Chemistry

Dance Club Patrons Describe Water Molecules

Written by Dave Sheldon, St. Clair County Community College

I approach the changing density of H₂O by having students imagine a high energy dance club. They imagine the loudest and most energetic night club that they can, and describe it in detail. They focus on the intense energy, the motion of the people and the number of people on the dance floor. I include some techno music to get them thinking! Once focused on the high energy and rapid motion of the patrons, I modify the scenario. First I drastically reduce the energy by cutting the music, killing the strobes and bringing up the house lights. My students describe a ringing of ears and a much slower movement by the club patrons. Second, I describe every person as an H₂O molecule with their arms outstretched in front of them at a 90° angle. Their hands represent two Hydrogen atoms and the middle of their back represents their Oxygen atom. Hydrogen bonding causes them to place their hands on the backs of two other people while two more people place one of each of their hands on your back. With low energy in the club, these bonds last for a relatively long time and the water molecules form a low density crystalline lattice. The people on the dance floor during the high energy rave would no longer fit in this low energy orientation. Some even note that they may actually be forced out of the club through windows and doors, which is what happens when water freezes.

The Earthquake Richter Scale and the pH Scale

Written by SuEarl McReynolds, Palo Alto College, San Antonio, TX

When discussing how the pH scale is logarithmic and a one number change is equivalent to 10 times as acidic or basic, I ask if they know what the Richter Scale is. Usually several people know that it has to do with measuring earthquake strength. So I ask how much difference they think there would be between earthquakes with a magnitude of 8 and a magnitude of 9. It’s “only one number.” In reality a magnitude 8 can cause serious damage over several hundred miles, but a magnitude 9 can cause devastating damage over several thousand miles. So one number change is a big difference in these kinds of scales.

When I get to buffers, I have a picture in my PowerPoint of a man working on a floor with a big buffering machine. He’s smoothing out the drastic high spots and low spots on the floor, making it more even. Or a person may act as a buffer between two friends with opposite personalities—again making things more smooth, the differences less pronounced—maybe helping the shy one feel freer to express opinions and the loud one less likely to interrupt and dominate the conversation.

Relating Chemical Bonds to Everyday Ideas

Written by SuEarl McReynolds, Palo Alto College, San Antonio, TX

I use several analogies when talking about chemical bonds. I compare them to different kinds of glue. I ask the students if they have a “junk drawer” at home. They smile, and I ask if it’s got some different kinds of glue in it—maybe paper glue, Elmer’s glue, wood glue, Super Glue. Just like you need different kinds of glue to stick different materials together, you need different kinds of bonds to hold different kinds of atoms together. Probably a lot of people compare the attraction of the oppositely charged ions in an ionic bond to the attraction of the opposite ends of a magnet. Another common analogy probably is to compare the sharing of electrons in covalent bonds to holding hands (as in carbon is an atom that has four hands sticking out to hold with other atoms). Covalent bonds could also be compared to kids who want to play with the same toy. So they set a timer and switch off who gets to play with the toy.

When I talk about polar covalent bonds, which result from an unequal sharing of electrons, such as in the water molecule, I tell this story: “Suppose I come to class with a big chocolate chip cookie. I tell you that I’m feeling generous and am going to share my cookie with you. You anticipate that I’m going to break the cookie in half and keep half and give you half. But that’s not what I do! I really, really like chocolate chip cookies—so I break off a little piece for you and keep most of it for myself! Well, I did share. I just didn’t share equally!”

When I get to the much weaker hydrogen bonds, I compare them to Post-It notes. I ask the students to visualize some inventor trying to formulate a new kind of Super Glue. He tries a lot of different variations and comes up with something that will hold things together when you want them held together but will release them without tearing them up or using a lot of energy when you want them separated. That’s the kind of adhesive that’s found in Post-It notes. It’s just a good thing he didn’t throw it in the trash because he had started out looking for a new kind of Super Glue! What would we do without Post-It notes? (I usually have one or two on my folders right there.) That’s what hydrogen bonds are like. An example would be the hydrogen bonds holding the two strands of DNA together. The strands need to be reliably held together most of the time, but sometimes they need to separate (of course I haven’t talked about replication or protein synthesis yet). It must happen without tearing the strands up or using an atom bomb’s worth of energy to make it happen. Then sometimes the strands will need to go back together (protein synthesis). That’s why the weak hydrogen bonds are important.