Section A Principles of Chemistry Mixtures and their Separation Pure substances are made up of one kind of material with consistent properties, This therefore means that it cannot be divided into parts by physical means as it is all made up of the same thing. Pure substances can be elements or compounds Elements are always pure substances, while compounds are chemical combinations of two or more elements and can also be pure Examples of elements include Nitrogen, Oxygen and Gold. Examples of Compounds include water (H2O) and Sodium Chloride What is a Mixture? Mixtures are a random blend, containing various types of two or more pure substances, which have not chemically reacted with each other. Mixtures can be physically separated (filtering, crystallising, evaporating, distillation and chromatography. The pure substances that mix keep their own physical and chemical properties – a mixture doesn’t have its own unique properties. Types of Mixtures Homogenous • Homogenous mixtures are evenly mixed, meaning that you cannot see the different parts. • Examples: saltwater, kool-aid Heterogenous • Heterogenous mixtures are unevenly mixed meaning that you can see the different parts. • Examples: chocolate cookie, pizza Differences Between Pure Substances and Mixtures The main difference between pure substances and mixtures is that pure substances cannot be physically separated while mixtures can be. A pure substance, any compound or element, cannot be separated into different atoms by physical methods. A chemical reaction must occur to separate a compound into its different atoms/elements. Remember that pure substances are compounds and elements (made up of the same atom or same molecule respectively) while mixtures are an assortment of different substances put together. Solutions, Colloids and Suspensions The size of the particles has a great effect on properties of a mixture. Mixtures get classified as solutions, suspension and colloids based on particle size. Solutions • When substances dissolve and form a homogenous mixture, the mixture is called a solution. Therefore, the solution is completely uniform throughout. • Whatever substance there is more off is called the solvent. The other is called the solute. E.g. Kool-aid drink – the solvent is the water and the solute is the kool-aid. Properties of a Solution • They do not separate into layers over time • If they are poured through a filter, none of the substances will get trapped. • Light passes through them. All of the particles in a solution are too small to become separated, filter out or scatter light. Colloids • A colloid is a mixture in which the dispersed particles are intermediate in size between those of a solution and a suspension. • Because the dispersed particles of a colloid are not as large as those of a suspension, they do not settle out upon standing • Examples include peanut butter, pudding, whipped cream, fog Properties of Colloids • Do not separate into layers • Particles will not become trapped by filter • Scatter light. Colloids and suspensions appear cloudy, unlike solutions which are much clearer. Colloids and solution will not separate into layers; suspensions will. Property Solution Colloid Suspension Particle Size Very small (less than one nanometre in diameter) Greater than that of a solution but they are not visible to the naked eye (between 1 and 1000 nanometres in diameter) Large enough so that they are visible to the naked eye (greater than 1000 nanometres in diameter) Type of Mixture Homogenous Heterogenous Heterogenous Appearance Generally transparent Usually opaque, some are translucent Opaque Can the components be separated by filtration? No No Yes Do the components separate out after the mixture has been standing for a while? No No Yes Transmission of light Transmits light appearing transparent Will scatter light Does not transmit light; it is opaque. Activity 1. What is a compound? 2. What is the difference between a homogenous mixture and a heterogenous mixture? 3. Classify each of the following as a solution, a suspension or a colloid: a) mayonnaise b) chalk dust in water c) fog d) white vinegar 4. Solutions, suspensions and colloids are example so mixtures. Explain the differences between these three mixtures in terms of particle size, sedimentation when left to stand and transmission of light. 1. Competently distinguish among the following: a)Elements b)Compounds c) Mixtures 2. Competently contrast Homogenous and heterogeneous mixtures 3. Outline separatory techniques for the following types of mixtures: a. Suspensions bSolutions Separating Mixtures • At home, we separate mixtures all the time. For instance, we strain fruit juice and filter tap water to remove impurities. • Since the components of a mixture are not chemically combined they can be separated using physical methods. Filtration • Filtration is a method used to separate suspended solids from a liquid (such as dirt from a suspension is dirt/soil and water) • 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 part that stays behind in the filer paper is called the residue. • The part of the mixture that passes through the filer paper s called the filtrate. 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. The method is not suitable if the solid to be collected s decomposed b heat. Evaporation is useful to obtain the sodium chloride from a sodium chloride solution. Like evaporation, crystallisation 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. Crystallisation is used if a hydrated solid containing water of crystallisation s required. Water of crystallisation is water that is incorporated with in the crystalline structure. Crystallisation Simple Distillation • Simple distillation is used to obtain a pure solvent from a solution • 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 Simple Distillation Fractional Distillation • Fractional Distillation is used to separate miscible liquids where the components of the liquid mixture have boiling points that are close together. A mixture of ethanol (boiling point = 78oC) and water (boiling point = 100oC) • 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. Separating Funnel • A separating funnel is used to separate a mixture of liquids that are immiscible and have different densities. • A separating funnel is a container that has a tap at the bottom, • 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. Paper Chromatography • Paper Chromatography is a technique used to separate, purify or identify substances. This method involves the use of a stationary phase and a mobile phase. • The substances are separated based on : • How soluble they are in 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 water molecules attached to the chromatrogarphy paper form the stationary phase and the solvent moving through the paper is th emobile phase Note the following about the process The stationary phase will tend to sopt he components of the mixture from moving, while the mobile phase will carry them along the paper. The components of the mixture will beheld to the stationary phase and will dissolve in the mobile phase to different extents. The components of the mxture wil therefore move along the paper at different rates. Activity • What is the main difference between using distillation and evaporation as a separation method? • For each of the following mixtures, explain how you would separate them into their component parts: • A mixture of salt, black pepper and water • A mixture of oil, sugar and water.
