EXPERIMENT 4: Separation of a Mixture of Solids Read the entire experiment and organize time, materials, and work space before beginning. Remember to review the safety sections and wear goggles when appropriate. Objective: To become familiar with the separation of mixtures of solids. Materials: Student Provides: Distilled water 2 Coffee cups 2 Small paper or Styrofoam cups 2 Sheets of paper Small spoon or stirrer Small saucer Piece of plastic wrap or a plastic baggie Crushed ice Paper towels From LabPaq: Magnet Funnel Graduated cylinder Digital scale Goggles 100-mL Beaker Burner stand Burner fuel From Experiment 4 Bag: Mixture of solids Circular filter paper Plastic weighing dish Discussion and Review: Many materials are actually mixtures of pure substances. How to separate mixtures into their component substances is a frequent problem for chemists. The essential distinction between mixtures and "pure" substances is whether or not they can be separated by physical means. Physical means of separation are those techniques that utilize the physical properties of a substance such as melting point and solubility. In this experiment you will separate a mixture of four substances: sodium chloride (NaCl, table salt); benzoic acid (C 6 H 5 COOH, a common food preservative); silicon dioxide (SiO 2, sand); and iron (Fe, powder and/or filings) into pure substances. The separation will be accomplished by utilizing the unique properties of each material and their differences in water solubility. Solubility is defined as the amount of the solute that will dissolve in a given amount of solvent. The extent to which a substance dissolves depends mainly upon the physical properties of the solvent and of the solute and to some extent upon the solvent s temperature. Sodium chloride (table salt) is an ionic substance that dissolves readily in cold water. Benzoic acid is a polar covalent compound that is only slightly soluble in cold water but is very soluble in hot water. The Handbook of Chemistry and Physics reflects that the solubility of benzoic acid in water is 6.8 g/100ml at 95 C and only 0.2 g/100ml at 10 C. 50
Table of Solubility of NaCl in Water at Various Temperatures: Temp in degrees C 0 10 20 30 40 50 60 70 80 90 100 Grams/100 ml H 2 0 35.7 35.8 36.0 36.3 36.6 37.0 37.1 37.5 38 38.5 39.2 From the above information and data, it is apparent that sodium chloride and benzoic acid can be dissolved at different temperatures in water. Further, we know that sand is not soluble in water. That leaves the iron filings or iron powder, but we also know that iron filings can be easily removed by a magnet. With this information we can now devise a plan or flow chart on how to separate the mixture. PROCEDURES: Some sections of this experiment may require several days for drying and evaporation. Plan your time accordingly. Remember to completely read all instructions and assemble all equipment and supplies before beginning your work. Before you start the actual separation, challenge yourself. Think about and prepare a flow chart on how the four substances might be separated. Then read the instructions and compare your proposed procedure or flow chart to the one presented here. 1. Separating out the Iron: A. Use your digital scale to determine the mass of your weighing dish. B. Empty the entire mixture of solids from the plastic bag into the weighing dish and determine the gross mass of the total mixture and weighing dish. Compute the net mass of the mixture: this is equal to the gross mass of the weighing dish with the mixture less the mass of just the weighing dish determined in 1-A. C. Spread the mixture into a very thin layer over a full sized piece of paper. D. Cut a second piece of paper into a 10-cm square. Weigh and record its mass and set it aside. E. Wrap a small square of clear plastic over the magnet. Remove the iron powder/filings by passing the magnet closely over the surface of the entire mixture. Repeat several times to make sure you ve collected all the iron. F. Holding the magnet over the 10-cm square of paper, carefully remove the plastic and allow all the iron to fall onto the paper. Weigh and determine the net mass of the iron powder/filings. 51
