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Exploring Intermolecular Forces (11 Favorites)

LAB in Solute & Solvent, Intermolecular Forces, Scientific Method. Last updated October 24, 2019.


Summary

In this lab, students will investigate the idea that “like dissolves like” by discovering which liquids are best suited for dissolving various substances. This can serve as a great inquiry lab prior to teaching intermolecular forces.

Grade Level

High school

Objectives

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

  • understand that every substance has specific requirements in order to be dissolved and that a solvent must be chosen based on the properties of substance to be dissolved.
  • predict which solvent is suitable for dissolving a particular solute, given the chemical structure and intermolecular forces of each.

Chemistry Topics

This lab supports students’ understanding of

  • Intermolecular forces
  • Solubility
  • Solute and Solvent
  • Physical Properties

Time

Teacher Preparation: 20 minutes

Lesson: 50 minutes (one class period)

Materials

  • glass microscope slides (3-5 per group)
  • wax crayons
  • selection of solvents of various polarities, such as water, rubbing alcohol, vinegar, paint thinner, acetone, etc (make sure to include a nonpolar solvent)
  • glass beakers, 50-mL (3-5 per group, depending on how many solvents are used) or disposable plastic pipets for containing the solvents
  • cotton swabs (e.g. Q-tips)
  • paper towels

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.
  • Glass slides may have sharp edges.
  • Students should handle solvents with care, avoiding skin or eye contact.
  • Provide appropriate ventilation and personal protective equipment depending on the solvents selected.
  • Depending on student ability and experience, the teacher may wish to dispense solvents into glass beakers or disposable plastic pipets to avoid spills.
  • Follow local practices for disposing of unused solvents.

Teacher Notes

  • This lesson may be run as a guided inquiry lab or as a directed lab experiment.
  • This lesson is intended as an introductory experience to justify exploration of intermolecular forces. The student will explore the effectiveness of different solvents of varying polarity in dissolving wax crayon. In the directed lab format, the teacher provides most of the experimental procedure; in the guided inquiry format, the students design their own experimental procedure.
  • The activity may be extended to other types of ink, such as permanent marker (Sharpie®) or water-soluble marker (Vis À Vis®).
  • “Like dissolves like” is a phrase often repeated in chemistry classrooms and carried over into everyday experience. But what exactly does this phrase mean? The act of dissolving a material (the solute) in a liquid (solvent) involves attractive forces between the solvent and solute molecules that overcome the attractions among the solute molecules. There are three generally recognized intermolecular forces. From weakest to strongest they are: London (dispersion) force, dipole-dipole force, and hydrogen bonds. The forces between molecules of a given substance depend on the molecular structure of the substance and the distribution of electron density in the molecules. In order for a solute to dissolve in a solvent, the attractive force between the solute and solvent molecules must be sufficient to overcome the attraction of solute molecules for one another. In addition, the solvent molecules must be separated from one another, as shown in Figure 1. This balance of forces leads to the rule of thumb “like dissolves like.”
File
  • Introduction: Ask students if they have had an experience where they wanted to remove or dissolve some kind of residue, such as adhesive, ink, or a stain. After they share experiences, ask if they have heard the expression “like dissolves like.” Explain briefly that the phrase means that one material (solvent) will dissolve another (solute) if the two share certain characteristics. In particular, the two substances should have similar types of intermolecular forces, a topic they will learn about in coming lessons. Tell them that today’s activity will provide them with practical application of this expression.
  • After introducing the topic, explain that students will try to determine if one cleaner (solvent) is better than the others in removing crayon from glass. Give the class the appropriate handout, depending on whether they are doing the directed experiment or the guided inquiry activity.
    • For the directed experiment option, instruct students how to prepare their test specimens (crayon-coated microscope slides) and how to test different solvents and record the results.
    • For the guided inquiry option, depending on the class, either discuss test procedures together as a class or allow each group to develop their own procedure. Students should design a procedure that is quantitative and reproducible, to their best ability. Results can be shared among groups.
  • As an extension to this activity, students might explore other types of ink, such as permanent markers (e.g. Sharpie® permanent markers) or water soluble markers (e.g. Vis À Vis® water-soluble markers). Do they expect the solvent that was best for crayon to be best for marker?

Cross-Disciplinary Extensions

Connect to Math
Students can share results between groups, leading to a discussion of uncertainty and statistics

Connect to Writing
Students may write up their results in a more formal laboratory report using the information recorded in the laboratory worksheet.

