Summary

In this lab, students will separate the component pigments of a water-soluble black marker using paper chromatography.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations

Objectives

By the end of this lab, students should be able to

• Separate component pigments found in a marker.
• Identify the component pigments of a black marker.
• Calculate the Rf value of the component pigments.
• Explain how chromatography can work to separate the components of a solution.

Chemistry Topics

This lab supports students’ understanding of

• Solutions
• Solubility
• Polarity
• Separating Mixtures
• Intermolecular Forces

Time

Teacher Preparation: 15 minutes
Lesson: 60 minutes

Materials (per pair of students)

• Coffee filter paper
• Water
• 125mL or 250mL beaker
• 1 black or brown marker (vis a vis black or brown dry erase markers work best)
• Pencil
• Ruler
• Calculator
• Scissors

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Students should be careful when using the scissors, do not point sharp ends toward anyone.

Teacher Notes

• These videos might be a helpful way to engage students with this topic:
  • CSI Chromatography
  • Paper Chromatography

Background information:

• Solubility: Not all substances can dissolve in one another. We can use information about type of bonding and molecular shape to predict whether one substance is capable of dissolving in another. The general rule of thumb is that “like dissolves like”. The phrase, “like dissolves like”, refers to similarities in intermolecular forces. Intermolecular (between molecules) forces are the forces which attract one molecule to the next. These forces are different in different types of molecules.
• Ionic compounds have electrostatic forces caused by oppositely charged ions. The larger the charge, the stronger the force.

• Covalent compounds have a variety of intermolecular forces that are determined by their molecular shapes and polarity.



• Polar molecules have uneven distribution of electrons (charge) across the molecule making one side more negative and one side more positive. The molecule itself remains neutral (has no charge) but behaves like a small magnet (dipole).

*Fluorine is more electronegative than hydrogen so bonding electrons shift toward fluorine. This makes the hydrogen end slightly positive and the fluorine end slightly negative.



Nonpolar molecules have an even distribution of electrons (charge) across the molecule most of the time. Their strength of attraction is due mainly to the movement of electrons (Dispersion forces or van der Waals forces) and increases as the number of electrons in the molecule increases. There are no significant charges or magnetic forces in these molecules overall.

*Carbon is more electronegative than hydrogen so the bonding electrons shift toward carbon. Because all four bonds are the same and the electrons are shifting toward the center, there is an even distribution of electrons and no polarity.

When mixing substances together the interaction of their intermolecular determines whether they will dissolve. “Like dissolves like”, means that if their intermolecular forces are similar they will dissolve, if they are not similar then they will not dissolve.

Polar dissolves polar: Both HF and water are polar and just like magnets they attract each other with their opposite poles and are they capable of dissolving each other.

• Nonpolar dissolves nonpolar: Both methane, CH₄, and carbon dioxide, CO₂, are nonpolar, so they will dissolve in each other.

• Polar dissolves ionic: Water is polar and has magnetic properties. NaCl is ionic and has charged ions. Because polar molecules have dipoles (like a magnet) they can attract ions (charged particles) because opposite charges/poles attract. This allows most ionic compounds to dissolve in polar compounds.

• Polar will not dissolve nonpolar: Methane, CH₄, is nonpolar and water is polar. They do not have similar intermolecular forces and are not attracted to each other. Methane (nonpolar) will not dissolve in water (polar).

• Nonpolar will not dissolve ionic: Methane, CH₄, is nonpolar and sodium chloride is ionic. There is no dipole (magnetic forces) in methane to attract and pull apart the ions in NaCl. Therefore, methane (nonpolar) cannot dissolve NaCl (ionic).

• Chromatography is a physical method of separating a mixture into its components. The components to be separated are distributed between two phases, a stationary phase, and a mobile phase. In paper chromatography, the stationary phase is paper, and the mobile phase is a liquid solvent. Separation is based on intermolecular attractions and the polarity of the sample.
• Polarity determines if sample will dissolve in the solvent. (Like dissolves like) and intermolecular attractions hold the molecules together.
• How It works:
• The retention factor, R_f, defined as the distance traveled by a component divided by the distance traveled by the solvent, is calculated for each component.

• Mixture is placed on stationary phase.
• Mobile phase passes through the stationary phase.
• Mobile phase solubilizes the components.
• Mobile phase carries the individual components a certain distance through the stationary phase, depending on their attraction to both of the phases.
• The result is a chromatogram which shows the distance each component travels.

• The retention factor, R_f, defined as the distance traveled by a component divided by the distance traveled by the solvent, is calculated for each component.

• Example, if a component travels 2.1 cm and the solvent front travels 2.8 cm, the R_f is 0.7.
• Sample Chromatogram Obtained Using a Black Vis ´a` Vis Marker.

• Extension Activity: Give each group 3 markers: a primary color (red, yellow or blue) a secondary color (orange, green, purple) and the last color is black or brown. Have students put three spots of color on the starting line, compare the Rf values, accounting for any differences they may observe.