh 100 g of water. In determining solubility of a solute in water in the laboratory, we make use of the fact that pure water has a density of 1 gcm³. In other words, 1 cm³ of water has a mass of 1 g, or 1 g of water has a volume of 1 cm³. This means that 100 g of water has a volume of 100 cm³, and it is easy to measure 100 cm³ of water in the laboratory. When we plot solubility of a solute against temperature, we draw a graph known as a solubility curve. The solubility curve for potassium chlorate(v), KCI₃, is shown in Figure 2.3.1. Looking at the solubility curve in Figure 2.3.1, you can see that the solubility of potassium chlorate(v) increases with an increase in temperature. Solubility curves such as this can be used to obtain various useful pieces of information, for example: - The solubility of a solid at any temperature within the range of the graph. ## A2.4 Separating Mixtures Since mixtures form a part of our everyday lives, the separation of these mixtures into their component parts is important. Examples include the purification of drinking water and the making of filter coffee. If you were stuck on a desert island without any drinking water, your knowledge of separating mixtures could help you to make pure water from sea water. Mixtures are a type of matter where the components can be separated by physical means. The method by which a mixture can be separated into its component parts is determined by the physical properties of these parts, e.g. particle size, boiling point, solubility. Table 2.4.1 summarises the methods of separation that you will be studying, together with the physical properties of the component parts that allows the mixtures to be separated by each method. **Table 2.4.1 A summary of the methods used to separate mixtures.** | Separation method | Physical properties of component parts | |---|---| | Filtration | A mixture of a solid and a liquid where the solid does not dissolve in the liquid. The components are separated based on their different particle sizes. | | Evaporation and crystallisation | A mixture of a solid which is dissolved in a liquid where the boiling point of the liquid is lower than that of the solid. The methods only allow for collection of the solid. The components are separated based on their different boiling points. | | Simple distillation | A mixture of a solid which is dissolved in a liquid where the boiling point of the liquid is lower than that of the solid. Both the liquid and the solid can be collected. The components are separated based on their different boiling points. | | Fractional distillation | A mixture of two (or more) miscible liquids which have different boiling points, i.e. there is a difference in volatility. Miscible liquids are ones that mix completely. The components are separated based on their different boiling points. | | Separating funnel | A mixture of two (or more) immiscible liquids which have different densities. Immiscible liquids are liquids which do not mix. The components are separated based on their different densities. | | Chromatography | A mixture of dissolved substances which will travel through a material. The components are separated based on their different solubilities in a solvent and attraction to the material. | ### Filtration Filtration is used to separate a mixture of a solid and a liquid where the solid does not dissolve in the liquid, i.e. it is used to separate the components of a suspension. Filtration makes use of filter paper to separate the solid from the liquid. The filter paper works like a sieve, keeping back the solid particles. The solid particles are too big to pass through the filter paper, whereas the liquid particles are small enough to pass through. The part of the mixture that stays behind in the filter paper is called the residue. The part of the mixture that passes through the filter paper is called the filtrate. Filtration is one of the steps used in the purification of drinking water. Most simple water purification devices that you might use in your homes use filtration as their main method of purifying the water. They separate the solid impurities from the water. A coffee machine makes use of a filter in the form of filter paper to separate the coffee granules from the coffee. ### Evaporation Evaporation is used to separate a solution of a solid dissolved in a liquid, but it only allows the solid to be collected. During evaporation, the solution is boiled allowing the liquid to vaporise into the air. The solute is left behind in the container. Evaporation is a fairly rapid process and if all the liquid evaporates, the solid remaining lacks any crystalline structure. The method is not suitable if the solid to be collected is decomposed by heat. Evaporation is useful to obtain the sodium chloride from a sodium chloride solution. ### Crystallization Like evaporation, crystallization is used to separate a solution of a solid dissolved in a liquid, and it only allows the solid to be collected. Unlike evaporation, the solution is not boiled, it is left in a container at room temperature for the liquid to vaporise into the air. Crystallization is a slow process and the solid remaining has a distinct crystalline structure. It is used if a hydrated solid containing water of crystallization