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I love Gas Laws Demo Day! |
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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.
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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!
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Here's the ammonium chloride cloud! |
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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.
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Can filled with hot gas. |
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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.
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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. |
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The egg is sliding into the flask as the gas cools. |
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Heating the air in the flask with the Bunsen burner. An egg is born! |
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Taping the balloon into the bell jar so it doesn't cover the hole. |
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Mr. Richards is considering the increase in volume of the balloon under low pressure. |
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There are easier ways to open a bag of potato chips... |
Thanks for all the pictures and explanations! I love how excited you are!
ReplyDeleteGreat pictures! You might be interested in my practical science teaching blog at: https://wilsonevescience.blogspot.co.uk/ 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.
ReplyDeleteDo you have any worksheets for these experiments for students to follow?
ReplyDelete