Thursday, February 23, 2012

Demo Show for the Third Grade

My son and I are testing the hydrogen gas.  Nice to end with a bang!

Today I did a demonstration show for my son’s third grade class.  The kids are studying the states of matter in school.  I asked him if he would like me to come in and do some demos for his class about this interesting topic.  He was very excited about the idea, so I set it up with his teacher and here we go.

Water and Ice Balloons
The first thing I did was pass around three balloons; one was filled with air, another was filled with water, and the third was filled with ice.  If you ever want a fun party favor for kids, just throw some water balloons in the freezer.  They were absolutely thrilled by the frozen balloon.  I described to the kids how I made it so they could repeat the experiment at home.  The three balloons provided a nice platform for comparing and contrasting the properties of the three states of matter.

Siphoning the blue water.
Next I turned my attention a common property of liquids and gases:  they both flow.  It’s easy to see how liquids flow and take on the shape of their container, but not as easy to see gases do this.  First I set up a siphon with some blue water to show the kids that water will flow through a tube to the lowest point.  Then I generated some carbon dioxide gas in the same container and tried to siphon it.  I did this demo successfully with my classes about a month ago, so I was confident that I could repeat it here today.  Although we confirmed the presence of carbon dioxide in the source container with a candle, I did not wait long enough for the gas to flow through the tube into the lower container.  I guess the high-energy third graders got the best of me on this one.

Making carbon dioxide
Pour the gas.

The next thing I did was another demo with carbon dioxide gas.  I filled a small fish tank with the gas by dumping a large amount of baking soda in the bottom and then pouring vinegar on top of it.  The kids loved watching the bubbling reaction.  Many kids asked what would happen if I added food coloring to the mixture, so I dropped in some green food coloring.  I think they were disappointed that the bubbles weren’t green.  I tested the carbon dioxide with the candle again to show the kids how much gas we had made.  Then I blew soap bubbles into the tank.  This demo is always a big hit because the soap bubbles hover on the top of the carbon dioxide.  I’m used to doing this for teenagers, who are mildly amused by the whole thing.  But I didn’t think about how excited kids get when they see soap bubbles.  The front row jumped up and started chasing the bubbles and popping them before they went into the tank.  Once I convinced them to let the balloons go into the tank, we got to see the effect.  But, honestly, I think they were more interested in the ones that missed the tank!
Making more carbon dioxide.
The floating bubble trick.

The finale was the electrolysis of water demo.  I wanted to show the kids a chemical reaction that would demonstrate the chemical formula of water.  Once I got the Hoffman appartus set up and going, I let them come up in small groups to look at the reaction going.  I’m not sure that the kids could make the connection between the two to one ration of hydrogen and oxygen gas to the water molecule.  However, I think they all enjoyed seeing the reaction and observing the color changes.  They also got to see the water displacement technique as we trapped the gases at the top of the tubes.  I finished off the day with the pop of the hydrogen gas.  Overall, it was a lot of fun to work with the kids.  I hope that they learned something from my show, but I’m sure that they walked away thinking that science experiments are fun to do.

Here I'm trying to get the third graders to understand that water is 2 parts hydrogen and one part water.  They were mostly interested in the colors, the bubbles, and the water displacement.  I'm okay with that.

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...

Tuesday, February 7, 2012

Are you the Limiting Reactant?

The Limiting Reactant Demo

No one wants to think of himself as the limiting reactant, but is it any better to be considered the excess? 

Before the reactions started.
This week I pulled out the baking soda and vinegar again for a great visual demonstration of the limiting reactant concept.  In this scenario, I measured out a varying amount of baking soda (6 g, 12g, 18 g, 24 g, 30 g, and 36 g) to pour into six balloons.  The balloons were attached to 1 liter plastic bottles, each containing 250 mL of vinegar.  I actually used a 1 molar solution of acetic acid because I was out of vinegar.  This substitution turned out to be a bonus because we could very easily calculate the moles of acetic acid in each bottle without getting bogged down in the molarity calculation.
How much more gas will be produced?

I asked for six volunteers, one for each balloon.  The kids flipped up their balloons, starting the famous fizzing reaction that produces carbon dioxide gas.  Each balloon inflated quickly.  Everyone wanted to have the balloon that exploded, but to everyone’s disappointment, they all stopped reacting before popping.  However, they noticed that the size of the balloons was different for some and the same for others.
Everyone is concentrating on their reaction.

The first three balloons each inflated more, which makes sense because of the increasing amount of baking soda.  But the second set of three balloons appeared to be roughly the same size.  How could that be, they each contained increasing amounts of baking soda too.  Ah Ha!  The limiting reactant switched from the baking soda to the vinegar somewhere between the pink balloon (18 g of baking soda) and the red balloon (24 g of baking soda).  If you look carefully, you can see a white film at the bottom of the jars in the three big balloons.  That’s the left over baking soda that did not react. 
You can see the three on the left get bigger, but the there three on the right are the same size.

I couldn’t get enough of the balloons, as you can tell by the many photos.  I love this demo because it gives the students a visual to refer back to throughout the stoichiometry unit.   So the bottom line is that sometimes you’re the limiting reactant and sometimes you’re the excess, it all depends on who else shows up.