1 Identify the different types of solutions Identify the different types of solutions Investigate experimentally the effect of structure and temperature on solubility of solids in water Investigate experimentally the effect of structure and temperature on solubility of solids in water Distinguish among solutions, suspensions and colloids Identify suitable separation techniques based on difference in properties of the components of mixtures Distinguish among solutions, suspensions and colloids Identify suitable separation techniques based on difference in properties of the components of mixtures Describe the extraction of sucrose from sugar cane Describe the extraction of sucrose from sugar cane Mixtures and Separations Chapter 8 Learning Outcomes

Copper(II) sulphate Pure substances A pure substance is a single substance not mixed with anything else. Sugar and table salt are examples of pure substances. Pure substances usually exist in crystal form. For example, table salt, sugar and copper(II) sulphate all exist as crystals. A pure substance can be an element or a compound. Sugar crystals Chapter 8 Mixtures and Separations

Pure and impure substances A pure substance has a fixed melting point and boiling point. For example, pure water boils at 100 o C, and melts at 0 o C. b.p. 100 o C m.p. 0 o C Chapter 8 Mixtures and Separations

Impure substances or mixtures A mixture on the other hand, is an impure substance. It does not have a fixed melting point and boiling point. It melts or boils over a range of temperatures.   For example, sea water boils at about 102 o C and freezes at around – 2.5 o C. In general, an impurity lowers the melting point and increases the boiling point of a substance. Chapter 8 Mixtures and Separations

Impure substances or mixtures In nature, most substances are impure. They consist of two or more substances mixed together. Such substances are called mixtures. Examples of mixtures are air, sea water, petroleum and granite rock. Mixtures can be purified or separated by physical methods. Chapter 8 Mixtures and Separations

6 A Mixture Air is made up of different gases: nitrogen, oxygen, carbon dioxide and noble gases such as argon, neon, and helium. Air is made up of different gases: nitrogen, oxygen, carbon dioxide and noble gases such as argon, neon, and helium. The gases in air can be easily separated by liquefaction followed by fractional distillation. The gases in air can be easily separated by liquefaction followed by fractional distillation. Air is an example of a mixture. Air is an example of a mixture. A mixture is made up of two or more substances not chemically combined together. Mixtures and Separations Chapter 8

7 Composition of a mixture A mixture can be made up of two or more elements. E.g. Brass is a mixture of copper and zinc. A mixture can be made up of two or more elements. E.g. Brass is a mixture of copper and zinc. A mixture can be made up of two or more elements or compounds. E.g. Air is a mixture which contains both elements and compounds. Copper atom Zinc atom Air contains both elements and compounds. Chapter 8 Mixtures and Separations Brass Element Compound

8 3.Hold a magnet over the mixture of iron filings and sulphur. The iron filings are attracted by the magnet and can be separated from the sulphur. 4.Heat the mixture of sulphur and iron filings strongly in an evaporating dish. Examine the substance formed carefully. A black solid is formed. It does not look like iron filings or sulphur. Compound of iron and sulphur Mixture of iron and sulphur Experiment Chapter 8 Mixtures and Separations

9 5.Hold a magnet over the black solid. What happens? The black solid is not attracted by the magnet. The iron can no longer be separated from the sulphur in the black solid. 6.When iron filings and sulphur are heated, a compound called iron(II) sulphide is formed. The equation for the reaction is: Fe(s) + S(s)  FeS(s) Experiment Chapter 8 Mixtures and Separations

Homogeneous mixtures A solution is a homogeneous mixture. Characteristics of solutions: When left to stand, the solute and solvent do not separate. Solutes are not visible (particle size ≤ 1 nm). All parts of the solution have the same chemical composition, chemical and physical properties. Chapter 8 Mixtures and Separations

Homogeneous mixtures Solubility of a solution refers to the amount of solute that is able to dissolve in a given quantity of solvent at a fixed temperature and pressure. The rate of dissolving of a solute depends on: Temperature (e.g. Solubility increases with temperature.) Surface area of solute (e.g. Fine sugar dissolves faster than rock sugar.) Rate of agitation (e.g. Stirring the solution increases the rate of dissolving.) Chapter 8 Mixtures and Separations

Heterogeneous mixtures Heterogeneous mixtures have non-uniform compositions which can be mechanically separated. Examples of heterogeneous mixtures are colloids and suspensions. Colloids include fog, smoke, shaving cream, milk, blood, styrofoam, gelatin, and cheese. Suspensions include muddy water, paint and chalk powder suspended in water. Chapter 8 Mixtures and Separations

13 Quick check 1 1.Explain why air is a mixture but water is a compound. 2.(a), State two reasons why a mixture of iron filings and sulphur before heating is a mixture. (b) When the mixture of iron filings and sulphur is heated strongly, state two reasons why the solid formed is a compound. 3.State whether the following diagrams on the right represent elements, mixtures or compounds. Solution Chapter 8 Mixtures and Separations

14 Solution to Quick check Air is a mixture because it is made up of many gases such as oxygen, nitrogen, etc. not chemically combined together. The gases in air can be separated by physical means. Water is a compound because it is made up of hydrogen and oxygen chemically joined together. We cannot separate the gases in water by physical means (a) It is a mixture because the iron and sulphur can be separated by physical means e.g. by using a magnet. Also, no heat or light is given out when the iron filings and sulphur are mixed together. (b) When the mixture of iron filings and sulphur is heated strongly, a compound is formed because a chemical reaction occurs e.g. heat and light are given off. Also, the sulphur and iron in the compound can no longer be separated by physical means such as by using a magnet (a) compound, (b) compound, (c) mixture, (d) mixture Return Chapter 8 Mixtures and Separations

