Experiment 4: separation of a mixture of solids
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® cups2 Sheets of paper Small spoon or stirrerSmall saucer Piece of plastic wrap or a plastic baggieCrushed ice Paper towelsFrom LabPaq: GogglesMagnet 100-mL BeakerFunnel Burner standGraduated cylinder Burner fuelDigital scaleFrom Experiment 4 Bag: Mixture of solidsCircular filter paper Plastic weighing dishDiscussion and Review: Many materials are actually mixtures of pure substances.How to separate mixtures into their component substances is a frequent problem forchemists. The essential distinction between mixtures and “pure” substances is whetheror not they can be separated by physical means. Physical means of separation arethose techniques that utilize the physical properties of a substance such as meltingpoint and solubility.In this experiment you will separate a mixture of four substances: sodium chloride(NaCl, table salt); benzoic acid (C6H5COOH, a common food preservative); silicondioxide (SiO2, sand); and iron (Fe, powder and/or filings) into pure substances. Theseparation will be accomplished by utilizing the unique properties of each material andtheir differences in water solubility.Solubility is defined as the amount of the solute that will dissolve in a given amount ofsolvent. The extent to which a substance dissolves depends mainly upon the physicalproperties of the solvent and of the solute and to some extent upon the solvent’stemperature. Sodium chloride (table salt) is an ionic substance that dissolves readily incold water. Benzoic acid is a polar covalent compound that is only slightly soluble incold water but is very soluble in hot water. The Handbook of Chemistry and Physicsreflects that the solubility of benzoic acid in water is 6.8 g/100ml at 95°C and only 0.2g/100ml at 10°C.Hands-On Labs SM-1 Lab Manual51Table of Solubility of NaCl in Water at Various Temperatures:Temp in degrees C 0 10 20 30 40 50 60 70 80 90 100Grams/100 mL H20 35.7 35.8 36.0 36.3 36.6 37.0 37.1 37.5 38 38.5 39.2From the above information and data, it is apparent that sodium chloride and benzoicacid can be dissolved at different temperatures in water. Further, we know that sand isnot soluble in water. That leaves the iron filings or iron powder, but we also know thatiron filings can be easily removed by a magnet. With this information we can now devisea plan or flow chart on how to separate the mixture.PROCEDURES: Some sections of this experiment may require several days fordrying and evaporation. Plan your time accordingly. Remember to completely read allinstructions and assemble all equipment and supplies before beginning your work.Before you start the actual separation, challenge yourself. Think about and prepare aflow chart on how the four substances might be separated. Then read the instructionsand 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 anddetermine the gross mass of the total mixture and weighing dish. Compute thenet mass of the mixture: this is equal to the gross mass of the weighing dish withthe 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 andset it aside.E. Wrap a small square of clear plastic over the magnet. Remove the ironpowder/filings by passing the magnet closely over the surface of the entiremixture. 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 plasticand allow all the iron to fall onto the paper. Weigh and determine the net mass ofthe iron powder/filings.Hands-On Labs SM-1 Lab Manual522. Separating out the Sand:A. Put the remaining mixture, containing sand, benzoic acid, and table salt into yourbeaker and add 50 mL of distilled water.B. Set up the beaker stand and burner fuel and heat the beaker of solids and waterto near boiling. Stir the mixture to make sure all soluble material dissolves. At thispoint, the benzoic acid and the sodium chloride should have dissolved and beenextracted 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. Thisassures 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 acoffee cup or similar container that is large enough to hold your paper cup ofbenzoic acid and salt solution. Make sure the ice bath level is higher than thesolution level but low enough so that no additional water can pour into thesolution cup.F. Place the cup containing the water solution of benzoic acid and salt into the icebath. Observe the benzoic acid crystallizing out of the solution as it cools. Setthis water bath assembly aside until the next section.G. Heat the sand in the beaker over low heat until the sand is completely dry. Sandhas a tendency to splatter if heated too rapidly. The possibility of sample loss canbe 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 towelsand 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 toa 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).Hands-On Labs SM-1 Lab Manual532) Set the paper cup inside a slightly larger coffee cup orsimilar container to give the paper cup support andprevent it from tipping over when you add a funnel.3) Fold a sheet of filter paper in half and then in half againas illustrated. Weigh it.4) Open one section of the folded filter paper as shown inthe bottom illustration.5) Place the opened filter paper into the funnel and thefunnel into the paper cup supported by the coffee cup.B. Remove the paper cup of salt and benzoic acid crystalsfrom Step2-F from its ice bath. Fill a graduated cylinder with about 5 mL ofdistilled 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 anycrystals 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, slowlypour 2 to 5 mL of chilled distilled water around the inside surfaces of the filterpaper-lined funnel to make sure all the sodium chloride has been removed fromthe benzoic acid crystals.E. After all the liquid has drained from the funnel lay the filter paper containing thebenzoic acid crystals on folded layers of paper towels and put this someplacewhere 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 thebenzoic acid crystals.NOTE: A very small amount of benzoic acid may not have precipitated but rathermay 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 ofsodium chloride solution someplace where it will not be disturbed while the waterevaporates. Depending upon the humidity in your area this might take severaldays. When all the water has completely evaporated only sodium chloride will beleft in the paper cup.Hands-On Labs SM-1 Lab Manual54B. Weigh the paper cup with the dried salt crystals inside and then subtract theweight 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 recordboth their masses in grams to at least 1 decimal place, (i.e., .1 or 1/10th of agram) and their percentage of the total mixture.Sample data table:Grams Percent of mixtureIron 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 %Questions:A. How did your proposed procedures or flow charts at the beginning of this experimentcompare to the actual procedures of this lab exercise?B. Discuss potential advantages or disadvantages of your proposed procedurecompared 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.
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