For the Student

Directed Experiment Option

Background

“Like dissolves like” is a phrase often repeated in chemistry classrooms and carried over into everyday experience. But what exactly does this phrase mean? The act of dissolving a material (the solute) in a liquid (solvent) involves attractive forces between the solvent and solute molecules that overcome the attractions among the solute molecules. The forces between molecules of any given substance depend on the molecular structure of the substance and the distribution of electron density in the molecules. In order for a solute to dissolve in a solvent, the attractive force between the solute and solvent molecules must be sufficient to overcome the attraction of solute molecules for one another. In addition, the solvent molecules must be separated from one another, as shown in Figure 1. This balance of forces leads to the rule of thumb “like dissolves like.”

File

Objective

In this experiment you will determine which solvent is most effective at removing wax crayon from a glass slide. The most effective solvent is the one that has similar intermolecular forces to the intermolecular forces between the wax molecules.

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Glass slides may have sharp edges.
  • The materials used in this experiment are common household materials. However, these materials can pose hazards if not handled carefully.
  • Avoid skin contact; wash with soap and water and notify your teacher if skin contact occurs.
  • Avoid breathing the solvent vapors and work with adequate ventilation. Do not expose these materials to fire or flames.

Materials

  • selection of solvents
  • glass microscope slides (one for each solvent)
  • glass beakers, 50mL, or disposable plastic pipets, one for each solvent
  • wax crayon
  • cotton swabs
  • paper towels

Procedure

  1. Record your prediction below: which solvent do you expect to be best at removing wax crayon, and why?

Prediction: I predict that ____ will be best at removing crayon from glass because __________.

  1. Coat the middle one-third of one microscope slide with the wax crayon.
  2. Obtain 10-20 mL of a solvent to be tested in a labeled 50-mL beaker or in pre-filled plastic pipets.
  3. Dip a fresh cotton swab into the solvent.
  4. Rub the cotton swab back and forth on the crayon-coated slide for a set number of times. You decide how many. Try to keep the pressure on the swab constant.
  5. Note how effectively the solvent removes the crayon and record your results in the data table.
  6. Repeat steps 2-6 for each solvent to be tested

Data

Slide Number Solvent Type Used Cleaning Effectiveness

Guided Inquiry Option

Background


“Like dissolves like” is a phrase often repeated in chemistry classrooms and carried over into everyday experience. But what exactly does this phrase mean? The act of dissolving a material (the solute) in a liquid (solvent) involves attractive forces between the solvent and solute molecules that overcome the attractions among the solute molecules. The forces between molecules of any given substance depend on the molecular structure of the substance and the distribution of electron density in the molecules. In order for a solute to dissolve in a solvent, the attractive force between the solute and solvent molecules must be sufficient to overcome the attraction of solute molecules for one another. In addition, the solvent molecules must be separated from one another, as shown in Figure 1. This balance of forces leads to the rule of thumb “like dissolves like.”

File

In this experiment you will determine which solvent is most effective at removing wax crayon from a glass slide. The most effective solvent is the one that has similar intermolecular forces to the intermolecular forces between the wax molecules.

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Glass slides may have sharp edges.
  • The materials used in this experiment are common household materials. However, these materials can pose hazards if not handled carefully.
  • Avoid skin contact; wash with soap and water and notify your teacher if skin contact occurs.
  • Avoid breathing the solvent vapors and work with adequate ventilation. Do not expose these materials to fire or flames.

Materials

  • selection of solvents
  • glass microscope slides (one for each solvent)
  • glass beakers, 50-mL or disposable plastic pipets, one for each solvent
  • wax crayon
  • cotton swabs
  • paper towels

Procedure

  1. Record your prediction: which solvent do you expect to be best at removing wax crayon, and why?

    Prediction:
    I predict that ___________________ will be best at removing crayon from glass because _____________________________________________________.

  2. Coat the middle one-third of each microscope slide with the wax crayon.
  3. Obtain 10-20 mL of each solvent to be tested in labeled 50-mL beakers or in pre-filled plastic pipets.
  4. Design an experiment to determine which solvent is most effective at removing wax crayon from glass microscope slides. Be sure to create a reproducible procedure and a reliable way to collect data and determine the results. Record your procedure, data table and results below.

Experimental Procedure (continued by student)

Data and Results

Analysis

  1. Which solvent was most effective at removing crayon? Did it match your prediction?
  2. Do you expect this solvent to be most effective at removing every kind of stain? Why or why not?
  3. If you had a stain to remove, how could you figure out what stain remover is best to use?
  4. Were your results consistent with other groups? If not, why not?
  5. What were some advantages of the procedure you chose?
  6. What were some disadvantages of the procedure you chose?
  7. How could you improve this experiment so you have more confidence in the results?