Separation of a Mixture
Mixtures are not unique to chemistry; you use and consume them on a daily basis. The beverages you drink each morning, the fuel you use in your automobile, and the ground you walk on are mixtures. Very few materials that you encounter are pure. Any material made up of two or more substances that are not chemically combined is a mixture. The isolation of pure components of a mixture requires the
separation of one component from another. Techniques needed to do this separation take advantage of the differences in physical properties of the components. The techniques you will use in this lab include the following:
1. 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 is called deposition. 2. Extraction. This uses a solvent to selectively dissolve one component of the solid mixture. With this technique, a soluble solid can be separated from an insoluble solid.
3. Decantation. This separates a liquid from an insoluble solid 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 is a good filter. Filters allow the liquid to, pass through but not the solid.
5. Evaporation. This is the process of heating a mixture in order to drive off a volatile liquid and make the remaining component dry. The mixture that will be separated in this lab contains three components: naphthalene, C10H8, common table salt, NaCl, and sand, SiO2. The separation will be done according to the scheme shown on the next page and involves three basic steps:
1. Heating the mixture to sublime the naphthalene.
2. Dissolving the table salt with water to extract.
3. Evaporating water to recover dry NaCl and sand.2
NaCl heat to naphthalene
sand 250ºC sublimes
solution wet sand
Weigh a dry 150 mL beaker and record the weight on the report sheet. Obtain about 3 grams of the mixture and using a mortar and pestle, grind the mixture into a fine powder. Transfer about 2 grams of the mixture into the weighed 150 mL beaker. Record the weight of the beaker and mixture.
Place an evaporating dish on top of the 150 mL beaker containing the mixture. Place the beaker with the evaporating dish on a wire gauze with an iron ring and ring stand. Put some ice cubes into the evaporating dish (don’t get any in the beaker). Carefully heat the beaker with a Bunsen burner until vapors appear in the beaker. A solid should collect on the underside of the evaporating dish. After about 10 minutes, remove the Bunsen burner from under the beaker, then carefully remove the evaporating dish and collect the solid by scraping it off with a spatula. Drain any liquid water in the evaporating dish and add a couple more ice cubes. Stir the contents of the beaker with a glass rod. Place the All heating with Bunsen burners or hot
plates is to be done in the fume hoods!3
evaporating dish back on the beaker and apply heat again. Continue heating and scraping off solid until no more solid collects. Place all the naphthalene that you scrape off in a collecting jar on the lab cart. Let the beaker cool to room temperature and then weigh the beaker with the contained solid. Add 25 mL of distilled water to the solid in the beaker. Heat and stir for 5 minutes. Weigh a second dry 150 mL beaker with 2 or 3 glass-bead boiling chips in it and assemble the apparatus shown below where a funnel is supported by a iron ring and the funnel tip is arranged so that the filtrate will run down the wall of the beaker: Fold a piece of filter paper in halves then quarters and place it inside the funnel. Wet the paper with water...
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