Chemistry Separation Techniques When scientists investigate samples of chemicals collected in the environment there's often more than one substance present in the sample and the different substances must be sorted out before an effective analysis can even begin. This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmoslessons.com Introduction: Separation Techniques It’s not often that scientists have to go to the front line of a war zone, but that’s what happened recently in Syria. The government there, which is ghting in a civil war, had used poisonous gas against the rebels. But before the United Nations could demand that Syria give up its chemical weapons it had to prove what was in them. That’s where the scientists came in. Travelling in very dangerous areas where they could be shot at any time, the chemists moved through the war zone collecting samples from where the gas had been used. Fortunately they did so safely and got the samples back to the lab. The next job was to analyse the samples to see exactly what gas had been used. They used a two-step technique called gas chromatography–mass spectrometry. The rst step separates the sample into its components. The second step identies the components by identifying signature chemical properties. The scientists proved that the poisonous gas sarin had been used and so the United Nations was able to force Syria to destroy its chemical weapons. Read the full Cosmos Magazine article here. Question 1 List: Have you ever noticed that you're surrounded by both pure substances and mixtures? List three things from around your home or school that you think might be pure substances and another three things that you think might be mixtures. Pure substances Mixtures 1 Gather: Separation Techniques Classifying matter Already today, your body has processed matter in the form of solids, liquids and gases: your breakfast, a drink and the air around you. Understanding what all these types of matter are composed of helps scientists to solve real problems, such as investigating the presence of the deadly toxin, sarin. This requires knowing whether any sample of matter is pure or mixed – and if it is mixed, how to separate out the dierent components. All matter is made up of extremely tiny particles. These particles can be atoms of one type of element or two or more dierent elements that are chemically combined into a compound. A pure substance can be either an element or a compound. For example, gold is a pure substance composed of only gold atoms while distilled water is a pure substance composed of hydrogen and oxygen atoms chemically bonded together to form water molecules. A mixture is a combination of two or more pure substances that can be physically separated. For example, a soft drink is a mixture of water, sugar, carbon dioxide gas and other chemicals. Air is also a mixture, composed of nitrogen, oxygen, carbon dioxide and other gases. Question 1 Question 2 Recall: Only elements are pure substances. Dene: What is a mixture? True A pure substance False A combination of two or more substances that can be physically separated I'm not sure A compound of two or more elements I'm not sure 2 Question 3 Label: Each sphere below represents an atom and each colour represents a dierent element. Some of the atoms are joined together to form molecules. There's no matter in between the particles – just empty space! Identify each sample of matter as either a mixture or a pure substance by dragging and dropping the labels. Two have been done for you. Homogeneous and heterogeneous mixtures We can group mixtures further by classifying them into those that are heterogeneous and those that are homogeneous. 0:00 / 4:54 Credit: ADLC – Elementary Science: Mixtures by ADLC Educational Media (YouTube). 3 Question 4 Notes: Use this space to take notes for the video. Note: This is not a question and is optional, but we recommend taking notes – they will help you remember the main points of the video and also help if you need to come back to answer a question or review the lesson. Question 5 Dene: After watching the video, dene the terms homogeneous mixture and heterogeneous mixture. Question 6 Summarize: Based on information in the video, complete the following table. Mixture Homogeneous Heterogeneous e.g. corn starch and water yes vinegar and water pepper and salt sand and water salt and water Solutions A solution is a homogeneous mixture in which one or more substances (the solutes) are dissolved in another substance (the solvent). Solutions are made up of elements or compounds mixed together at the molecular level. Apple juice is a solution. The juice looks and tastes the same whether it comes from the top, middle, or bottom of the glass. 4 Question 7 Discuss: Do you agree or disagree with the statement "All solutions are mixtures, but not all mixtures are solutions"? Give a reason for your choice. Question 8 Review: Before we look at how various mixtures can be separated, it's important that you're comfortable with how mixtures are classied and how they're distinguished from pure substances. Consider the following terms relating to matter that you've encountered in this lesson: pure substance | mixture | homogeneous mixture | heterogeneous mixture atom | element | compound | solute | solvent | solution Revisit these concepts and then construct a detailed mind map that shows your understanding of how these terms are related. 5 Process: Separation Techniques Gas masks contain sophisticated lters to separate out toxic particles from the air to protect the wearer. Combustion engines, such as those in cars and buses, also contain lters. These lters separate out tiny carbon particles from the exhaust. Separating mixtures It's often important for human and environmental health that the components of a mixture are separated away from each other. As reported in the Introduction, scientists investigated mixtures suspected of containing microscopic particles of a lethal substance called sarin. The combustion engines in cars and buses have been designed to separate out very tiny carbon particles and prevent them from polluting the atmosphere. A similar, more sophisticated, separation system can be found in the gas masks worn by the scientists in Syria. Mixtures can be separated into their dierent pure substance components based on the physical properties of the components. Pure substances have unique physical properties such as dierent density, state of matter, melting point, boiling point and solubility. Scientists need an understanding of these dierent physical properties to be able to distinguish one type of particle from another. Techniques used to separate mixtures rely on these dierences in the physical properties of the components. 6 0:00 / 2:29 Credit: Separating Mixtures by Anthea Barbara (YouTube). Question 1 Notes: Use this space to take notes for the video. Note: This is not a question and is optional. 7 Filtration Filtration can be used to separate insoluble solids from a liquid. It uses a lter. The holes in a lter are usually too small to see, but solid pieces get trapped whereas the liquid and anything soluble pass through. Example: When a mixture of sand and water is poured through lter paper in a funnel, the sand particles remain as a solid residue in the lter paper, while the water is collected in the beaker below as a liquid ltrate. If the water has salt dissolved in it, the salt passes through the lter paper into the ltrate. Filtration separates on the basis of dierences in particle size and solubility. Evaporation Evaporation is a method that recovers solid substances dissolved in a liquid. It recovers the solute from the solution by evaporating away the solvent, but not the solute. Example: Salt can be collected from a salt water mixture by evaporation. The liquid water evaporates to leave the salt behind, since water has a much lower boiling point than salt. Evaporation separates on the basis of dierences in boiling point. Question 2 Explain: What happens to the particles in a solution during the process of evaporation. 8 Simple distillation Simple distillation is an extension of evaporation. It's used to recover a pure liquid substance from a solution. The solution is heated to evaporate the solvent. But unlike simple evaporation this solvent, which is now in gaseous form (a vapour) is collected. It is cooled to condense it back into liquid in a separate container. Example: A mixture of salt and water is heated to just over 100°C in a special distillation ask. 100°C is over the boiling point of water so the water turns into vapour. 100°C is well below the boiling point of salt, so it stays in the ask. The water vapour passes into a cooling tube which condenses it back into liquid water so it can be collected as distillate. The salt has been left behind. Simple distillation separates on the basis of dierences in boiling point. Question 3 Sort: Sequence the following processes into their correct order as they occur during the distillation of pure water from salty sea water. 9 Flotation Flotation can be used to recover a solid or a liquid from another liquid depending on which components sink or oat. Example: If sawdust and sand are stirred into water, the sawdust oats and the sand sinks, because the sawdust is less dense than the water and the sand is more dense than the water. Another example of this is if oil and water are stirred together.