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.

Siphoning A Gas

Siphoning carbon dioxide gas.  It's hard to know how long to wait!

Pouring the carbon dioxide.

Testing for the carbon dioxide gas.

No, not gas from someone’s gas tank, although my students now know how to do it.  We are heading into the gas laws unit with my classes.  As a “warm up”, I teach the kids about the states of matter, including some physical characteristics of all three.  One thing that is hard to show to students is that gases can flow, like a liquid.  Using my “go to reaction” of baking soda and vinegar to produce carbon dioxide, I showed my students that you can pour a gas and actually siphon it. 

I actually used HCl instead of vinegar today.

I got this demo from Flinn as one of their monthly chemistry activity emails.  I love those guys!  This particular month was especially helpful because they sent along the video of the demo along with the instructions.  Watching the video got me excited about trying the demonstration.  (ChemFax! Publication No. 91636, Pouring and Siphoning a Gas)

Pouring gas directly onto the lit candle to see what happens.

To start off, I cut the tops off of two 2-liter soda bottles.  In one of the bottles, I poured some baking soda and then added vinegar to create carbon dioxide gas.   We never get tired of watching the fizzing from this reaction.  The problem here is that the carbon dioxide is clear and colorless.  How do we know it’s in there?  I created a “gas detection apparatus” out of a piece of wire coat hanger and tea candle.  By lowering the lit candle into the bottle, it is very obvious that the gas is in there because the candle is extinguished as it touches the carbon dioxide.  What I love about this simple test is that we can now quantify the amount of CO₂ in the bottle.

Yes, there is carbon dioxide in there.

Once we established the presence of CO₂, I poured the gas into the “empty” bottle (of course, it isn’t empty because it contains air).  Using the gas detection apparatus, we can easily see that the gas filled up the second bottle.  The hard part about the demo is waiting long enough for the gas to flow because there is no way to know when it’s done.  With the candle, we measured the level of the CO₂ in the bottle.  Not all the gas was transferred, but we got over half of it.

For the finale, I set the bottle with the CO₂ on a block, and put a rubber tube into it leading to another empty bottle at a lower level.  I sucked in some of the CO₂ to get it started and then we waited.  Let me tell you that the gas was not tasteless; it had a terrible taste of acid that filled up my mouth.  Once again, it was hard to know how long to wait, but using the gas testing apparatus; we confirmed that the gas flowed through the tube into the lower bottle.  

The taste was terrible.  Don't try this at home!

Thanks to Kelley, my  student photographer for this demo.

Thanks to my student assistance during this part of the demo.