Separating the Components of a Mixture

Transcription

1 Separating the Components of a Mixture Introduction: Mixtures are not unique to chemistry; we encounter them on a daily basis. The food and drinks we consume, the fuel we use in our vehicles, building materials, soil, air, and ordinary water are all mixtures. In fact, very few common materials in the world are pure. Any material made up of two or more substances that are not chemically combined is a mixture. It is not difficult to separate the components of a mixture if you take advantage of differences in physical properties of the components. 1. Sublimation. Some substances change directly from a solid to a gas when they are heated. The reverse process, when the vapor goes back to the solid phase without a liquid state in between, is called deposition. Some solids which sublime are iodine, caffeine, and paradichlorobenzene (mothballs). Dry ice sublimes also. 2 Extraction. This uses a solvent to selectively dissolve one component of the solid mixture. With this technique, a soluble substance can be separated from an insoluble substance. 3. Decantation. This separates a liquid from insoluble solid sediment by carefully pouring the liquid from the solid without disturbing the solid. 4. 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 trap the solid but let the liquid pass through (see diagram in Procedure section). 5. Evaporation. This is the process of heating a mixture in order to remove a liquid that evaporates easily, leaving a dry residue. Purpose: 1. To separate the components of a mixture using physical methods. 2. To calculate the percent composition of a mixture.

2 Procedure: The mixture that will be separated in this experiment contains three components: Naphthalene, C10H8 (does not dissolve in water; sublimes) common table salt, NaCl (soluble in water) sea sand, SiO2 (not soluble in water) Briefly, the process involves 1. Heating the mixture to sublime the naphthalene 2. Dissolving the table salt with water to extract it from the sand by filtration, and 3. Evaporating water to recover dry NaCl and dry sand. A flow-chart summarizes the sequence of steps that you will use. Identify each of the materials present after each step in the boxes. To begin: 1. Label a clean, dry 150-mL beaker with #1 and your initials. Obtain its mass to the nearest 0.01 g. Record this mass and all other masses on the data chart. 2. Obtain a small vial (small glass or plastic container with cap) containing the mixture to be separated and empty all of the mixture into beaker #1. 3. Record the mass of beaker #1 with the mixture.

3 Sublimation: 4. Move all combustible materials away from your work area. Plan the next steps carefully. 5. Place a clean, dry evaporating dish on top of beaker #1 containing the mixture. Place the beaker and evaporating dish on a wire gauze with an iron ring and ring stand assembly as shown. Place ice in the evaporating dish, being careful not to get any water on the underside of the evaporating dish or inside the beaker. 6. Carefully heat the beaker with a Bunsen burner. Vapors will appear in the beaker and solid crystals should collect on the underside of the evaporating dish. CAUTION: NAPHTHALENE IS FLAMMABLE. BE SURE THAT THE FLAME IS KEPT LOW DO NOT LET IT COME INTO CONTACT WITH VAPOR THAT ESCAPES FROM THE BEAKER. TURN OFF THE BURNER EACH TIME YOU REMOVE THE EVAPORATING DISH FOR SCRAPING. 7. After 5 min. of heating, turn off the Bunsen burner. Drain away any water from the evaporating dish. Collect the crystallized solid by scraping it off the dish into a piece of paper towel with a scoopula. We will discard the naphthalene. The edges of the evaporating dish might be hot handle it with crucible tongs if necessary. 8. Stir the contents of the beaker with a glass rod. Add more ice to the evaporating dish and place it back on the beaker. Re-light the burner and continue to heat/scrape it as described above until no more solid collects on the underside of the dish. CONTINUE TO USE CAUTION WITH THE FLAME. 9. Let the beaker cool until you can hold it comfortably. Then obtain the mass of beaker #1 with contents. DO NOT DISCARD THE NAPHTHALENE IN THE SINK. THE TEACHER WILL TELL YOU WHERE TO PUT IT.

4 Decantation and Filtration: 10. Add approximately 25 ml of deionized water to the solid in the beaker. Stir the mixture with a glass rod to thoroughly dissolve the salt. 11. Label a clean, dry 250-mL beaker with #2 and your initials. Obtain its mass. 12. Label a piece of filter paper with your initials, and fold it as shown. Obtain the mass of the filter paper. 13. Assemble the apparatus for filtration. Place the pipestem triangle across the iron ring to hold the funnel. Separate three edges of paper away from one edge to make a cup 14. Position empty beaker #2 under the funnel. Let the stem of the funnel touch the glass. 15. Decant most of the liquid from beaker #1 into beaker #2 (do not spill out any sand). Then transfer any remaining salt water/sand directly into the funnel. Use a small amount of water from your wash bottle to help transfer all of the residue from beaker #1 into the funnel. 16. When all of the liquid has passed through the filter paper, gently remove the paper from the funnel and place it in the drying oven overnight. You will obtain the mass of the sand and filter paper together after the paper is completely dry.

5 Evaporation: 17. Obtain the mass of a clean, dry watch glass. 18. Place beaker #2 and its contents on wire gauze with an iron ring and ring stand assembly as shown. Begin to heat gently with a Bunsen burner. If the contents of the beaker begin to spatter, remove the burner for a few minutes. You may place a watch on top of the beaker to prevent spattering of the contents out of the beaker. 19. When all of the liquid is gone, allow the beaker to cool to room temperature. Obtain the mass of the beaker (and watch glass, if used) and the solid residue. 20. Clean up your lab area and wash your hands. Complete the analysis and questions.

6 DATA: 1. Mass of beaker #1: g 2. Mass of beaker #1 and original mixture: g 3. Mass of cooled beaker #1 and residue after sublimation of naphthalene g 4. Mass of beaker #2: g 5. Mass of filter paper: g 6. Mass of watch glass: g 7. Mass of filter paper and dry sand: g 8. Mass of beaker #2 and dry salt g (including watch glass, if used) ANALYSIS: Show the calculations, even if they seem trivial; use unit labels, work neatly. 1. Initial mass of mixture, before separation: 2. Mass of naphthalene removed by sublimation: 3. Mass of sand recovered by filtration: 4. Mass of salt recovered by evaporation: 5. Express each of the components of the mixture in terms of its percent composition. A percentage is a calculation that expresses part over whole. Use the data you collected to determine what part of the whole each substance contributed. Naphthalene: Sand: Salt: If the sum of the percents is not 100%, explain.

7 QUESTIONS: 1. From an g sample containing sodium chloride, naphthalene, and sand, the following components were recovered: 3.64-g sodium chloride, 1.56-g naphthalene, and 5.92-g sand. Calculate the percentage of each substance in the sample. 2. A student started this experiment with a mixture weighing 2.30 g. After separating the components using the same procedure as you did here, a total of 2.35 g of material was recovered. Assume that all the massing and calculations were done correctly. What is probably causing the apparent increase in mass of the recovered material? 3. The mass of naphthalene in your sample could be determined either by difference (as in this experiment) or by directly weighing the amount of solid scraped off of the evaporating dish. Which method do you think would provide more reliable results? Explain your answer. 4. Study the diagram and chart on pg. 713 of your textbook. Use it to explain how crude oil is separated ( refined ) into simpler components. Does fractional distillation involve physical changes or chemical changes? CONCLUSION: Summarize in a well-written paragraph (3-5 sentences) how your results support the Law of Conservation of Matter.