CHEMISTRY: Chromatography

Transcription

1 CHEMISTRY: Chromatography By Darby Sloss and Marianne Smith Edited by Anne Starace Abstract Chemistry is an important part of our lives. Kitchen Chemistry uses some common household items to introduce a few basic chemical concepts. This module demonstrates chromatography, separation, of ink. Keywords Chemical, molecule, chromatography, compound, element, capillary action Funded by the National Science Foundation and the University of Nebraska

2 Content Standards K History & Process Standards K Skills Used/Developed: 2

3 Table of Contents I. OBJECTIVES 4 II. SAFTEY 4 III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS 4 IV. LIST OF MATERIALS 4 V. INTRODUCTION 5 VI. PROCEDURE 6 VII. FREQUENTLY ASKED QUESTIONS 8 VIII. TROUBLESHOOTING 9 IX. HANDOUT MASTERS 9 X. REFERENCES 9 Everything in the world is made of CHEMICALS! One of the most amazing things about chemicals is the way they can join together to make different new chemicals. That s why chemicals can make up all the zillions of different things in the world! WonderScience magazine 3

4 I. Objectives: Students will: -grasp the basic concept of chromatography which is the separation of a mixture. -see that black ink is composed of many colors put together. -be able to explain capillary action. II. Safety General Safety Never allow anyone to taste the chemicals used in this module. Isopropyl alcohol is highly flammable. Do not use near hotplates, open flames, etc. If isopropyl alcohol gets onto one s skin, one should thoroughly wash one s skin with soap and water. If isopropyl alcohol gets into one s eyes, one should thoroughly flush one s eyes with running water. If isopropyl alcohol is spilled on the floor, it should be wiped up using a dry towel and then washed with soap and water. Be careful not to get the chemical on yourself. III. Level, Time Required and Number of Participants Level: K-Adult Time Required: 20 to 30 minutes Number of Participants The activity is appropriate for groups of people. IV. List of Materials 4 black, felt tip pens from different manufacturers Beakers ( mL sizes work well) 50% Isopropyl Alcohol /Water Solution (available at drug stores) Paper Clips Magic Markers Straws 4

5 V. Introduction The periodic table organizes and includes the all the known elements. For example, sodium (Na) and nitrogen (N) are elements. The smallest amount of an element is an atom. Only a few elements, called the noble gases, consist of individual atoms that move about independently of one another. In other words, the noble gases do not usually react with other elements. These elements are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Notice that all of these elements are in the same column. Other elements will usually react with each other, forming molecules. A chemical reaction is the breaking or forming of chemical bonds. A molecule is a group of atoms (2 or more) joined together by chemical bonds. Molecules move as a single unit. Molecules may consist of identical atoms, as in oxygen (O 2 ), or different atoms, as in water (H 2 O). The terms molecule, compound and chemical can be used interchangeably in this activity. A chemical bond is the force that holds two atoms together. For example, water is a molecule composed of one oxygen atom and two hydrogen atoms. The hydrogen atoms are bound to the oxygen. Chemists use several different notations to represent a chemical bond. The most common is a line between two atoms. The molecular structure of water using lines to represent bonds is depicted in Figure 1. O H H Figure 1 Chromatography Chromatography is the separation of a mixture into two or more of its components. The technique derives its name from its original use for the separation of colored plant pigments. Chromatography is based upon the partitioning of a mixture s components between the 5

6 stationary phase and the mobile phase. Kitchen chemistry separates the dyes that compose black ink using paper chromatography. The individual dyes in the ink are the components of a mixture. The stationary phase is the filter paper and the mobile phase is the rubbing alcohol (isopropyl alcohol - water solution). Different colors of ink are made of different molecules. These different molecules of the ink are separated into color bands because they interact differently with the paper and the rubbing alcohol, as the filter paper absorbs the rubbing alcohol. Chromatography does not alter the structure of the molecules in the mixture, i.e. no chemical bonds are broken, nor formed. Chromatography simply allows one to separate (or partition) the components (molecules) of a mixture. Chemists use chromatography all the time because it is often easier to work with solutions consisting of only one sort of molecule than a mixture of many molecules. Chemists must also use chromatography because often the molecules they want to use and study are mixed up with other molecules. For example, if a chemist wanted to study the phenolic acids which are found in soybeans, he/she would not do experiments with whole soybeans. He/she would go through a long process of purification and chromatography to extract the phenolic acids from the soybean and separate each of the acids. Capillary action Capillary action is the movement of a liquid through a paper towel, up the roots of a plant, and in this activity, up a piece of filter paper. Capillary action occurs with water because there are hydrogen bonds between molecules of water. Alcohols also have hydrogen bonds between molecules. These bonds cause the molecules to stick together. Hydrogen bonds make the surface tension and capillary action of water possible. VI. Procedure Chromatography 1. Pour 50% isopropyl alcohol - 50% water solution into a beaker until it is about 1 cm deep. 2. Bend a straw over the top of the beaker and attach with a rubber band if nessessary. 3. Place a black dot (0.25 cm diameter) on a piece of filter paper for the chromatography demo. 4. Hand out the pen you used to make the first dot as well as 2 or 3 other black pens to volunteers. 5. Ask the volunteers to add ink spots beside the original ink spot. The size of the ink spots should not exceed 0.25 cm, otherwise, the experiment may fail. 6. Stand the piece of filter paper in the beaker. Use a paper clip and the straw to help the paper remain upright. The bottom of the paper must touch the alcohol, but the dots on the paper must not touch the alcohol. 7. Allow the solution to absorb up the filter paper. This occurs by the process of capillary action. This process takes 5-10 minutes. 8. When you can see color separation, remove the paper from the beaker and ask the volunteers to describe what they see. Black ink is made of several colors put together. 2 cm Piece of filter paper used for chromatography showing the location of the original ink spot. Add volunteer ink spots to the right and left of the original. 6

7 Different colors of ink are made from different molecules which interact differently with the paper and alcohol. The color patterns of dots made by the same marker are very similar, so you can tell which two dots are made from the same marker. Upon drying the filter paper will be safe for the audience to handle. Here is the set-up: Here is a picture of filter papers after ink has been chromatographed. Here is a closer picture: 7

8 Human Chromatography (This activity can be done while you are waiting for the filter paper to absorb the liquid.) 1. Form two parallel lines of 8-12 volunteers each. The lines should be far enough apart for two people to walk between. 2. One person (not in the lines) is going to shake hands with each of the volunteers. The second person is not going to shake hands with anyone. 3. This works best if the second person walks slowly. This is not a race. 4. When the second person is one person from the end of the line, stop the experiment. 5. You will notice that the second person has walked by nearly everyone and the first person has only passed 2, 3 or 4 pairs of people. The two people have been separated, i.e. chromatographed. Relating this demonstration to the previous one: Different inks go different distances up the filter paper because they like the paper molecules to different degrees. If a certain ink likes the paper, it will not move as quickly through the paper as an ink that does not like the paper as much. VII. Frequently Asked Questions Q Where can I get Isopropyl alcohol? A. At a drug store. 8

9 VIII. Troubleshooting If chromatography does not work, try using different pens and placing dots lower down on the filter paper so that enough alcohol solution will reach them. IX. Handout Masters X. References For general chemistry information: Chemistry by Steven S. Zumdahl For more detailed information about chromatography: For a variation of this demonstrations see the Kitchen Chemistry Script 9