Are You Feeling Dense?

My beautiful density column.

Density is an introductory concept that comes up several times throughout the year in chemistry class.  Every year I have this “what, you mean you don’t know this” moment with my students.  I know that they have measured density many times throughout their school careers.  Eighth grade science could be renamed “An In-depth Look at Density”.  But my chemistry students give me the blank stare when I start talking density.

I have a set of classic density demos that I enjoy doing for my kids to investigate the relative density of solids, liquids, and gases.  This year I decided to stretch my repertoire with a multilayer density column. I used Demo 9.2 from Shakhashiri’s Volume 3 as my inspiration, along with two of my colleagues who do variations of the density column. I decided to use glycerin, water, antifreeze, corn oil, and ethanol for my column.  Adding a little food color to the water and the alcohol gave me five nicely colored layers.

 Liquids and objects for the column.

Look for the square piece of plastic in each beaker.

My plan was to construct the density column in a 1-liter graduated cylinder (my favorite piece of glassware for demos) with an object floating on each layer.  I did some tests of the liquids and objects in beakers to determine what would sink and what would float.  If you look closely you can see that the plastic square sinks in the antifreeze but floats on the glycerin.  Perfect!  An object at the intersection of the bottom two layers.  I tried a plastic piece from my molecular model kit; it floats on the water but sinks in vegetable oil.  I left my oak sample at home, but I expected it to float in the oil but sink in the alcohol.  Top it all off with a cork that floats on the alcohol. 

I’m feeling pretty confident as I begin to construct my column.  I grabbed the oldest sample of glycerin from the stock room for my demo because I don’t need to use it for a chemical reaction.  It turned out that the old glycerin has a great amber color, which made the column that much more interesting.  First I poured in 200 mL of the golden glycerin (density = 1.25 g/cm³).  Next came the antifreeze, made mostly of ethylene glycol (density 1.11 g/cm³) with a great neon green color.  As I poured in the third liquid, water with some red food coloring (density = 1.0 g/cm³) it mixed with the antifreeze.  Wait a minute; the antifreeze was soluble in water.  Of course!  When you take one look at the structure of ethylene glycol, with an –OH group on each end, then it’s obvious that this polar molecule will feel right at home in an aqueous solution. 

 Here's the antifreeze dissolved in the red colored water.  No good.

So I started again without the antifreeze.  Glycerin, red colored water, then vegetable oil (density = 0.91 g/cm³), and I topped it off with some blue colored ethanol (density = 0.789 g/cm³).  Beautiful!  The layers kept to themselves very nicely.  Then I had a lot of fun dropping objects into the column.  Sadly, the oak piece floated on top of the alcohol.  The black rubber stopper dropped to the bottom but the white one stopped on top of the glycerin.  The visual impact of this demonstration is worth 1,000 notes on the topic.

Here’s a run-down of my favorite density demos that I use every year.

Diet Coke floats, regular Coke sinks.  Constant volume, different mass.

The floating egg!  The egg is resting on a layer of brine, tap water is on top.

Soap bubbles float on a layer of carbon dioxide gas.  This one is really exciting to see.

Don’t Go Changin’ …

The beginning of the year is wide open for fun demonstrations of introductory concepts.  I like to capitalize on this opportunity to keep my kids engaged and have some fun with demos that do unexpected things.  The topic of the day was physical and chemical properties.  I decided to try to boil water in a paper cup to demonstrate the high heat capacity of water, a characteristic physical property of water.  This demo is described in Demo 9.4 Boiling Water in a Paper Cup:  Heat Capacity of Water found in volume 3 of Shakhashiri’s Chemical Demonstrations, A Handbook for Teachers of Chemistry.  First I put an empty paper cup in the flame.  It did just what everyone expected, it burned quickly.   I filled the second cup half way with water.  When I placed it in the burner, the top part of the cup caught fire and burned off, but the bottom portion that was in contact with the water remained unaffected.

 

The top portion of the cup is burning while the bottom half stays intact.

I heated the water in the paper cup to the boiling point and then proceeded to boil it all off in the paper cup. 

The water is boiling in the Dixie cup!

I was very excited to see the water boiling in the cup.  As a bonus, I got the chance talk about the phase change happening to the water.  The paper cup provided a great visual display that the water, not the cup, absorbed the energy from the burner.