Methods of separating Mixtures Filtration Evaporation Crystallisation Sublimation Simple distillation Fractional distillation Use of separating funnel Chromatography Chapter 8 Mixtures and Separations

Filtration Filtration is the method used to separate an insoluble solid from a liquid. An example would be to separate a mixture of sand and water. The liquid (water) that has passed through the filter paper is called filtrate. The solid (sand) left on the filter paper is called residue. Chapter 8 Mixtures and Separations

Evaporation Evaporation is the method used to separate a solute from a solution. It can only be used for solids which do not decompose under heat e.g. table salt (sodium chloride). It is done by heating the solution in an evaporating dish until it is completely dry. Crystals of salt remain after the solution is evaporated to dryness. Chapter 8 Mixtures and Separations

Crystallisation Crystallisation is the method used to obtain pure crystals from a solution. It is done by heating the solution in an evaporating dish until it is saturated. The hot solution is then allowed to cool. Crystals will be formed on cooling. They are then dried between sheets of filter paper. Chapter 8 Mixtures and Separations

Sublimation Sublimation is the method used to separate a substance which sublimes from a mixture. Examples of substances which sublime are: iodine, ammonium salts, naphthalene and dry ice. The mixture is heated in an evaporating dish covered with a filter funnel. The substance which sublimes will be changed into a vapour and then formed back on the inside of the funnel. To separate ammonium chloride from sodium chloride, this method can be employed. Chapter 8 Mixtures and Separations

Simple distillation Distillation is the method used to obtain a pure solvent from a solution. E.g. pure water from seawater. It is done by heating the solution in a distillation flask and collecting the vapour that boils off. A condenser is used to condense the hot vapour and change it to a liquid. The condenser is cooled by the flow of cold water (in the condenser) from the tap. The pure liquid collected from distillation is called distillate. Chapter 8 Mixtures and Separations

Fractional distillation Fractional distillation is used to separate two or more miscible liquids with different boiling points. A fractionating column and a condenser are used. Example: Mixture of ethanol and water When the thermometer reaches a steady temperature of 78 o C, ethanol is collected. Water is collected when the thermometer reaches a steady temperature of 100 o C. Chapter 8 Mixtures and Separations Separation of ethanol-water mixture by fractional distillation

Use of separating funnel This method is used to separate a mixture of immiscible liquids. The mixture is placed into a separating funnel and allowed to settle into two layers. Example: Mixture of oil and water The tap is first opened to allow the water to drain out of the funnel. After all the water has drained out, another beaker is placed below the funnel and oil is now drained out. Separation of oil-water mixture by using a separating funnel Chapter 8 Mixtures and Separations

Chromatography Chromatography is a method used to separate and identify small quantities of substances. There are different kinds of chromatography techniques, such as gas chromatography, liquid chromatography and paper chromatography. It is based on the principle that different substances have different solubilities in the same solvent. The more soluble substance will get carried along faster by the solvent and move further ahead than the less soluble substances. Chromatography can be used to detect and identify very small quantities of substances. It is used to detect banned dyes used in food colourings, and detect illegal drugs used by athletes during major competitions. Paper Chromatography Chapter 8 Mixtures and Separations

Setup for paper chromatography -The starting line must be drawn in pencil, not ink. This is because unlike ink, pencil lead is insoluble in the solvent and will not interfere with the chromatogram. -The spots of mixtures must be placed above the solvent level, so that they will not immediately dissolve in the solvent, and the solvent has time to slowly move up. starting line solvent solvent front chromatography paper large beaker glass cover -The solvent front must be allowed to move as far up the paper as possible to ensure that all the dyes are separated. Chapter 8 Mixtures and Separations

The R f value of a substance R f value = distance moved by a substance distance moved by solvent front (7 cm) (10 cm) E.g. R f value of red dye = 7 cm = cm Sometimes the chromatogram needs to be sprayed with a locating agent to make the colourless spots in the chromatogram appear. Chapter 8 Mixtures and Separations

Paper chromatography Worked example The chromatogram shows 3 single dyes red, green and blue, and also four unknown samples P, Q, R and S. Identify the dyes present in each of the samples, P, Q, R, S. Solution Sample P contains green dye and one unknown dye. Sample Q contains only blue dye. Sample R contains green, blue and red dyes. Sample S contains green and red dyes. Chapter 8 Mixtures and Separations

27 Identify the various method separation methods used in the diagram below. Chapter 8 Mixtures and Separations

Quick check 1. 1.State the method you will use to separate the following substances. (a) calcium carbonate from table salt (b) iodine from sodium chloride (c) table salt from seawater (d) sugar from sugar solution (e) pure water from sewage water (f) ethanol from beer (g) yellow dye from durian ice cream 2. 2.Explain the following in chromatography. (a) Why is the starting line not drawn with ink or a ball point pen? (b) The spots of samples on the start line should be small. (c) What is the biggest advantage of chromatography? 3. 3.A sample of ink was analysed using paper chromatography (see diagram above). Identify the dyes present in the ink. Solution Chapter 8 Mixtures and Separations

Solution to Quick check State the method you will use to separate the following substances. (a) dissolution, followed by filtration (b) sublimation (c) evaporation (d) crystallisation (e) distillation (f ) fractional distillation (g) chromatography Explain the following in chromatography. (a) Ink contains dyes which could dissolve in the solvent and interfere with the chromatogram. (b) So that they would not smudge the paper. (c) It can detect and identify very small amounts of substances Blue and yellow dyes Return Chapter 8 Mixtures and Separations

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