Separation of a Mixture

Transcription

1 Separation of a Mixture The isolation of pure components of a mixture requires the separation of one component from another. Chemists have developed techniques for doing this. These methods take advantage of the differences in physical properties of the components. The techniques to be demonstrated in this laboratory are the following: Sublimation. This involves heating a solid until it passes directly from the solid phase into the gaseous phase. The reverse process, when the vapor goes back to the solid phase without a liquid state in between, is called condensation or deposition. Some solids that sublime are iodine, caffeine, and carbon dioxide (dry ice). Extraction. This uses a solvent to selectively dissolve one or more components from a solid mixture. With this technique, a soluble solid can be separated from an insoluble solid. Decantation. This separates a liquid from an insoluble solid sediment by carefully pouring the liquid from the solid without disturbing the solid. Filtration. This separates a solid from a liquid through the use of a porous material as a filter. Paper, charcoal or sand can serve as a filter. These materials allow the liquid to pass through but not the solid. Evaporation. This is the process of heating a mixture in order to drive off, in the form of vapor, a volatile liquid, so as to make the remaining component dry. The mixture that will be separated in this experiment contains three different components: sodium chloride (table salt), silicon dioxide (sand), and naphthalene (an organic solid). The separation will be done following the scheme illustrated below. The scheme involves 1. Heating the mixture to sublime the naphthalene 2. Dissolving the salt and filtering out the sand 3. Evaporating the remaining water to dry the sand

2 Procedure 1. Obtain a clean, dry 150mL beaker and record the mass of the beaker. 2. With the beaker still on the balance, carefully transfer approximately 2g of the unknown mixture to the beaker. Record the mass of the beaker and the unknown mixture. 3. Obtain an empty weigh dish and record the mass of the dish. 4. Set up a ring stand with an iron ring and wire gauze. Place the beaker on top of the wire gauze and cover the top of the beaker with an evaporating dish. Fill the evaporating dish with ice. See figure below. 5. Carefully heat the beaker with a Bunsen burner. Start off with gentle heating and increase the intensity until vapors begin to form in the beaker. A solid should start to collect on the underside of the evaporating dish. 6. After 10 minutes of heating, remove the burner from under the beaker and carefully remove the evaporating dish. Remove the solid from the bottom of the dish by scrapping it into the weigh dish from Step Drain any water from the evaporating dish and refill the dish with ice if necessary. With a glass rod, carefully stir the contents of the beaker. Place the evaporating dish back on the beaker and repeat the procedure beginning with Step 5. Continue this process until no more solid collects on the bottom of the dish. 8. Weigh the dish with the collected solids and record the mass. 9. Allow the beaker to cool to room temperature. Weigh the beaker with the remaining solid and use this to calculate the mass of naphthalene that sublimed. Compare this to the mass of naphthalene you collected. 10. Add approximately 25mL of distilled water to the beaker and, while stirring, heat it gently for about 5 minutes. Do not let the liquid boil. 11. Weigh a new clean, dry 150mL beaker along with 2 or 3 boiling chips. 12. Obtain a piece of filter paper and record the mass of the paper. 13. Assemble the equipment needed for gravity filtration as described by your instructor. Use the filter paper from Step 12. Place this second beaker under the funnel of the apparatus to collect the filtrate.

3 14. Pour the warm solution from Step 10 through the filter, being careful not to spill any of the liquid. Make sure to transfer as much of the solid to the filter as possible. 15. Rinse the beaker into the filter using 5-10mL of water; repeat with an additional 5-10mL of water. At this point there should be no solid left in your beaker. Make sure not to use too much water in this step! 16. Place the beaker with the filtrate and boiling stones over a Bunsen burner and begin to heat gently. Control the heating so that the solution does not boil over or begin to splatter. As the water boils off you should begin to see solid sodium chloride start to form. 17. While the beaker with the water and sodium chloride is heating, CAREFULLY remove the filter paper from the funnel and open it up onto a watch glass. Place the watch glass and paper under the heat lamp to dry the sand. 18. Once all of the liquid has boiled off from the beaker, turn off the flame and cool the beaker to room temperature. Once the beaker has cooled record the new mass. 19. Once the filter paper and sand have dried, CAREFULLY determine the mass of the paper/sand combination. 20. Using the data you collected determine the percent yield of material recovered and the percentage of each component in the original mixture using the formulas below.

4 Name: Separation of a Mixture Mass of Beaker Mixture Mass of Mixture Collected Mass of Beaker After Sublimation Sublimed Lost Boiling Chips Mass of Filter Paper Sodium Chloride Mass of Filter Paper and Sand Mass of Sodium Chloride Mass of Sand Totals Mass Recovered % Yield % Napthalene % Sodium Chloride % Sand

5 PRE LAB QUESTIONS Lab 3 Separation of components of a mixture 1. Define sublimation and describe how it differs from evaporation. 2. If someone prepares a glass of iced tea by brewing a seeping a tea bag in water, adding sugar, and ice cubes, which if any of the five techniques described in the background section was used to prepare the tea. 3. The difference in which physical property is used in the process of filtration? 4. Do any chemical changes occur in any of the five separation techniques described in the introduction to this lab? Explain your answer.