Friday, September 30, 2011

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.

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