Getting Out of a Slump

I haven't written in the blog since my return from spring break. I've had a science teacher's equivalent of a writer's block. I returned from break to two of the most difficult sections of my course to show through demonstration: stoichiometry and electron configurations.

 I would love to hear from anyone out there with some ideas for making the electron configuration unit more interesting to teach. I struggle with it every year, trying to put on a good positive attitude and drawing on my own appreciation of the structure of the atom to get me through. It's so hard to get the kids to wrap their heads around the abstract world of quantum numbers. There are always a a few kids, the puzzle solvers, who really like this stuff. The rest of the class is glassy eyed and bored. It's hard for me to keep my spirits up when my class feels like a death march with no clear destination. I always think to myself "Do I have to teach this section? Can my students walk away from this course without any knowledge of electron configurations?" My guilty conscience always wins out. This year I delayed attacking this unit until the spring term, rationalizing this decision with the following logic: spring term means everyone is happy, it will be better. Wrong. I was just as miserable as usual during this unit. My kids grades dropped during this part of the course, it was a disaster (as usuaal).

Stoichiometry isn't any better for demonstrations. I love this part of the course because it presents great lab applications, and I did just that this year: tons of labs during the stoichiometry unit. But I get discouraged when my kids say, "Another lab, noooo." How can that be the response? Labs are what make my job fun and interesting. I don't understand why the kids would prefer to sit in their seats taking notes and doing more practice problems rather then getting into the lab. So I really like the lab work involved with the stoichiometry unit, but I find that the students are not energized by the topic. Again, I would love to hear from anyone out there with ideas for making the stoichiometry unit more lively.

Titration of vinegar, part of the stoichiometry lab work.

 I made it through, and now we're moving on to chemical bonding and molecular geometry. Still very little to offer for lab work or demos, but a topic that is easily digested by sophomore minds. We do lots of white boarding during this section and the kids get a lot of satisfaction from mastering Lewis Dot diagrams. I'm heading straight into the intermolecular forces concept. This concept is the "biggie" that the biology teachers want the kids to know when they move up to their class. I like the idea of doing it last so that it's fresh in their minds in the fall (theoretically). If the kids don't know what a hydrogen bond is, then I feel that I have failed the biology guys. Four more weeks, so much chemistry left.

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.

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...

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.

Yet another reason why I do demos

Sharing the fun of chemistry at home!

I received these photos from a parent of one of my students.  She has two daughters, one who is in my chemistry class this year and another who took my class two years ago.  In the photos you can see the younger sister showing her older sister a fun demo I did in class just before Christmas vacation.  It made my day to see the girls sharing the excitement of chemistry with each other (without me telling them how exciting it is) and with their parents while on school vacation.  It just doesn't get better than that for a science teacher.

Dipping the dollar bill in the liquid.

Who gets to light it this time?

In the demo I dip a dollar bill in a clear, colorless liquid and then light it on fire.  The bill appears to be engulfed in flames, but it quickly burns out and the money is left unharmed.  "What?!  How did you do that?  Do it again!" my students exclaim when they see the money burn.  After I repeat the demo a few more times, I give them the explanation.  It's simple, the liquid is half rubbing alcohol and half water.  The flammability of the alcohol creates the impressive flames, but the high heat capacity of water keeps the dollar bill from being damaged.  When I'm working with household products I usually say the Myth Busters line, "Kids, don't try any of these demos at home, I'm what they call an expert."  But maybe this time I said, "This one is so easy you can do it home and impress your family."  That sounds like something I would say.  Anyway, the girls took my advice and did the demo at home.  You can too, in a well ventilated area, next to the sink, with your hair tied back, and safety goggles on if you have them!  Enjoy.

I get by with a little help from my friends

The guys in the dining hall are great.  I frequently ask for food items for my demonstrations.  Today I asked for hard-boiled eggs and bags of potato chips for my demos with the vacuum pump.  They've given me jugs of vinegar, baking soda, food coloring, fruits and vegetables, a pumpkin for the exploding pumpkin demo, drinks to test for pH or density, and the list goes on.  They just shake their heads and smile.  "What are you blowing up today", they ask me when I go through the line at breakfast.  Here's the email I received today from the head of the dining hall:

 He called me a "mad scientist", but I responded that I'm actually a happy scientist because they are always willing to help me out.

Electrolysis of Water, Old School Syle

Introducing the class to the Hoffman apparatus.

Sometimes you have to pull out a classic demo to keep things real.  A few years ago I found a Hoffman apparatus in a back corner of the stock room.  I cleaned it up, ordered new stopper electrodes, and gave it a try.  This classic demonstration of the electrolysis of water is a great way to “see” the chemical formula of water.  We all learned at an early age that water is “H-2 O”.  But do you really believe it?  Now my students can say with confidence that water is definitely 2-parts hydrogen to 1-part oxygen.

Bleeding out the rest of the air.

Pouring in the sodium sulfate solution.

Today I asked for student volunteers to set up the Hoffman apparatus.  The blue liquid is water with sodium sulfate dissolved in it (a catalyst) and bromothymol blue (a pH indicator).  The apparatus is designed to collect hydrogen in one side and oxygen in the other.  Each side is marked off in milliliters so you can quantify the amount of gas produced in each chamber.   The rubber stopper in the bottom of each of the two tubes contains a metal electrode that I connect to a dc power supply.  The electric current that is passing through the water causes a decomposition reaction to occur.  Oxygen is collected at the anode, while hydrogen is collect at the cathode. 

Connecting the electrodes to the dc power supply.

Watching the color change.

You can see right away the ratio of hydrogen to oxygen produced.  At first one of my student volunteers thought he had done something wrong.  He said, “Mrs. Geyer, I think this side is leaking.”  I waited a while longer to see what the other students thought about the reaction.  As the volumes increased, constantly in a 2 to 1 ratio, another student said, “No, that’s what’s supposed to happen.  I have seen this before in my science class in Germany.”  He proceeded to explain the demo to the class.  Perfect!  I love it when a plan comes together.

The bromothymol blue is a nice addition to the demo.  Not only does it give the solution a beautiful color, but it also distinguishes the two electrodes.  This indicator turns yellow when it is in an acid solution and blue when it’s in a basic solution.  The color change happens very quickly after turning on the power supply. As you can see in the photos, the side with the small amount of gas is yellow and the larger volume of gas is blue. The reaction at the anode produces oxygen gas and hydrogen ions, which cause the pH to drop to the acidic range.  The cathode side produced hydrogen gas and the hydroxide ion, which increases the pH to the basic range.

Here you can see the 1:2 ratio of the gases.

The culminating event is the test of the two gases.  The video shows our simple test of the oxygen and hydrogen gases.  We opened the stopcock at the top of each tube and collected the gas in a test tube.  A glowing splint will flare up in the presence of oxygen.  A burning splint will cause the classic “hydrogen pop” with the hydrogen gas.  My student volunteers were eager to do the tests, even if one had to conquer her jitters about the pop.

 video of our test of the gases

Waiting for the color to change again.

When it was all done, we set it up again, but switched the electrodes.  We wanted to see if the pH indicator would change colors if we switched the anode and the cathode.  We waited for a while and finally the blue turned yellow, but we did not wait long enough to see the yellow one turn blue.  Time was running short and two more students were ready to test the oxygen and hydrogen produced.

This classic demonstration with the Hoffman apparatus is a great way to connect my students to the chemistry class that their parents took back in the day.  I hope that a few of these kids will mention it over the dinner table tonight.