Chalk Chromatography

Advance Preparation


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:

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

Materials (per lab station)

  1. Two beakers -- 100 ml and 250 ml
  2. Small metric ruler
  3. Food coloring (green, blue, yellow, red)
  4. Toothpicks
  5. Methanol ( 25 ml per group, per color )
  6. White chalk


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.
  1. 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.
  2. 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.)
  3. 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.
  4. Carefully invert the 250 ml beaker to cover the first beaker. (methanol evaporates quickly so cover the first beaker immediately after adding the methanol)
  5. 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.
  6. 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.)
  7. 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)