Expt 002 -- Separation

Description

A mixture of iron filings, salt, benzoic acid, and sand is provided. The task is to design a scheme to separate the components into four separate containers.

Safety

Wear goggles and apron. Remember that hot objects and cold objects look alike; hold a finger near a previously heated object to sense for heat before touching the object. Do not ingest chemicals. Have all plans for experiments approved by the instructor. Wash hands after the experiment.

Procedure

  1. The following equipment is available for your experiment.
  2. Using this equipment, devise a procedure for the separation of the four substances.
  3. Design experiments, and write procedures for the designs. Ask the instructor to approve each experimental design.

Questions

  1. Explain why the sodium chloride is contaminated with benzoic acid.
  2. Suppose sawdust from wood less dense that water were incorporated into the mixture. Suggest a way to revise the procedure so as to be able to separate the sawdust.
  3. Suppose copper metal replaced iron. Explain how this would have affected the separation procedure.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 002 Separation

A mixture of iron filings, salt, benzoic acid, and sand is provided. The task is to design a scheme to separate the components into four separate containers.

  1. The following equipment is available for your experiment.
    Solubility (g/100g H2O)
    Substance Cold (0 ºC) Hot (95 ºC)
    Iron 0 0
    NaCl 35.7 38.8
    SiO2 0 0
    Benzoic Acid 1.7 68
  2. Design a procedure for the separation. Give your procedure to the instructor.
  3. After turning in the procedure, view the movies to see a sample separation technique. Compare it to the one you proposed.
  4. Answer the questions.

Curriculum-

Instructors regard this as an early activity in the course. It also fits in very well after the temperature effects upon solubility are introduced.

Safety-

Time-

Teacher Preparation: 10 minutes

Class Time: 50 minutes (Ask students prepare and hand in procedure one period, and do the experiments the next period.)

Materials-

Disposal-

The solids used in the experiment may be disposed of with ordinary solid trash. The liquids may be discarded at the sink with large amounts of water.

Sample Procedure-

  1. Examine the sample.
    !!!Click here to See Picture.
  2. Turn a plastic bag inside out. Grasp a magnet in one hand, and slip the bag over that hand. Spread the mixture on a piece of paper. Pass the covered magnet through the mixture, tapping often to dislodge nonmagnetic particles.
    !!!Click here to See Movie.
  3. Only the iron filings adhere to a magnet.
    !!!Click here to See Picture.
  4. Invert the bag, restoring the original inside-outside relationship. The filings are now contained in the bag with the magnet on the outside. Remove the bag from the magnet. Either seal the bag, or transfer the filings to a tube and stopper the tube.
    !!!Click here to See Movie.
  5. Place the mixture (less iron) into a small test tube. Add a small amount of distilled water, say 3 mL for every mL of mixture. Heat and stir occasionally.
    !!!Click here to See Movie.
  6. Heat until all but the sand dissolves.
    !!!Click here to See Movie. Click |> to advance slides.
  7. While the mixture is heating, set up a filter cone with a second small test tube to catch the filtrate. Filter the hot solution. Allow the filter to drain. Transfer any remaining solid to the filter paper by washing with several small portions of hot water.
    !!!Click here to See Movie.
  8. The filter cone contains sand; the hot filtrate contains benzoic acid and sodium chloride dissolved in water.
    !!!Click here to See Picture.
  9. Cool the filtrate in a bath of cool tap water. Once cooled, place the tube of filtrate in an ice-water bath.
  10. Wash the sand in the filter cone five times with 5-mL portions of hot distilled water. Allow the filter cone to drain. Fold a paper towel in half. Remove the sand from the filter cone to the folder paper towel, and set aside to dry.
  11. When the tube in the ice bath is cooled to ice temperatures, fine, needle-like white crystals appear. Allow to stand for five minutes after the crystals have formed keeping the temperature very cold.
    !!!Click here to See Picture.
  12. Set up a filter cone with a second small test tube to catch the filtrate. Filter the solid. Transfer the solid to the filter paper by washing with several small portions of water.
    !!!Click here to See Movie.
  13. The filter paper contains benzoic acid. The filtrate contains dissolved sodium chloride. Retain this filtrate.
    !!!Click here to See Picture.
  14. Fold a paper towel in half. Wash the solid 3 times with 1-mL portions of ice cold distilled water, discarding the washing filtrate. Remove the benzoic acid from the filter cone to the folder paper towel, and set aside to dry.
  15. The filtrate contains dissolved sodium chloride. This can be recovered by heating the solution to evaporate water. Another approach is to set the filtrate aside in an open Petri dish for several days. The water evaporates, and cubic crystals of NaCl, often large ones, form. Salt recovered by evaporation is usually contaminated with brown ferric salts from the iron filings.
  16. Transfer the dried sand to a tube and stopper. Transfer the dried benzoic acid to a tube and stopper. If the Petri dish evaporation procedure has been employed, examine the crystals. Use the spatula to gather the cubic crystals into a small test tube. [Some benzoic acid needles also will be present.] Return the iron filings in the plastic bag or in a stoppered test tube.
    !!!Click here to See Picture.

Answers-

Q1. Explain why the sodium chloride is contaminated with benzoic acid.
A1. Even though the concentration is low, the solution removed from the solid benzoic acid crystals still is saturated with benzoic acid. This remains with the sodium chloride in the absence of some additional separation procedure.
Q2. Suppose sawdust from wood less dense that water were incorporated into the mixture. Suggest a way to revise the procedure so as to be able to separate the sawdust.
A2. When the water is first added, the sawdust floats. A mechanical separation can be accomplished by skimming the floating sawdust, and then washing the sawdust with water.
Q3. Suppose copper metal replaced iron. Explain how this would have affected the separation procedure.
A3. The magnetism supports a nifty separation procedure. If copper is used, a much more complex approach is needed. The difference in density between copper and sand is large. These two can be separated together in a first step by adding hot water. The solid mixture can be swirled quickly, allowed to settle for a moment, and the top poured off. This will tend to pour of the sand and keep the copper. (A better procedure would be to use a dense liquid to separate the solids. Sand has a density in the range of 2.2-2.6 g/cmL. The sand would float, and the copper sink. For example, 1,1,2,2- tetrabromoethane, C2H2Br has a density of 2.964 g/mL.)

Literature Data-

Substance Cold (0 ºC) Hot (95 ºC)
Iron 0 0
NaCl 35.7 38.8
SiO2 0 0
Benzoic Acid 1.7 68

The solubility data for benzoic acid were taken from "The Merck Index," 8th Edition, Merck & Co., Rahway, NJ 1968.

Solubility data for NaCl were calculated from equations from "Solubility in Inorganic Two-component Systems", M. Broul, J. Nyvlt, & O. Sohnel, Elsevier, Amsterdam, 1981. The data are plotted below.

The equation for the mole fraction of NaCl is of the form log x = A + B/T + C log T where x is the mole fraction of NaCl, T is the absolute temperature, log is the base-10 logarithm function, and parameters specific to NaCl are A = -5.89477, B = 199.6575, and C = 1.707143.

Key Words 1-

solubility, magnetism, solute, solvent, separation, filter, filtration, temperature dependence

Elements-

Fe C Na Cl Si O H