University of Virginia
Physics Department

Chromatography

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

Motivation for Learning

Demonstration

Materials

Procedure

  1. Prepare each of the following mixtures in a clear plastic cup:
    sand and water
    salt and water
    sand and iron filings
  2. Ask students if there is a way to separate each of the mixtures without changing any of the substances into new substances.
  3. Using input from students, demonstrate that each of the mixtures can be separated fairly easily.
  4. Separate the sand and water mixture by filtering.
  5. Separate the salt and water mixture by evaporating the water.
  6. Separate the sand and iron filing mixture using a magnet. (For ease in removing the iron filings from the magnet, cover the magnet with plastic wrap.)
  

Background Information

All matter can be placed into two categories - pure substances and mixtures. Pure substances are either elements or compounds, and mixtures can be classed as suspensions, solutions, or emulsions. The substances in a mixture are not chemically combined, so therefore they can be separated through some physical process.

Chromatography is a useful method for separating the substances in certain mixtures. There are many physical processes which may be used to separate mixtures, but chromatography will not be as obvious to students as others such as filtering, evaporation, or using a magnet. Because the substances which make up certain mixtures dissolve at different rates, these substances can be separated by chromatography.

Chromatography was developed in the early 1900's when a Russian scientist, Mikhail Tsvett, was searching for a way to separate the hidden red and yellow pigments from green leaves. Like many students, Tsvett knew that the colored pigments were present in green leaves, but remained hidden until the chlorophyll broke down in the fall, allowing the leaves' true colors to appear. He crushed green leaves into a thick solution, and discovered that when this solution was mixed with a certain powder, different colors appeared in specific areas of the powder. The hidden colors in the leaves separated to different areas, depending on how easily they were absorbed by the powder.

Years later, two British researchers, Martin and Synge, improved upon Tsvett's procedure and developed a process called paper chromatography. They were able to use this technique to separate the amino acids in a protein, and were awarded the Nobel prize in chemistry in 1952 for their work in paper chromatography.

Many mixtures, such as inks and food colorings, consist of two or more dyes. To separate the dyes, a small portion of the mixture is placed onto an absorbent material, such as filter paper. A liquid called a solvent is absorbed onto one end of the filter paper. The solvent soaks the filter paper, dissolving the ink. If a dye in the ink dissolves well, it will move along the paper at the same rate as the solvent. If another dye in the ink doesn't dissolve as well, it will not move as far.

In a short time, a pattern of colors will appear on the filter paper. Each color will be a single dye that was in the ink. The distance that a particular dye traveled on the filter paper is a physical property of that dye. Dyes that are most soluble will travel fastest and will therefore reach the highest level on the filter paper. Dyes that are less soluble in the specific solvent will travel more slowly and not reach as far on the paper.

Commercial chromatography paper may be purchased for this activity, but laboratory filter paper may be cut into strips, as can coffee filters or even pieces of paper towels or tissues. Coffee filters are particularly inexpensive and the number used by each group of students will not need to be restricted. Various types of inks may be used, but the type solvent must be matched to the ink. Water-based inks such as washable markers and overhead projector pens work well with water as a solvent. If permanent type inks are used, such as Magic Markers or ball point ink, another solvent, such as isopropyl alcohol or acetone must be used. Students may be allowed to try any markers or pens that they happen to have with them, or they may be told in advance to bring a variety of pens and markers.

Students will color a small dot (about 2 mm in diameter) near the end of their filter paper. There should be several mm between the dot and the end of the paper. A very small amount of solvent will be placed in the beaker. The paper will be placed in the beaker so that the end of the paper is in the solvent, but the dot is not touching the liquid. Students may place several filter papers in their beakers at once, but they should be cautioned against the solvent becoming colored from any of the dyes. If this happens, the solvent should be replaced. Filter papers may be anchored by folding the edge down over the rim of the beaker.

Use appropriate cautions when using solvents other than water. Be sure to remind students that acetone is the active component in nail polish remover and will damage nail polish or acrylic nails.

 

Student Activity

To print out the Student Copy only, click here.

Materials

 

Procedure

Part I

  1. Place enough water in the beaker to just cover the bottom.
  2. Using a water-based ink, make a dot on the filter paper a couple of cm from the bottom.
  3. Place the filter paper into the water so that the end of the paper is wet, but the dot is not touching the water.
  4. Watch the colors separate as the solvent (water) is absorbed and travels up the filter paper. This may take several hours.
  5. Repeat using several other water-based inks.
  6. Use a toothpick to place a drop of food coloring on a piece of filter paper and repeat.

Part II

  1. In a second beaker, place a small amount of another solvent, such as isopropyl alcohol or acetone.
  2. Repeat the above process, but make your dots with permanent inks. Follow the same procedure as in Part I.
  3. Again, watch as the components of the inks separate and travel up the paper.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

 

For a copy of Assessment questions only, click here.

  1. Describe what happened when the filter paper with the water-based ink was placed in the water.

  2. Describe what happened when the filter paper with the permanent ink was placed in the solvent.

  3. Is separating an ink by chromatography separating by physical means? Why or why not?

  4. Why do some colors rise farther on the filter paper than others?

  5. List some other ways that some mixtures can be separated.