High School Biology-Chemistry SMILE Meeting
19 March 2002
Notes Prepared by Porter Johnson

Pushpa Bahl (Collins HS) -- Handout: Paper Chromatography of Food Coloring
explained that paper chromatography is used to separate individual substances in a mixture.

"Chromatography is a process which separates the substances in a mixture. The relative sizes of molecules or the charges on ions influence the rates of separation. Gas chromatography separates mixtures of gases and volatile liquids, using metal columns which are thin and very long. Column chromatography uses a liquid medium to separate complex substances such as vitamins, proteins, hormones and DNA. Gel electrophoresis is a form of chromatography used to separate fragments of DNA by their size and electrical charge. Paper or chalk chromatography is used to separate the components of dyes, inks, food coloring and other mixtures by their molecular size and their solubility in polar solvents-- such as water and alcohol."
Source Lawrence Livermore National Laboratory Science & Technology Education Program: http://education.llnl.gov/

The plan is to investigate various colors of food coloring, to see whether any contain multiple components with different colors. We put about 1/2 ml of each color into a ceramic dish with little wells, from which we were able to make "spots" using inexpensive plastic "bulb" type droppers. [Ken Schug said that toothpicks dipped into each color are a great way to produce "tight" spots.]  We laid out a piece of special chromatographic paper, and put pencil marks on it for four types of food coloring [see picture]. Then we put the paper into a beaker containing 0.5 cm of water, being careful to keep the paper from touching the sides of the beaker to reduce "smiling" of the lines, and making sure that only the bottom of the paper, but not the spots themselves, was submerged.
We made the following determinations about the various food colorings:
 Yellow  Predominantly yellow, with a little orange mixed in
  Green A mixture of yellow and blue, as expected
   Blue Blue with a little (unexpected) pink
   Red Predominantly red, with a little pink
It also appears from the values of Rf, which is defined as the distance from the spot to the center of the color track divided by the distance traveled by water on the chromatogram, that the "blue" in blue and green food coloring and the "yellow" in yellow and green food coloring are probably the same dyes. In other words, the food coloring factory probably uses relatively few basic colored compounds in various proportions to produce all the colors.

This was a terrific PA, Pushpa!! It was lots of fun, very easy and safe, and showed lots of real science. Ken continued with a discussion of how the migration rate of a spot depends upon its relativity affinity for the paper and water (its "partition coefficient"), and how you can do the same type of experiment by rubbing M&Ms and Skittles on the paper as sources of pigments.

For additional details see the websites  http://www.rpi.edu/dept/chem-eng/Biotech-Environ/CHROMO/chromintro.html and http://www.kids.union.edu/fsnChromatography.htm.

Ben Stark (IIT Biology) -- Bernoulli Principle
took a strip of paper, folded it in half, and held it vertically at the crease.  He then blew (X) through the crease:

           Paper held at crease
(high P) / \ (high P)
/ \
/ X \
/(low P)\
/ \
(Blow through crease)
We noted that the two halves moved together as he did this. The Bernoulli Principle explains this as well as the flight of birds and airplanes. According the Bernoulli Principle:
  1. A  moving fluid has lower pressure than a stationary fluid.
  2. The faster the movement of the fluid, the lower the pressure in the fluid.
He laid out a cross-section profile of an airplane wing on the floor, about 10 feet long, with a "rounded" leading edge and a "sharp" trailing edge. For streamline flow, and given the shape of the wing, air must move more rapidly above than below, resulting in greater pressure below than above the wing.  (See http://www.eskimo.com/~billb/miscon/wing2.gif.)  To illustrate this point, Barbara Pawela and Karlene Joseph walked along the profile of the wing, starting together at the leading edge and arriving together at the trailing edge. But Karlene's speed (along the top of the wing) was larger than Barbara's (along the bottom of the wing).  Karlene had to walk faster than Barbara to reach the end of the walk at the same time.  At "walking speed" (say, 5 miles per hour), the pressure differential is rather small, but at a take-off speed (say,  150 miles per hour) the pressure differential is large enough to lift the airplane.  Very interesting, Ben, and just how do birds take off?  For additional information on airplane flight, see the NASA webpage Bernoulli versus Newtonhtttp://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html.

Ken Schug (IIT Chemistry) -- More on the Bernoulli Principle
held up a thread spool and a small piece of cardboard (index card) for us to see.  He alternately blew air into and sucked air out through the hole in the spool, so we would understand what he was about to do next. With the spool held vertically, he held the card flat against the bottom of the spool, and centered it on the opening of the hole. When he sucked air through the hole and released the card, it remained stuck on the bottom of the spool. We anticipated that. But then he blew down through the hole and released the card. Surprise! The card remained stuck to the bottom of the spool as long as Ken could blow air through the hole. When he ran out of breath, the card dropped off! How come?  The explanation for this feat is, once again, the Bernoulli Principle. [See http://home.earthlink.net/~mmc1919/venturi.html.]  He also mentioned that one may reduce air pressure in a flask by connecting it with tubing to the side of a pipe through which there is a fast flow of water.  Because of the lowered water pressure inside the pipe, the pressure at the hole is reduced, and the flask may be partially evacuated.  Very good, Ken, but just how do we learn to fly like the birds?

Notes taken by Ben Stark