2. Separating out the Sand: A. Put the remaining mixture, containing sand, benzoic acid, and table salt into your beaker and add 50 ml of distilled water. B. Set up the beaker stand and burner fuel and heat the beaker of solids and water to near boiling. Stir the mixture to make sure all soluble material dissolves. At this point, the benzoic acid and the sodium chloride should have dissolved and been extracted from the insoluble sand. C. Decant (pour) the liquid while it is hot into a small paper or Styrofoam cup. D. Pour another 10 to 15 ml of distilled water into the beaker containing the sand, bring the mixture to a boil, and decant again into the same cup used in 2-C. This assures that any remaining salt and benzoic acid is removed from the sand. E. Make an ice bath by placing a small amount of crushed ice and tap water into a coffee cup or similar container that is large enough to hold your paper cup of benzoic acid and salt solution. Make sure the ice bath level is higher than the solution level but low enough so that no additional water can pour into the solution cup. F. Place the cup containing the water solution of benzoic acid and salt into the ice bath. Observe the benzoic acid crystallizing out of the solution as it cools. Set this water bath assembly aside until the next section. G. Heat the sand in the beaker over low heat until the sand is completely dry. Sand has a tendency to splatter if heated too rapidly. The possibility of sample loss can be reduced by covering the beaker with a small saucer and heating it very slowly. You might accomplish this also by placing the beaker in a warm oven. Alternatively, you may dump the wet sand onto a double layer of paper towels and let it air-dry. H. When the sand is completely dry allow the beaker to cool to room temperature. I. After the sand and any paper towels used are completely dry transfer the sand to a weighing dish of known mass and determine the net mass of the sand. 3. Separating out the Benzoic Acid: A. The benzoic acid crystals from Step 2-F above can be separated out by filtration. Use the following instructions to set up a filtration assembly: 1) Weigh a paper cup and record the weight (mass). 52
2) Set the paper cup inside a slightly larger coffee cup or similar container to give the paper cup support and prevent it from tipping over when you add a funnel. 3) Fold a sheet of filter paper in half and then in half again as illustrated. Weigh it. 4) Open one section of the folded filter paper as shown in the bottom illustration. 5) Place the opened filter paper into the funnel and the funnel into the paper cup supported by the coffee cup. B. Remove the paper cup of salt and benzoic acid crystals from Step2-F from its ice bath. Fill a graduated cylinder with about 5 ml of distilled water and place the cylinder in the ice bath to chill the distilled water. C. Swirl the cup containing the salt and benzoic acid crystals to dislodge any crystals from the sides. Then, while holding the filter paper in place and open, pour the contents of this cup into the filter paper-lined funnel. D. After the sodium chloride solution has fully drained through the filter paper, slowly pour 2 to 5 ml of chilled distilled water around the inside surfaces of the filter paper-lined funnel to make sure all the sodium chloride has been removed from the benzoic acid crystals. E. After all the liquid has drained from the funnel lay the filter paper containing the benzoic acid crystals on folded layers of paper towels and put this someplace where it will not be disturbed while the filter paper and its contents air dry. Depending upon the humidity in your area this can take several hours or days. F. When the filter paper containing the benzoic acid crystals is completely dry, weigh it and subtract the weight of the filter paper to obtain the net weight of the benzoic acid crystals. NOTE: A very small amount of benzoic acid may not have precipitated but rather may have remained in the salt solution and have passed through the filter paper. This will cause a very small experimental error in your final results. 4. Separating out the Salt: A. Remove the funnel from the above filtration assembly and set the paper cup of sodium chloride solution someplace where it will not be disturbed while the water evaporates. Depending upon the humidity in your area this might take several days. When all the water has completely evaporated only sodium chloride will be left in the paper cup. 53
B. Weigh the paper cup with the dried salt crystals inside and then subtract the weight of the cup to get the net weight of the table salt. C. Prepare a data table listing the various components of the mixture and record both their masses in grams to at least 1 decimal place, (i.e.,.1 or 1/10 th of a gram) and their percentage of the total mixture. Questions: Sample data table: Grams Percent of mixture Iron filings.8.8/4 * 100 = 20 % Sand 1.4 1.4/4 * 100 = 35 % Table salt 1.2 1.2/4 * 100 = 30 % Benzoic acid.6.6/4 * 100 = 15 % Total 4.0 100 % A. How did your proposed procedures or flow charts at the beginning of this experiment compare to the actual procedures of this lab exercise? B. Discuss potential advantages or disadvantages of your proposed procedure compared to the one actually used. C. What were potential sources of error in this experiment? Cleanup: Thoroughly clean, rinse, and dry all equipment and return it to the LabPaq. Throw all used paper cups and paper towels in the trash. 54