Summary

In this activity, students will learn about solubility, saturation, polarity, and intermolecular forces through chromatography techniques.

Grade Level

High School

NGSS Alignment

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

• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data

Objectives

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

• Explain the concept of chromatography.
• Identify the solute, solvent, and medium used.
• Understand the concept of polarity.
• Comprehend difference between polar and nonpolar, as well as the progression from one to the other.
• Identify the polarity of each of the three components in chromatography.
• Understand the concept of “like dissolves like” (polar solutions can dissolve other polar solutions and nonpolar solutions can dissolve other nonpolar solutions) through understanding the concept of intermolecular forces (IMFs).
• Explain that substances are not just polar or nonpolar; there is a range of polarity based on the strength of the substance’s IMFs.
• Infer why certain colors spread more than others.
• Interpret why isopropanol is used in this activity instead of water.

Chemistry Topics

This activity supports students’ understanding of

• Solution
• Solute
• Solvent
• Solubility
• Saturation
• Molecular compounds
• Polar covalent and nonpolar covalent bonding
• Molecular Polarity
• Intermolecular forces

Time

Teacher Preparation: 25 minutes

Lesson: 45 minutes

Materials

• White t-shirt
• Pipet (disposable pipet preferred)
• Rubber band
• Permanent markers of different colors
• Rubbing alcohol (isopropanol)
• Receptacle to collect excess isopropanol (like a beaker)
• Paper towels

Safety

• Students should always wear safety goggles and aprons 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.

Teacher Notes

• My school pays for me to buy each student a t-shirt. If yours does not, ask each student to bring in one. As long as it is mostly white, the activity will work.
• Student must use at least four colors to be able to analyze the possible differences.
• Designs can be as simple as making dots of color, being sure to saturate the ink on the t-shirt, but it can be as complicated as tying the t-shirt with rubber bands in the same fashion as real tie dye. Then students can color on the t-shirt while it is all tied up, really saturating it with the ink. Once the alcohol has been added, the outcome looks very similar to a real, normal tie dyed t-shirt.
• It is not about how much surface you can cover with the ink. It is about how much ink you can saturate in any one spot. More color in a particular spot, more spreading of color.
• Students can be as creative as they like. Some can draw pictures; some can write words; others just imitate the example that I show them with dots of color to create colorful pinwheels that look like the back of CDs.
• When applying the alcohol to the t-shirt, point the pipet with alcohol in the direction you want the alcohol to travel.
• Do not use the marker to color on wet alcohol as it tends to dry out the marker very quickly.
• Any receptacle to catch the excess alcohol can be used. I use beakers since they are readily available in a chemistry classroom.
• Be sure to use a rubber band to pull the t-shirt taut when applying the alcohol, otherwise the color will not spread well.

• Misconceptions
  • Many students use the words “polar” or “nonpolar” even when they are not referencing molecular compounds.Molecular compounds are held together by covalent bonds.Be sure they understand that the term “polar” is short for “polar covalent” and “nonpolar” is short for “nonpolar covalent”.
  • Solubility of the solute is based on that substance’s IMFs using the concept of “like dissolves like.”Don’t let students just quote this phrase.They should be able to explain what it means: Polar solutions can dissolve other polar solutions and nonpolar solutions can dissolve other nonpolar solutions through understanding the concept of intermolecular forces (IMFs).
  • All molecules are not just fully polar or fully nonpolar.There is actually a progression or a range of polarity based on the strength of the molecule’s IMFs.Some can be mostly polar while some are only slightly polar.Some can be nonpolar while some are only slightly nonpolar.Students have a hard time recognizing this concept and might need help to reach this conclusion.