Chalk Chromatography

Advance Preparation

Prep Time Needed: Twenty minutes
Safety Precautions: Methanol, if ingested, is poisonous, and it is also flammable. Do not use near an open flame, dispense methanol from small plastic bottles at lab stations. Be sure to evaporate used methanol in fume hood.

Introduction

Chromatography is a method used to separate similar molecules by moving a mixture over a column or strip of material that will absorb the molecules at a different rate -- the larger the molecule, the slower it moves. When molecules are similar in size or absorption rates and are difficult to separate, an electric current is applied to complete the separation. In 1949, the use of chromatography helped identify the amino acids in milk protein. Scientists believed that the three-dimensional configuration of molecules affected their activity during separation. In the early 1950's American Chemist, Linus Pauling showed that proteins have a helical structure. In 1953, English biochemist Frederick Sanger determined the nature of insulin. Also in 1953, American biologist James L. Watson and English biochemist Frances Crick showed that the DNA molecule is a double helix. Gel electrophoresis is a form a chromatography that uses electricity to separate pieces of DNA. The dyes in food coloring are all different sizes. The chromatography process allows smaller molecules to move the greatest distance. Chromatography is presently used in a number of ways:

to match unknown inks, dyes or controlled substances in crime lab investigations.
to match blood types or DNA in body fluids using gel electrophoresis.
to match combustibles with volatile substances in arson cases.
to separate or purify large molecules like vitamins, proteins and hormones.

Class time needed

Maximum of two (45 - 55 minute) class periods, one for the procedure, and one for the analysis of data and questions. If analysis is done as homework, the second day class discussion should take twenty minutes.

Student Objectives

Students will demonstrate safe laboratory techniques during the lab, including proper use and disposal of methanol.
Students willanalyze the components of food coloring by separating the colors.
Students will recognize and identify two or more characteristic properties of the component colors in food coloring.
Students will design a lab to identify inks or dyes.
Students will compare his/her data with that of other class members to compile a class data table.

Materials (per lab station)

Two beakers -- 100 ml and 250 ml
Small metric ruler
Food coloring (green, blue, yellow, red)
Toothpicks
Methanol ( 25 ml per group, per color )
White chalk

Procedure

Depending on the time available, students can run two pieces of chalk in each beaker. They can analyze two food colorngs and then share data to get the third color.

Stand chalk upright in the 100 ml beaker, try scraping the bottom of chalk, or use small amount of clay on the bottom of the chalk to keep it upright.
Pick up chalk and use the broad end of a toothpick to apply 3 small dots of food coloring at equal distances around the chalk. Apply the dots 1.5 cm from the bottom. (use only one color per piece of chalk.)
Place the chalk back into the beaker and carefully pour 25 ml of methanol into the beaker. (methanol is very volatile - have students recap bottle immediately.
Carefully invert the 250 ml beaker to cover the first beaker. (methanol evaporates quickly so cover the first beaker immediately after adding the methanol)
Observe the movement of the dyes in the food coloring as the methanol travels the entire length of the chalk. Remove the chalk as soon as the methanol reaches the top, if it flows over the top, the recorded distances will be too long.
Repeat steps 1 - 5 with two other food colorings. (Do the red food coloring if time permits. It is interesting since it is only one dye, not a mixture.)
Collect all the used methanol in a container in a fume hood. If no fume hood is available, collect the waste methanol in a well-ventilated area.

Analysis Questions

It is important that the student groups finish three food colorings and fill out the data table before answering questions 1 - 4. (Included are some answer guidelines)

Using the information in the data table, describe the results for each piece of chalk. (include the distances too)Students will interpret the data to explain the movement of colors on each piece of chalk, and include the distance each color traveled.
What physical characteristics of food colorings cause them to travel different distances? The relative sizes of the molecules affects their movement, the smaller a molecule the farther it will move.
Were the colors on each piece of chalk visible in the food coloring when you placed it on the chalk? This question asks the students to consider the components of color and the color combinations in food colorings. They will be surprised by the color mixtures.
How do the results from each food coloring compare? Do the same colors always travel the same distance? If not, how close are they? Individual colors should travel he same distance on a piece of chalk. Students will have to compare color mixtures chalk pieces, 1,2 and 3) to decide. i.e., yellow appears in both yellow and green food colorings, and the distance it travels on chalk should be the same.
Predict the results of repeating this experiment with red food coloring. Show your prediction to you teacher, then repeat the experiment with red food coloring. Since the other colors were mixtures, the students will think that red food coloring is a mixture.
Design a lab to identify inks or dyes using chromatography. The students should be able to design a lab procedure to identify the components in dyes or inks. Chalk or paper chromatography would work.
List some other applications for chromatography Applications include those listed in the teacher introduction.
Write a conclusion explaining chromatography and what you learned by using it in this lab. By way of conclusion the students should be able to:

explain the chromatography process.
identify some physical characteristics that allow separation to occur.
list the color(s) in each food coloring.
explain what they have learned.