is required. Water of crystallization is water that is incorporated within the crystalline structure and you will learn more about this in Unit 8.4. Crystallization is useful to obtain hydrated copper(II) sulfate crystals (CuSO₄.5H₂O) from a copper(II) sulfate solution. ### Simple Distillation Simple distillation is also used to separate a solution of a solid dissolved in a liquid. It allows both the solid and the liquid to be collected. Simple distillation is an appropriate method of separation only if the liquid has a lower boiling point than the solid, i.e. the liquid becomes a vapour before the solid. One of the key components of the apparatus used in distillation is a Liebig condenser. The method by which the distillation apparatus works is as follows: - The mixture is heated in the distillation flask until it boils and vaporisation occurs. - The vapour rises up the distillation flask and as it passes into the Liebig condenser it cools and condenses back to a liquid. The liquid passes down the condenser and is collected as the distillate. - The concentration of the solution in the distillation flask gradually increases and when most of the liquid has vaporised, the solution can be poured into an evaporating basin and left to crystallise to obtain crystals of the solute if required. ### Fractional Distillation Fractional distillation is used to separate a mixture of two or more liquids which are miscible and have different boiling points that are close together. Miscible liquids are liquids that are able to mix. The apparatus used in fractional distillation is similar to that used in simple distillation. However, a fractionating column is attached between a round-bottomed flask and the Liebig condenser. Fractional distillation can be used to separate a solution of ethanol and water since the boiling point of ethanol is 78°C and that of water is 100°C. The method by which the apparatus for fractional distillation works is as follows. - The mixture boils and vapours of both liquids enter and move up the fractionating column where they condense and vaporise many times. - As the mixture of vapours moves up the fractionating column, it becomes increasingly richer in the more volatile component, i.e. the one with the lower boiling point (ethanol), until the vapour reaching the top of the column consists only of the more volatile component. This vapour passes into the condenser, condenses and is collected as the distillate. - The vapour of the less volatile liquid, i.e. the one with the higher boiling point (water), condenses in the fractionating column and returns to the round-bottomed flask. ### Separating Funnel A separating funnel is used to separate a mixture of liquids that are immiscible and have different densities. Immiscible liquids are liquids which are unable to mix. A separating funnel is a container that has a tap at the bottom, allowing one liquid to be drained off before the other. It is usually conical in shape to reduce the amount of liquids lost at the interface of the two liquids. Oil and water are two liquids that are immiscible and they have different densities. If a mixture of oil and water is placed in a separating funnel, the oil with a lower density floats on the water which has a higher density. The method by which the separating funnel works is as follows: - The tap is opened to allow the liquid with the higher density (water) to run into the container below. - The tap is closed as the liquid interface almost reaches it and the first container is replaced with a second. - The tap is opened again to allow a very small amount of the liquid with the lower density (oil) to run into the container and then closed. The contents of the second container are discarded. ### Paper Chromatography Paper chromatography is used to separate a mixture of dissolved substances which are colored or can be coloured, and which will travel through a material, e.g. filter paper. The substances are separated based on: - how soluble they are in the solvent used - how strongly they are attracted to the paper. Many inks and food colourings are mixtures of two or more dyes, which can be separated by paper chromatography. The method by which paper chromatography works is as follows: - A drop of the dye mixture is placed 1 cm from the bottom of a rectangular piece of absorbent paper, e.g. filter paper. The paper is then hung in a beaker containing solvent so that the lower edge is below the surface of the solvent and the dye is above. - The solvent moves up the paper and dissolves the dyes in the mixture, carrying them with it. However, the different dyes travel up the paper at different rates. - The dyes that are the most soluble in the solvent and least attracted to the paper travel the fastest and the furthest distance. - The dyes that are the least soluble in the solvent and most attracted to the paper travel the slowest and the least distance. - Once the solvent has completed its movement up the paper, the paper is allowed to dry. There will be a pattern of different coloured dyes on the paper, each one representing a part of the mixture. This pattern is known as a chromatogram. ## A2.5 Extraction of Sucrose From Sugar Cane Sugar cane was first introduced to the Caribbean by the Dutch in about 