Friday, September 23, 2011

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”. 

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