 The "after" photo of the cup.

How do you narrow down the possibilities for demonstrating chemical properties?  There are so many great demos to choose from for this one, but in the spirit of this blog project I decided to try something new.  In my email inbox I found my inspiration from Flinn Fax!, Vol. 11-4; an excellent publication from Flinn Scientific Inc. with demonstration ideas that are timely for the curriculum and the season.  “The Carbon Soufflé, A Sweet Exothermic Reaction” was on the cover.  I did this reaction in year two of my teaching career, I think.  Let’s just say it’s been a long time since I’ve done this one.  I poured 18 molar sulfuric acid (highly concentrated acid) on sugar (sucrose).  It worked great!  After a short delay, the reaction began to boil, it produced heat, and the carbon soufflé emerged from the beaker.  You can watch the video of the reaction to see how cool it is and to hear the voices of my students as they watch this unexpected event.  I made one mistake in my explanation, which you will hear on the video.  The reaction produces carbon and water.  The carbon column grows from the beaker as water vapor forms gas bubbles in the hot carbon.  At one point in the demo I said that carbon dioxide is also formed in the reaction, but after further reading, I realized that this is not the case.  Sugar is a carbohydrate made up of carbon, oxygen, and hydrogen in an interesting double ring structure.  The sulfuric acid causes a dehydration reaction of the sugar, which removes hydrogen and oxygen from the sugar molecule to form water and carbon.  In the end, I had a large carbon “popsicle”. 

http://www.youtube.com/watch?v=YuPawp5MSGc

Simple Distillation is Not So Simple

 Simple distillation is a deceptive name.  This week I conducted a demonstration of a simple distillation for my honors class as part of the Foul Water Lab from the ChemComm curriculum.   (I don’t use ChemComm, but I like this lab from the book.)  I used Demo 9.10 Separating Liquids: Fraction Distillation found in volume 3 of Shakhashiri’s Chemical Demonstrations, A Handbook for Teachers of Chemistry as a reference for the set-up and discussion. 

Here's the simple distillation apparatus.

The distillation demo was the final step in a water purification lab conducted by my Honors students.  They were challenged to purify a water sample that contained oil, coffee grounds, garlic powder, and salt.  I used the “Foul Water Lab”, which is an excellent introduction to separating mixtures, chemistry lab techniques, and the physical properties of matter.  The students did a series of purification steps:  separating the oil from the water, sand/gravel filtration to remove large particles, charcoal absorption to remove odors, and a distillation to remove the salt (and the rest of the impurities that they didn’t get out before).  I conducted the final step of the experiment as my demo this week because my lab tables don’t have sinks (very annoying) and we don’t have enough condensers (they are useless without sinks, by the way). 

My colleague suggested that I distill Cherry Coke first to introduce the technique, something I had never tried.  This was a lot of fun because the kids always love testing something they might actually eat or drink.  I decided to set up the apparatus during class (some of the groups were still finishing up the filtration, so half the class had some down time).  Most of the students had never seen a distillation.  I handed the tubing to a couple of boys and asked them to connect it for me, while I attached clamps to ring stands.  I asked for more helpers to hold the round bottom flask and condenser in place while I clamped it all together.  The set-up was a distant cousin to the fractional distillation apparatus in my college organic chem lab, with glassware that all fits together and bars in the back of the hood for clamps.  After a lot of adjusting, I managed to assemble the apparatus with the Coke in the flask, when I realized that I had forgotten to add the boiling chips.  So I loosened some clamps, lowered the round bottom flask, added the boiling stones, and manipulated the apparatus back together.  My students watched, gathered around the front bench, a few lending a hand to help me steady everything as I readjusted.   At the same time I explained how the system works to separate liquids with different boiling points.  Finally we were ready to light the Bunsen burner.  (As a side note, the first time they see me light the burner is a big deal every year.  Soon enough the magic is gone and this becomes just part of the regular lab routine.)

