Lesson note 5: Separation of Mixtures Mixtures Many different types of mixtures Examples: - air (gas-gas mixture), - sea water(liquid-solid mixture), - crude oil (liquid-liquid mixture) The mixtures can be separated depending on: 1 1 2 Mixtures Separation Methods: WHY do we want to separate the mixtures? To get pure substances & these pure chemicals are very important & useful How? Give some examples Magnetic attraction Filtration Evaporation Distillation Paper chromatography Crystallization Fractional Distillation Physical methods 3 4
Magnetic Attraction Used to separate magnetic materials from nonmagnetic materials by magnets. Salvaging of iron and steel Iron and steel are recovered from junkyards so that these metals can be recycled since they are limited natural resources. When the magnet is switched off, iron and steel are dropped into a collection bin. The metals are then sold and brought to recycling centres. Used in - food processing - salvaging scrap iron & steel RECAP: Click here for a video to see how heating changes a mixture of sulfur and iron filings. We can use magnetic attraction to separate a mixture of sulfur and iron filings. However, when the same mixture is heated, does magnetic attraction still work to separate the mixture? Why? A powerful magnet is used to separate iron and steel from other materials in a junkyard. Iron and steel are reused in new objects. 5 Food processing We use magnets to remove harmful objects from food. What is Filtration? During food processing, metal pieces may accidentally get into food products. Food is placed in magnetic separators that detect and remove any metal pieces in the food. Removal of metal pieces keeps the food clean and makes it safe to be eaten. The food is further processed before being sold in shops. Used to separate an insoluble solid from a liquid in a solid-liquid mixture residue filtrate 8
A suspension is passed through a filter paper as shown. The filter paper has tiny pores. Uses of Filtration The filter paper has tiny pores that allow the smaller liquid particles to pass through while trapping the larger solid particles. The trapped solid particles that remain on the filter paper is called the residue. Vacuum cleaners Coffee makers Water treatment filter paper The liquid that passes through the filter paper is called the filtrate. Filter jugs are used to prepare fresh coffee. 9 10 Water treatment Raw water from the reservoir is treated with chemicals to kill germs and bacteria. Some parts of our bodies also use filtration to control the types of substances entering and leaving our bodies. Water is filtered through layers of sand beds which contain sand grains of different sizes. Solid silt and sand particles are removed from the water.
Evaporation Used to separate a dissolved solid from a solid-liquid mixture mixture residue Evaporation makes use of the fact that the solvent in a solution can vapourise at any temperature, leaving behind a residue of the solid that was dissolved in the liquid. A solution is heated over a water bath. Salt solution When the mixture has completely evaporated, no water is left behind. (Salt) The solid left behind is the residue. 13 Click here to watch a video on how sea salt is made from seawater in Hawaii. How are the holding ponds containing the seawater designed in order to maximise the yield of salt obtained? Uses of Evaporation Distillation and its applications One survival technique involves learning how to obtain drinking water. In this technique, water from the ground evaporates into water vapour. The water vapour condenses on the cool underside of the plastic sheet, which then collects in the cup. The water in the cup is safe for drinking. To obtain salt from sea water 15
Distillation Used to separate a pure liquid from a solid-liquid or liquid-liquid mixture Distillation involves boiling and condensation. It works on the basis that when a liquid boils and evaporates from a solution, it will condense to form a pure liquid. mixture distillate The mixture is heated in a distilling flask until the mixture boils. To ensure smooth boiling The vapour formed is allowed to escape into the condenser where the vapour condenses. The pure liquid collected from distillation is called the distillate. The distillate is collected in a conical flask. 17 18 Uses of Distillation To make perfumes Desalination plant Paper Chromatography Used to separate small amounts of substances, especially pigments, from mixtures. Also used to separate 2 or more substances that are soluble in the same solvent. Makes use of the fact that different substances dissolve to different extent in a mixture Used to separate mixtures of coloured substances such as ink and dyes 19 20
A spot of ink is applied to a piece of chromatography paper, which is then lowered into the solvent. The solvent dissolves the dyes in the ink. The dyes move up the paper together with the solvent. solvent front (maximum height reached by solvent before the chromatogram is removed) The different dyes in the ink mixture travel at different rates along the paper. (continued on next slide) At the end of the experiment, the different dyes in the ink will be separated on the chromatography paper. The paper with the different coloured spots is called a chromatogram. 21 22 Uses of Paper Chromatography -Determine no. of dyes 3 dyes for A To identify the contents of a mixture E.g. To find the dyes used in ink; To identify food colourings To test the purity of a liquid. 23 A B C D E Pencil Original dye/substance (on pencil line) line
- Determine pure substance(s) - Answer: B - Why isn t there any separation of dye(s) for Substance E? - Answer: E is not soluble in the solvent used A B C D E Pencil Original dye/substance (on pencil line) line A B C D E Pencil Original dye/substance (on pencil line) line Crystallisation A technique used to obtain the pure crystals from a solution Used for substances which will decompose when heated Crystallisation process: 1) A solution is heated over a flame/water bath & left to saturate 2) The hot saturated solution is allowed to cool 3) On cooling, crystal will be formed 4) Crystal can then be filtered off, using filter paper & allowed to dry Crystallisation
Fractional Distillation Water out Used to separate more than two liquids in a mixture of miscible liquids with different boiling points. Oil refineries use fractional distillation to separate crude oil into useful product such as petrol, diesel and kerosene Fractionating column Water in distillate 29 30