Spotting Chromatography

Lauren Lindelof Leith
Avalon School
Saint Paul, MN 55104
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Summary

In this mini-lab students will use chromatography to compare the mobile phase and the stationary phases of different inks used in marking pens. They will also directly determine the polarity of the solvents and inks. Finally, the students will use their calculated information to solve a crime.

Learning Goals

Goals
· Students will understand the polarity of solutions and solutes will determine solubility.
· Students will calculate the ratio between the stationary phase and the mobile phase and determine the Rf values of different inks in different solvents.
· Students will be introduced to chromatography and its applications.

Concepts
A general rule of polarity in dissolving substances is – like dissolves like. In this case a polar solution will dissolve polar inks and the non-polar solution will dissolve non-polar inks.
In chromatography the distance the ink moves in dependent on its partition and absorption properties with respect to the solvent and the paper.

Terms
stationary phase, mobile phase, eluting solution, solvent, solubility, retention or retardation factor (Rf)

Context for Use

This activity will be used toward the beginning of the school year in a general high school chemistry class. It will be used after describing mixtures, compounds, physical and chemical properties, polarity, and to begin the chapter on solubility. This mini lab is designed to take place within one hour. It requires very little equipment and only goggles for safety equipment. It will give students practice in writing up lab reports and proper chemical handling and clean up.

Description and Teaching Materials

Students will work in groups of 2-3

Materials/group:
2-Chromatography paper (6 X 12 cm)
1- circle filter paper (10 cm)
2 -250 ml beakers
1-metric ruler
1-pencil

Materials/classroom
7 different black pens and one green pen - numbered 1-8
distilled water
acetone
25 cent coin

Students will be introduced to a possible problem a chemist may have to solve, such as how to determine which compounds (methyl orange) are in a solution.

Procedure
Preparing the paper
Using pencil, students will draw a line 2 cm up from one of the longer sides of the chromatography paper. They will divide the paper into 8 equal columns perpendicular to the line they drew. They will then put one dot in each column on the line. Each dot will be from a different pen. Have them record the number of the pen used in each column. They will do this twice.
Separating the inks
They will then fold the paper in half and place one in a beaker with 1 cm of water and one in a beaker with 1 cm of acetone. They should let the solutions run until they stop.
Measuring
Students will then take out the papers and mark the top of the solutions on each paper and the bottom of the first color separation. They will then measure the distance to the solvent boundary from the first line. Secondly they will measure the distant to the nearest color from the first line. The ratio of the color to the solvent is the Rf value for the ink. They will record this in their lab report.
Analysis
Students will determine the polarity of each solvent based its ability to dissolve.
They will also be given a sample of pen used in a ransom note and be asked to identify the pen that was used to write the note.
Class Discussion
Write the molecular model of water and acetone on the board to determine their polarities. Show the students how it is used to separate unknown metal ions in solution as another application.

Extra (just for fun)
For those students that complete the mini lab early, they could trace around the quarter on the round filter paper. They can then use 2 different pens to make alternating dots around the circle. Then they have to make a hole in the center and roll a piece of filter paper and place it in the hole to use it as a wick. If they allow the inks to separate in a solvent they will have a nice design. websites (Microsoft Word 23kB Aug3 09)

Teaching Notes and Tips

Students will be graded on their individual lab reports. Grades are determined on data collection, analysis and conclusion

Assessment

Students will be graded on their individual lab reports. Grades are determined on data collection, analysis and conclusion

Standards

MN Chemistry Standard
2. Chemical and physical properties of matter result from the ability of atoms to form bonds.
Benchmark 9C.2.1.2.7. Explain the role of solubility of solids liquids and gases in natural and designed systems

References and Resources