Measuring Surface Tension to Investigate Intermolecular Forces

Considering the foundational role intermolecular forces (IMFs) have when trying to explain and understand chemical phenomena, it is likely that this topic is addressed, to various degrees, in the classrooms of many chemistry teachers. Like most concepts in chemistry, this one takes a bit of imagination and critical thinking to fully comprehend and apply when explaining a variety of situations.

Though demonstrating the presence of these forces in a simple and explicit manner can easily be done, I wanted to change how I introduced IMFs a bit this year by focusing on a more data-to-concepts approach. It turned out the web-based Pivot Interactives come through, yet again, with a wonderful investigative activity that helped provide my students with a solid foundation to build knowledge from. Whether you expect your students to explicitly differentiate between IMFs or simply develop a basic understanding of how forces between molecules govern certain properties, this activity can fit a variety of needs while continuing to engage students in the valuable practices of science.

Like many of the activities within the Pivot Interactives library, the beauty of this one comes from its creativity in experimental design and overall simplicity. The knowledge developed from this entire activity stems from an investigation of one property—surface tension. Though I have certainly paid lip service to this property in the past, I had never really thought about having students quantify this in any way. Additionally, there were only a few liquids I had immediate access to or felt comfortable with letting my students use (ex: water and alcohol) that could easily demonstrate differences in surface tension. So, what does Pivot Interactives offer that might be worthy of your attention?

The experimental setup within their interactive videos is pretty straightforward. Follow the link to view a short video of the setup. Using a micrometer, a device used to measure small distances or thicknesses between its two faces, a small amount of a certain liquid is placed between a 1 mm gap. By turning the screw in the micrometer, the original 1 mm gap starts to widen, and the liquid begins to stretch (see figure 1). Eventually, the gap becomes too large and the stretched out liquid snaps back. By measuring the maximum distance each liquid can stretch, students can easily rank the surface tension of various liquids and make inferences about the magnitude of IMFs present.

Figure 1: Liquid being stretched by micrometer

Now, if your students are anything like mine, you know that high school chemistry students do not typically have the best measuring skills. How in the world are we to expect them to precisely and accurately measure something that happens rather quickly and at a level of only a few millimeters! Herein lies the simplicity. The entire process is filmed using a macro (closeup) lens, and embedded within the video is an interactive 4 mm ruler that can easily and consistently measure each liquid’s maximum stretch. Pretty cool, right? The GIF below (figure 2) provides a great visual of this process.

Figure 2: GIF of two liquids in Pivot Interactives activity (Image used with permission from Pivot Interactives)

Throughout different parts of the activity, students will measure a total of 12 different liquids; many of which are unlikely to be in any given stockroom.

• Alkanes → Pentane, Hexane, Octane, and Decane
• Alcohols → Methanol, Ethanol, Butanol, Pentanol, Hexanol, Octanol
• Water
• DMSO (Dimethyl sulfoxide)

Though Pivot Interactives allows for complete customization of any activity, I found the default questions and overall flow to work out just fine. Here is an overview of what students experience.

Part 1—Comparing Pentane and Octane

This provides a simple opportunity for students to get used to some of the logistics such as choosing a liquid, using the ruler appropriately, and determining the point in the video they will measure the stretch of the liquid. Additionally, since octane has a higher surface tension, they soon discover the macroscopic effects of having different surface tensions. The default activity then dives in to why octane has a greater surface tension than pentane by relating students’ macroscopic observations to particle-based models (figure 3). The concept of London dispersion forces is introduced by focusing on the momentary dipoles that appear within each molecule and the effect these forces have on surrounding molecules to account for the differences in attraction. Eventually, students are aware that the longer the alkane is, the greater the attraction will be. To check for understanding, Part 1 ends with asking students to apply their recent knowledge by ranking the surface tension of decane, hexane, octane, and pentane.

Figure 3: Models displaying pentane-pentane and octane-octane interactions. Red arrows represent the constant motion of the charges within the molecule that cause the momentary dipole. Blue arrows communicate how oppositely charged parts of each molecule are attracted to one another. (Image used with permission from Pivot Interactives)

Part 2—Ranking Surface Tension of Alkanes

Once students have made their prediction at the end of Part 1, they are now able to evaluate their prediction by actually measuring the surface tension of decane, hexane, octane, and pentane. Next, particle models of two alkanes (pentane and hexane) and an alcohol (pentanol) are introduced (see figure 4). Before they are able see whether the presence of an -OH group effects its surface tension, students must first predict whether pentanol will have more, less, or the about the same surface tension as pentane and hexane.

Figure 4: Models of pentane, pentanol, and hexane (Image used with permission from Pivot Interactives)

Part 3—Surface Tension in Alcohols and Dipole-Dipole Interactions

Once again, the beginning of this section allows students to see if their previous prediction was correct. Additionally, they see how pentanol compares to larger alkanes such as octane and decane. Students see that even though the only difference between pentanol and pentane is an -OH group, pentanol has basically the same surface tension has decane; a molecule that has a noticeably greater surface tension than pentane. Based on this, students now have evidence to support the idea that the -OH group must be playing some kind of significant role and their original claim that longer alkanes result in greater attraction is now in question and must be modified in some way. Again, this is what I like about this activity—data-to-concepts. To account for this unusual behavior, students dive back into the particle level and are shown the pentanol molecule with greater detail (figure 5) and how multiple pentanol molecules interact with one another (figure 6).

Figure 5: Model displaying pentanol and the difference in the magnitude of the dipole that forms on the OH group compared to the momentary dipoles between carbon and hydrogen. (Image used with permission from Pivot Interactives)

Figure 6: Visualizing how the strong dipole attraction differs from the weak London attraction when pentanol molecules interact with one another (Image used with permission from Pivot Interactives)

With new evidence and an increased understanding of how the surface tension of alcohols compares to alkanes, Part 3 ends by asking students to rank the surface tension of a greater variety of molecules like methanol, ethanol, butanol, pentane, pentanol, hexane, hexanol, octane, octanol, decane, and decanol.

Part 4—Water vs. Alcohols and Alkanes

The last part of this activity introduces students to the concept of hydrogen bonding by looking at how water molecules interact with one another (Figure 7).

Figure 7: Visualizing the presence of hydrogen bonding (Image used with permission from Pivot Interactives)

After discussing how hydrogen bonds are stronger than the dipole-dipole interactions between the -OH groups of alcohols, students are asked to make and eventually test one last prediction by ranking the surface tension of water, decane, and octanol. It is worth noting that students are able to measure the surface tension of DMSO (dimethyl sulfoxide) in this section as well. I found this to be a great extension to the activity for students to apply not only what they learned in this activity, but also their knowledge of Lewis structures.

Student understanding of intermolecular forces and their impact on the properties of substances has always been a bit tricky for me in the past. After completing this activity and allocating appropriate time for discussion during and after, there seemed to be a noticeable difference in comprehension and overall confidence with this abstract concept. Like any good scientific investigation, the evidence drove the understanding and I will certainly repeat this activity again in the future. If you are interested in this activity, check out the Pivot Interactives website!