The first thing we observed was rapid bubbling as we watched the carbon dioxide leaving the solution.  I bubbled the gas through limewater to confirm the presence of CO₂, another suggestion from my colleague.  Very cool so far.  Then we started collecting a liquid.  I wish that I had remembered to put a thermometer in the flask to note the boiling point of the distillate, but I didn’t think about that until it was too late.  It’s been a really long time since I’ve done a distillation.  It all started coming back to me as we were boiling the Coke.  Maybe I should have spent a little longer reading Shakhashiri, because a thermometer is clearly labeled in the diagram!  We collected a clear, colorless liquid form of the cherry/vanilla flavoring.  The strong odor of cherry coke was overpowering in the small sample we collected.  The second liquid we collected was also clear and colorless with a faint smell of cherry coke; this time I think we collected water with a small amount flavoring contaminate.  After we had about 100 mL of distillate, I removed the pot and let the students observe the smell of the liquid left behind, flat and flavorless Coke.  They enthusiastically passed around the three samples and commented on their odor and color.

 Distilling the foul water sample, the last step in the purification lab.

Next we distilled their foul water samples.  I combined several groups’ samples into the pot because we were only going to do this once.  The brownish, electrically conductive water sample in the pot started to boil vigorously as the bell rang ending the period.  No!  My kingdom for 20 more minutes.  We were already collecting distillate, so I decided to let it continue to distill after the period ended.  I saved the purified water sample into an Erlenmeyer flask and saved the remaining pot liquid.  During the next class period, I was a bit nervous to test the conductivity of the solution.  What if it didn’t work?  How would we explain the presence of salt in our purified sample?  I decided to let the anticipation build by showing the class a control test of the conductivity of distilled water (no light) and salt water (bright light).  We were all pumped up when the light did not come on when we submerged the electrodes in our purified water. The only disappointment was the lingering garlic odor in our water sample.  The purification would have been better with a fractional distillation set-up rather than a simple distillation (a second condenser above the pot makes all the difference), possibly separating out the organic substance in the solution responsible for the lingering odor.  However, we did the job we set out to accomplish; the salt was definitely gone from the final product.

 Compare the sample of untreated foul water to our purified sample.  Looks nice, but don't smell it!

 Here's the distillate we collected and the liquid that was left in the pot at the end of the distillation.

A Year of Chemical Demonstrations

This is my fifteenth year of teaching chemistry and physical science to high school students.   I love my job.  Each day I have something exciting going on in my classroom:  a demonstration, a lab, or a group activity.  But my science-teaching career didn’t start off this way; I had a horrible first year of teaching (don’t we all?). 

My first teaching job was in a small high school in rural Virginia.  I was the only chemistry and physics teacher at the school.  I had no one to turn to for help with all the issues that a new teacher faces, and more tragically, no other science colleagues to work with who loved teaching science.  I was on my own in my lab with 20 students for five periods a day.  I made all the mistakes that new teachers make in their first year.  After the first week, I lost the kids attention and I had no idea how to get it back.  My carefully crafted lectures were incredibly boring and way over my students’ heads.  By Christmas, I realized that my bad classroom management skills were making my life miserable.  I didn’t know what to do to make it better. Lab prep consumed so much of my time, and then the experiments were disasters.  Let’s just say that I was very discouraged by the spring term.  By the end of the year I was ready to throw in the towel.  I actually took three days of sick leave when I got my new contract.  I was immobilized by the thought of going back for another year, but I was certain that things shouldn’t be like this in my classroom.  After three days of crying, fretting, and soul-searching, I decided to sign on for another year at the school.  I vowed to make it a better year by seeking out help from other chemistry teachers.

Over the summer I attended a weeklong workshop for chemistry teachers in Virginia put on by the local American Chemical Society (ACS) chapter.  Excuse the cliché… this workshop changed my life.  On the first day, we met Bassam Shakhashiri, the guru of chemical demonstrations from the University of Wisconsin.  He did a demonstration presentation for the group and then followed it with a hands-on workshop so we could learn how to do these demonstrations.  That was the day my eyes were opened to the wonderful world of science education.

This blog is not only a celebration of my 15^th year in the classroom, but also a tribute to Dr. Shakhashiri who changed the trajectory of my teaching career (or some would say “created a monster”).  My plan is to perform one demonstration each week from Shakhashiri’s Chemical Demonstrations, A Handbook for Teachers of Chemistry, the “bible” of chemical demonstrations, a five-volume set of demonstration books.  Even though I do demonstrations in my class almost every day, this year I will incorporate one new demo a week from the Shakhashiri collection.  I will use the blog to reflect on the preparation, performance, and effectiveness of the new demonstrations.