1625 and has been a very important part of its economy ever since. The production of sucrose from the sugar cane plant is an industrial process that makes use of several separation techniques. The processes involved in the separation of sucrose from sugar cane are as follows: - The sugar cane stalks are harvested, transported to the factory, cleaned and cut into small pieces by revolving knives in the shredder. - The pieces are then crushed in the crushers as water is sprayed on them to dissolve the sugar present. This produces cane juice and cane fibre, or bagasse. The bagasse is taken to the boiler furnace where it is burnt to supply heat for the boilers. - The cane juice, which is acidic and contains impurities, enters the clarifier where precipitation occurs. The juice is heated and calcium hydroxide is added which neutralises any acids in the juice and causes the impurities to precipitate out, i.e. they are converted into larger, insoluble particles. - The juice then moves into the rotary filter where continuous filtration takes place to remove the insoluble impurities. This produces factory mud and clarified juice. The factory mud is returned to the fields. - The clarified juice, which is about 85% water, goes into a series of three or four boilers or evaporators where vacuum distillation occurs. These boilers are under successively lower pressures so that as the juice passes from one to the next it boils at successively lower temperatures. In this way the water evaporates and the juice is concentrated but not charred or caramelised by the boiling process. The juice from the last boiler is a thick syrup containing about 35% water. - The thick syrup moves into the crystalliser where crystallization takes place. Here the syrup is evaporated until it is saturated with sugar. As soon as the saturation point is exceeded, small grains of sugar, called ‘seed’, are added to serve as nuclei for the formation of sugar crystals. As the crystals form, the remaining syrup becomes thick and viscous and is called molasses. The mixture of crystals and molasses forms massecuite. - The sugar crystals and molasses in the massecuite are then separated by centrifugation in the centrifuges. Each centrifuge contains a perforated basket. The massecuite is placed in the basket and this revolves at high speed. The molasses are forced out through the holes in the basket and are collected in the outer drum of the centrifuge. The sugar crystals remain behind in the basket. - The damp, unrefined sugar crystals are collected and dried by being tumbled through heated air. ## Key Concepts - Matter can be classified into pure substances and mixtures. - A pure substance is composed of only one type of material. It has a fixed, constant composition, its properties are fixed and constant and the component parts cannot be separated by physical means. - Pure substances can be classified into elements and compounds. - An element is a pure substance that cannot be broken down into any simpler substances by any ordinary chemical or physical means. - A compound is a pure substance that contains two or more different types of element which are bonded together chemically in fixed proportions and in such a way that their properties have changed. - A mixture consists of two or more substances combined together in varying proportions. Each component retains its own independent properties and has undergone no chemical reaction with any other substance in the mixture. The components of mixtures can be separated by physical means. - Mixtures can be classified into homogeneous mixtures and heterogeneous mixtures. - A homogeneous mixture is a mixture in which the properties and composition are uniform throughout the sample and the components cannot be distinguished from each other. - A solution is a homogeneous mixture consisting of two or more components, one of which is usually a liquid. The particles in a solution are extremely small. - A heterogeneous mixture is a non-uniform mixture. - A suspension is a heterogeneous mixture in which minute, but visible, particles are dispersed in another substance, usually a liquid. - A colloid is a heterogeneous mixture in which the particles of one substance are dispersed in another substance, usually a liquid. The dispersed particles are smaller than those of a suspension, but larger than those of a solution. - Solubility is a measure of the mass of solute which will saturate a fixed mass of solvent at a given temperature. The unit of solubility is g per 100 g water. - A saturated solution is a solution that contains as much solute as can be dissolved at a given temperature. The solubility of most solids in water increases as temperature increases. - A solubility curve is a graph showing how the solubility of a solute varies with temperature. - The physical properties of the component parts of a mixture are used to determine the method by which a mixture can be separated. Possible separation methods are filtration, evaporation, crystallization, simple distillation, fractional distillation, a separating funnel and chromatography. - The extraction of sucrose from sugar cane involves the following processes: crushing, precipitation, filtration, vacuum distillation, crystallization and centrifugation.