2. Separations

    While most people think of chemistry as mixing two substances together, and then seeing something happen, a great deal of chemistry is involved in making use of various physical and chemical properties to "unmix" the substances. "Purification" or "isolation" are two other names for such separations. Letıs think of just four "mega-bucks" examples - (1) isolation of iron from iron ore on the way to making steel, (2) the making of quick lime for making concrete, (3) the historical making of lye from wood ashes leading to making soap, and (4) the isolation of antibiotics from cultures of molds and bacteria. ((1. iron from "rust" [Fe3O2], FeO, and pyrite [FeS = fool's gold]; 2. limestone to CaO; 3. Na2O and K2O to NaOH and KOH.))

    Today we shall be separating substances from both mixtures and solutions. What is the difference between a mixture and a solution? To make stainless steel, you add carbon and chromium to the vat of molten iron. Is steel a mixture or a solution? What if you stir a scoop of NaCl into a bucket containing a scoop of sand, do you get a mixture or a solution? What if you added a pinch of ferric chloride (whatıs its formula?) to 100 ml of water? Is that a solution or mixture?

Here are some harder "thought experiments" for you:

  1. You stir a teaspoon of starch into some cold water. Mix or Solution?
  2. You heat "a" to boiling, stir, cool back to room temperature. Mix or Sol?
  3. You add the requisite amount of water to a bag of concrete mix. M or S?
  4. Is cowıs milk a mixture or a solution?
  5. You add a small amount of a solution of baking powder to the ferric chloride preparation you made above. What happens?

    (A helpful hint for the future: as we progress through these techniques, think of a word or two for each, and make a quick-reference list of those words inside the back cover of your lab notebook. That way, in the future when you have an isolation or purification or separation problem, a quick look at the list should provide you with a few candidate methods you might try.)

    Today we shall devote our time to separating small molecules using what most people would call pure chemistry, and to separating larger molecules, which falls more into realm all by itself - not exactly physical chemistry. Most of these "larger" molecule techniques find themselves being used by biochemists and molecular biologists - and, to be sure, textile or polymer chemists.

    We will thus become acquainted with a number of different separation techniques, and many of the chemical separations are, in principle, similar to those you will use for "1" and "2", below. Most will be very obvious, and we shall breeze through those quickly using only up-front demonstrations. Others will be less obvious, and you will need to think and experiment a bit at your bench. But once you understand it, you will find it very difficult to forget it for the remainder of your lives.

  1. Rapid separation of lima beans, black-eyed peas, table salt and sand
  2. Separation of the sand from the NaCl.
  3. Separation of the iron from "e", above.
  4. Separation of silver from solution
  5. Separation of "b", above (directions)
  6. Separation of two food colors that have both been added to some water. (directions)
  7. Separation of the pigments in leaves. (directions)
  8. Thought experiments only based on your previous knowledge from watching CSI and other programs or readings:
    1. Identification of your DNA from that of someone else.
    2. Separation of ethanol from water.

    Plan of attack: In each case, you look at the following list of properties and decide which ones might be uniquely useful to use. Then move to the second list below and decide which types of equipment you might be useful.

Some PROPERTIES of substances that are used for separations:

Useful EQUIPMENT:

Discussion of Qualitative vs Quantitative Chemistry
and
The Importance of the Methods Used in this Lab Exercise

Separation of Dissolved Starch and Salt

  1. Per Group: Weigh out 2 grams of "soluble starch", add it to 200 ml of distilled water, and heat it to boiling with constant stirring. Pour the starch solution into a large beaker to liberate the pan for another group's use. Add 1 gram of NaCl; swirl to dissolve.
  2. Test this for a positive starch and positive chloride tests. You test half for starch by using a drop of tincture of iodine (KI3), and see if you can figure out how to test for chloride!
  3. Obtain a liter graduated cylinder, about a foot of dialysis ribbon, an air-pump (shared with another group), and some air tubing.
  4. In distilled water, soak your dialysis ribbon for a minute or so. With wet fingertips, wiggle the end of the ribbon, and you will soon discover it is not a ribbon but rather a tube. Pour distilled water through the tubing to open it its entire length.
  5. You are going to make a sausage of the tubing. Tie a knot about 1 cm from one end, and then another knot about 1 cm above that.
  6. Insert the tube of a funnel into the top end of the tubing, and pour your cooled starch/salt solution into the dialysis tubing - leaving about 3 inches of the tubing unfilled. Double knot that end to complete the starch/salt sausage. Tie a string "handle" to the tubing between one of the pair of end-knots. Tie the other end of the string to pencil. Rinse the sausage with distilled water.
  7. Put 500-700 ml of distilled water into the graduated cylinder. Note exactly how much water is there!
  8. Do a starch and chloride tests on another sample of the distilled water.
  9. Lower the sausage into the water leaving the pencil outside the cylinder, and note the total volume in the cylinder. (What's the volume of the sausage? Record it!)
  10. Install a bubbling air-tube down to the bottom of the cylinder to promote vertical mixing and massaging of the dialysis tubing. Allow to bubble for about an hour.
  11. Pull out the air-tubing, note the total volume. Any loss is due to what?
  12. Next lift out the sausage. What's the volume of liquid in the cylinder? Thus what is the volume of the sausage? Record all these numbers in your lab notes.
  13. Test the liquid in the cylinder for starch and chloride.

You should have noticed several things: was the starch able to escape the sausage? What about the NaCl? Did the volume of the sausage change over the course of this reaction period? And finally, how can you account for these various changes?

Separation of pigments and food colors