Thursday, February 16, 2012

Gas Laws: Where it all Began

I love Gas Laws Demo Day!
Setting up the fussy diffusion demo.

Gas Laws demos are so fun and easy to do.  This is where it all began for me back in my second year of teaching when I started using demos in my chemistry classes.  I learned how to crush a can using a Bunsen burner and a bucket of ice; from that moment on I was hooked.  Ever since that second year of teaching, I like to do “Gas Laws Demo Day” to showcase several ways to see the relationships between temperature, pressure, and volume.  It just so happens, by my good luck, that my new boss, Mr. Richards, decided to visit my class on Gas Laws Demo Day. 

Inserting the gaseous cotton ball.
I started off by setting up a very fussy demo that I have tried unsuccessfully for many years.  It’s called “The Ratio of Diffusion Coefficients:  The Ammonium Chloride Ring”, found in Shakhashiri’s Volume 2.  The idea here is to take two gases, put them in the ends of a clear tube, and then wait until they diffuse far enough to meet in the middle.  The two gases are hydrogen chloride (from concentrated hydrochloric acid) and ammonia (from concentrated ammonium hydroxide).  These two substances are both dangerous to handle and very smelly.  I carefully dipped a cotton ball into each of the two liquids, one is a strong acid and the other is a strong base, then I inserted the cotton balls in the ends of the tube, and closed it all up with rubber stoppers.  I re-read the demo, paying close attention to all the details, which helped me learn why it had failed all these years.  It takes about 15 minutes for the gas to travel down the tube!  I’m not known for being a patient woman, so it’s understandable why I thought it never worked.  Today, I got the apparatus all set up and then we left it there to do its thing for a while.  After several other demos, we came back to check on the diffusion, and there it was!  The white cloud had formed in the middle of the tube!  It worked!  I was so excited to see the cloud; Mr. Richards was a little frightened by my exuberance. The gases traveled the length of the tube and then reacted when they met.  It’s like a classic story of searching for your perfect match and then finding them right in front of you; it was love at first sight.  To Mr. Richard’s defense, my students were also a bit puzzled by all the fuss I was making.  It’s not a “whiz bang” demo with explosions, fire, colors, etc.  But, for me, I was so pleased to show my students diffusion, a difficult task with clear and colorless gases.  And, by the way, I tried it again later in the day, confident that I had mastered the demo, but it didn’t work! 
Here's the ammonium chloride cloud!

Mr. Richards does the can crushing demo.
I did several other classic gas law demos that day.  I forced a hard boiled egg into the mouth of an Erlenmeyer flask by cooling the steam inside the flask in an ice water bath.  The fun part is getting the egg back out of the flask.  Once again, relying on the thermal expansion of gases, I forced the egg back out of the flask.  I put the can crushing demo in the hands of the Head of School to get him into the action.  He successfully crushed the can by heating it on the Bunsen burner and then inverting the hot gases onto an ice bath.  I put a balloon in the vacuum pump so we could watch it expand under lower pressure; and then did the same thing with a bag of chips.

Can filled with hot gas.
Crushed can!

I revisited a classic bar trick today too.  I put a candle in the center of a shallow water bath.  If you invert a flask over the candle, trapping the gas with the water, the candle goes out and the water level rises.  Chemistry teachers love to tell their students that this happens because the candle is using up all the oxygen in the flask, thus decreasing the pressure inside the flask causing the water to be pushed into the flask.  This explanation is partially true, because the water level does rise some as the candle is burning.  However, the temperature change once the candle goes out causes a more dramatic change in the water level.  I tested this theory by conducting the experiment with a hot flask, no candle at all.  You can see that the blue water rose into the flask just from cooling the gas inside.   I still think this makes for a fun party trick if you’re out with some friends, regardless of how you explain it.

Here are some photos of the candle trick.  You can try this one at home with a bit of play dough, a birthday candle, a bowl, and a jar.

All the necessary supplies for some good gas law demos:  vacuum pump and bell jar, ice, vacuum grease, bags of chips, flasks, balloons, a Bunsen burner, and some empty soda cans.
The egg is sliding into the flask as the gas cools.

Heating the air in the flask with the Bunsen burner.  An egg is born!
Taping the balloon into the bell jar so it doesn't cover the hole.

Mr. Richards is considering the increase in volume of the balloon under low pressure.
There are easier ways to open a bag of potato chips...


  1. Thanks for all the pictures and explanations! I love how excited you are!

  2. Great pictures! You might be interested in my practical science teaching blog at: For the diffusion demo the ring actually forms closer to the hydrochloric acid end of the tube as the hydrogen chloride is denser than the ammonia.