Summary

In this lab, students will compare and assess the effects of polarity and intermolecular forces of different liquid samples.

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

Objectives

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

• Identify intermolecular forces that will act on a given substance.
• Explain why certain streams of liquids are deflected by a charged object.
• Recognize that increased strength of intermolecular forces results in increased surface tension.

Chemistry Topics

This lab supports students’ understanding of

• Polarity
• Intermolecular Forces
• Surface Tension
• Lewis Dot Structures
• Bonding

Time

Teacher Preparation: ~1 hour

Lesson: 1 hour

Materials

Activity 1 (make 3 sets of the following):

• 3 burets (labeled with contents)
• 3, 250 mL beakers (labeled to match the contents of each buret)
• 3 ring stands
• 3 buret clamps
• Approximately 60 mL of a Non-polar liquid in a buret (examples: cyclohexane, toluene, hexane)
• Approximately 60 mL Water in second buret
• Balloons
• Combs (optional)
• Label tape and marker

Activity 2 (make 3 sets of the following):

• 2 pennies
• 2 beakers (100 mL , 150 mL, or 250 mL)
• 2 pipettes
• Approximately 30 mL Alcohol for penny (choose different alcohol than what was used in activity 1 buret), (examples: propanol, ethanol, isopropyl)
• Approximately 30 mL Water
• Label tape and marker

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.
• Use caution, as Cyclohexane and alcohols are flammable.

Teacher Notes

• Teacher should gather 3 sets of materials for each of the two lab activities, labeling burets and beakers. This may take ~1 hour.
• This lab can be done prior to explaining intermolecular forces and polarity or as a further investigation into intermolecular attractions.
• I find that it is best if you set up 3 stations for Activity 1. Each station will have 3 burets filled up to the top with one of the liquids, 3 beakers and 3 balloons (or combs). I find it best to label the buret with the liquid in it and place a beaker under the buret to catch the liquid also labeled. This way students can pour the liquid back into the buret and the waste is minimal.
• I blow up a balloons in advance and leave one at each station for the students. However, you can allow students to do this to minimize teacher prep time.
• I also set-up 3 stations for Activity 2. Each surface tension station will have 2 pennies, 2 droppers and two labeled beakers. One beaker will be water and the other will be a different alcohol than in the buret used in Activity 1. The pipettes will be put into the beakers so that students can use one pipette for one solution.
• Three stations for each of the lab activities allows for small groups of students to do one activity and then move to the other one. The order of completion of the activities does not matter.
• If time is short you could also use this as a demo, but I find students enjoy it more when they can see the results themselves and then try to figure it out.
• During the activity, I do make sure students are getting the correct results for the streams of water, cyclohexane and alcohol. If they don’t I ask them to do it again. The students should see the biggest bend with water, then alcohol and nothing with cyclohexane. You will find that sometimes the students put the balloon under cyclohexane and the balloon pops. This is because of the interaction of cyclohexane and the rubber. The latex balloon will absorb the cyclohexane and swell causing it to pop. This is because the latex easily absorbs “oily solvents”.
• I make sure students are getting the correct results with the penny activity. The water should be about twice the amount of the alcohol. Sometimes students miss the drops falling off the side of the penny for the alcohols and therefore miscount.
• For the penny lab we do not do a non-polar liquid since it will have even fewer drops than the alcohol, but it is a great discussion to have after the fact to see if students understand this concept.
• This unit on Intermolecular Forces can be tied in too many everyday things the students encounter. I talk to my students about water bugs walking across the water. Everyday foods like ketchup, molasses, honey and viscosity or “thickness” of liquids