Bond with James: You're so dense!

Monday, September 7, 2015

You're so dense!

Many of the demos here are not new. However, if you're new to profession or teaching chemistry, perhaps these may help you.  

One engagement trick I learned early in my career was using duck tape, a brick (or piece of wood), and a piece of Styrofoam with similar dimensions.  If done properly, you may have students compare the objects and discuss their observations (e.g. why does one feel heavier if both objects have the same volume).   

I was surprised that none of the nearby stores such as Hobby Lobby or Michaels had Styrofoam and so I went with wet foam. For the record, I hated the wet foam. While it was easy to cut, it came apart easily. Additionally, the duck tape did not adhere to the surface of the foam.  I had to wrap long pieces of tape around the foam and stick pieces of tape to one another.  I recommend going with Styrofoam or cutting a piece of wood to match the dimensions of the brick.  Wood might be a better substitute for either the Styrofoam or wet foam because it is less difficult to puncture the wood than the other two pieces. After several successful years of keeping my Styrofoam piece safe, I finally had a student take a pencil to it.  I was not happy - although, all the pieces are cheap and easy to set up. 
This next part includes several videos of demos or activities you may do with your students.  Due to the fact that several of my students have seen some of these, I actually have these set up as stations in the lab area.  Students would rotate from station to station and would have an opportunity to take notes, observe, and explain what they thought was happening.  Depending on class size, the entire station activity would take no more than 10 - 20 minutes. 
Feel free to use any of these density videos with your students - whether to show in class, have absent students view, or to embed on your personal site. 

Density: Coke and Diet Coke
Most students in my area have seen this by the time they get to high school. This demo is cheap and easy to set up. If you don't have a small tank, use a large beaker or other clear container.  One year I even brought in a cooler with several different drinks. I asked students if they had ever used a cooler and noticed the drinks in it  (i.e., why are some floating and others at the bottom). 

Density: Ice in Alcohol and Water
This is cheap and easy - all you need is a clear container, ice, alcohol, and water. 

Density: Ice in Vegetable Oil
If you decide to do this one, I suggest using a smaller piece of ice and allowing some time for the process to take place. If the students are rotating from station to station, then it's not a big deal. However, have something for the students to do if you are not considering a station rotation. 

Density: Eggs in Tap Water, Salt Water, and Tap/Salt Water
There are two versions of this video (normal and slow-mo).  I recommend setting this up for the students to observe as one of your stations. Try using smaller volume beakers or containers so that you are not wasting salt. Additionally, I once tried using 1000mL beakers with no success on the tap/salt water combination - perhaps I was not patient enough. Also, make sure you have fresh eggs as rotten eggs will float in tap water. 

For the tap/salt water combination container, pour the salt water in first and see if the egg will float.  If it does, then pour in the tap water; or take the egg out and then pour the tap water in so that you can place the egg in later. You could add a drop of food coloring after the egg is placed and gently swirl (the very top only) to help students distinguish between the two layers - the dye will diffuse through the tap water. 

Another station you can create is the density column demo. From bottom to top: Karo syrup, dish washing liquid, water with food dye, vegetable oil, and alcohol with food dye.  You can add more liquids and even add solids to the mix. 

As mentioned earlier, all of the activities shown above would be part of a station set up and would last no more than 20 minutes.  I would then move into the concept of density and calculations as those are specific standards in my state (calculations in 8th grade and the concept of an intensive physical property at HS chemistry level). I've done this several ways.  One method involves the use of blocks. While the blocks don't fit in a 100mL graduated cylinder, students can practice measuring the volume of a cube.

To get in more density calculation practice, as well as show in the intensive property of a substance, I would assign each group - made up of 4 students - a different tube of density cylinders (refer to photo below). The student data and graphs were amazing every time I used these in class and the activity helped students with the concept that the density of a substance is an intensive property. I would caution the use of glass graduated cylinders and the steel and copper pieces.  

You may always rely on the use of water if you are not able to obtain the blocks, the classroom density assortment, or you if simply wish to get another lab in. Have students collect data on different volumes of water (20mL, 40mL, 60mL, 80mL, and 100mL).  I did this the year I moved to a new campus and we did not have the density cylinders at the time.  My students still obtain great data this way, however, they sometimes get bored using only the water. Furthermore, students do not get to practice the displacement method to calculating the density of an object.

The blocks and cylinders come in handy if you like doing lab practicals. You can assess students on their measuring skills, as well as their density calculations. For example, students might get 1 point for their accuracy (how close they were to the true value of the unknown substance), 1 point for showing their calculation, 1 point for significant figures, and maybe 1 point for lab safety.  I always explain to students that they will be taking part in a lab practical down the road and so it benefits them to participate and pay attention during all labs. 

As always, thanks for stopping by and reading. Please feel free to share, comment, and/or ask a question. Enjoy the rest of your day and have an excellent week! 
-Bond with James-

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