My favorite way to showcase the effects of intermolecular forces in a lab activity is with the very classic penny drop lab. In this activity, different substances are tested for their surface tension in sitting on the surface of a penny. It’s an amazing lab for helping students to see the real life effects of intermolecular forces.
Background:
The stronger an intermolecular force is, the greater the surface tension of the substance. Water, for example, has hydrogen bonds, and therefore its molecules stick together very well. It’s hard to get them to boil. It’s tough for the particles to evaporate (vapor pressure). And finally, it’s easy for them to bubble up and stick together – and you can see it happen on the surface of a coin.
This is the final lab that I do in my Intermolecular Forces Unit. We would have already learned bond polarity, molecule polarity, dipole-dipole forces and hydrogen bonds, dispersion forces and lastly the effects of intermolecular forces. You can read more about the demos that I do for that entire unit in this post.
The students would have also done the like dissolves like lab activity. You can also see how this plays out in my chemistry curriculum outline.
Up until this point, there’s not a lot of labs that allow students to really see the chemistry. I like to teach the “micro” units before the “macro” units. This is really the first taste of how chemistry looks in the real world for my students. And for that reason it’s a lot of fun. It’s also a super low maintenance lab which means its not a lot of set up for me!
Set Up:
For this lab I set the students up with a penny, a few paper towels, beakers with chemicals and some droppers. I’ve switched up the chemicals depending on what I have in my stock closet. I will always use water, and then I use a high percentage rubbing alcohol. I prefer to use baby oil, because it is a very thin oil and pretty easy to clean up. I’ve also tried using salt water and soapy water. Although I never measure how I make these solutions…
Students put the penny on the paper towel and test how many drops of each solution will fit on the surface before it falls off. They keep track of the data and calculate the averages. It’s a really simple lab for seeing the effects of intermolecular forces. It certainly beats tracking vapor pressure like I did in my quantitative analysis class!
My Tips:
- I prefer to give my students all of the materials that they need and have them work at a single lab station for the duration of the experiment. But this lab could also work well as a stations labs activity if you prefer that. I can understand in terms of cleaning up the mess and limiting materials that this could make sense for you.
- Have students hold the dropper vertically. If they hold it to the side, the often wind up squirting the liquid across the lab bench. For that reason, even though these are household materials, be sure students are wearing goggles.
- When I have a lot of time, I often ask students to share their data with me. I like to collect the averages for the class. The next day I will share the data with all my classes and see what they have to say about the numbers. It’s a really great lab for discussion because you can’t really argue with the data!
You can purchase this lab activity in my TPT store. It’s also included in my full year chemistry lab manual!
