High School Biology-Chemistry SMILE Meeting
19 March 2002
Notes Prepared by Porter Johnson
Pushpa Bahl (Collins HS) -- Handout: Paper
Chromatography of Food Coloring
Pushpa explained that paper chromatography is used to separate individual substances in a mixture.
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.
"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/
|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|
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
Ben 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 creaseWe 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:
(high P) / \ (high P)
/ X \
(Blow through crease)
Ken Schug (IIT Chemistry) --
More on the Bernoulli Principle
Ken 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