Banner
    School Year Ended With A Bang
    By Enrico Uva | July 16th 2012 02:00 AM | 4 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

    View Enrico's Profile
    School years are cyclical, so over the years teachers could end up feeling like Sisyphus, forever condemned to rolling a boulder up a hill, watching it go down, only to have to push it up again. Of course, things don't have to be that way. Once in a while, one can get creative, or at least whimsical and spice things up.  And what better time for some carefully orchestrated, chemistry antics than on the last day of school?

    This year's finale might turn out to be the most memorable, so we'll save that story for last.

    A few years ago, we did a variation on the unimaginative but ever popular dump-the-potassium-in-the-puddle trick. We went outside and chose a spot that was far away from the staff parking lot and, not to attract too much attention, also not within the sight of any classroom. Unfortunately, the seemingly secluded spot we chose was within the sight of some balconies of nearby apartment buildings.

    In the days when our parking lot's moon crater-like pot holes had not been filled yet, it was easy to find or create puddles. The morning had been rainy, so instead of bringing buckets of water, one of our students' carried vinegar (an aqueous solution of  CH3CO2H ) . By acidifying the puddle we would not only create a more vigorous reaction, but we would eliminate, or at least reduce, the base produced from the reaction between potassium and water.

    This was the reaction with vinegar:
    2 K + 2 CH3CO2H  --> 2 KCH3CO2 + H2 + heat

    and the heat would ignite the hydrogen and create a second exothermic reaction:
    2 H+ O2--> 2 H2O + heat 

    We were rewarded with an anticipated and emphatic explosion. But the heat created a large amount of water vapor, which soon condensed and formed a thick cloud that drifted towards the apartment buildings. Of course the sound drew some people out onto their balconies. And one of those people was a pregnant woman.
    When you carry out that type of reaction indoors, the vapor is not something you want to inhale. Since the potassium is stored in oil, during the reaction some of the oil ignites and injects some nasty throat-irritating compounds into the mist above the reaction vessel.  So I watched the cloud, jumping frantically and like Carlton Fisk, I waved my arms, trying to will it away from the pregnant spectator.  My students were also shouting, "Go inside-- just in case!" When she stayed put, just leaning over the railing and looking at us, we switched languages. Some tried, French, Arabic, maybe Italian, but all in vain.

    Luckily for her, other onlookers and my employment status, the cloud dissipated before it reached their floor. Nobody coughed. We apologized and explained that we were performing an outdoor experiment. An elderly passerby who did not speak English, stared at us sternly, pointed at me and pretended his hand was screwing and unscrewing a large bolt into his head. (photo credit: student Patrick Callahan)

    The following year, after sharing that experience with my class, we had to find a way of making it more spectacular but without involving the neighbors. My students suggested we perform the reaction with a large bucket of water in the baseball field. To avoid missing the bucket with the chunk of potassium, they thought of attaching a wooden platform to a pulley. A lot of us resent the fact that the government has sneaked in shop-elements into our grade 10 science course, but hands-on skills came in handy! To add a little bang, we relied not only on the hydrogen generated from the reaction but on H2-filled balloons, which had also been prefilled with a little copper sulfate to add a green colour to the explosion. Expecting the colors to be likely bleached in daylight, we first demonstrated the explosion of a few balloons  indoors.


    This year, on the final day of school, we resurrected our old tennis ball cannon. In the old days, beer cans were sturdier and their diameter was such that a tennis ball would fit in beautifully. We would cut the bottom out of 5 or 6 cans, carve a two-inch hole into the top of the base-can, make an additional pea-size hole at the bottom for ignition purposes and then connect them with duct tape. Rather than buying old beer cans on ebay, a few years ago I had asked my brother-in-law to weld a supportive base to two feet of three inch pipe. 

    The problem with our cannon was that the pipe was slightly too wide for the tennis ball. But a few of my students had seen the use of Vaseline demonstrated in a similar context on Mythbusters. First we taped up the ball to slightly increase its diameter, and then greased it up so that none of the exhaust gases would escape through little gaps between the ball and the inside walls of the pipe. Our fuel was 95% ethanol. We used a syringe to inject a small amount of the liquid through the ignition hole and shook it to create some vapors. My student Maariya had thought of also injecting pressurized oxygen. She originally intended to use the pressure as a combustion-less way of projecting the ball, but it got us thinking about speeding up the reaction rate by increasing oxygen's concentration.

    Our trial run was a smashing success.  It left a ring of gunpowder on the wall. (One year the students had used gunpowder as a fuel, and the cannon had never been cleaned properly.) That in itself sparked Luis' idea of sealing the ignition hole with a firecracker, which of course would act as a fuse! We took the cannon outside, and, when it did blast the tennis ball almost halfway across the field, the big thrill was not only that it worked, but that the venture was not something out of a manual but mostly conceived by our students. 






































































































































    Comments

    That reminds me of the chemistry teacher in high school (Gymnasium), in the late 1960ies. He wanted to demonstrate, that reactions in metastable mixtures could not only be started by fire, but also with other forms of energy, eg light. No need to go outdoors for such a simple demonstration. So he prepared in the classroom his mixture of chlorine and hydrogen in a big glass jar. The green colour of the chlorine should absorb the light and start the reaction. He needed 2 or 3 litres of it, to ensure enough absorption. To prevent the gasses from escaping, he closed the jar with a glass plate. He wanted to start the reaction with a flash of light from burning magnesium powder on an asbestos plate nearby, amplified by oxygen from a bottle. It worked very well and showed us, additionally, some other fundamental laws of nature.

    The magnesium ignited and produced the flash. The mixture also ignited, got hot and that lifted (the ideal gas law plus Newton’s F=ma law) the glass plate to the ceiling, making a big and enduring dent. Then the plate fell down again (Newton’s law of gravity) and crashed to the floor with the usual result. In the meantime the magnesium was blown away by the oxygen and burnet the wooden desk producing a lot of smoke. A schoolmate got the fire extinguisher and saved the teacher (not in danger, but nevertheless). 20 seconds later, the dean of the school (his office was just on the other side of the hallway, probably he awoke by the “Big Bang”) entered the room and found 20 students behind their desks (but on the floor), a teacher slightly shocked and completely white (the shock, but chiefly the foam of the extinguisher), and the room filled with smoke (and some hydrochloric acid), which slowly penetrated into the hallway (the second law of thermodynamics).

    Needless to say, that I didn’t study chemistry.

    UvaE
    I've had fun and painful experiences with chlorine but always in the absence of students. And I plan on keeping it that way.
    Needless to say, that I didn’t study chemistry.
    In the real world, there are a lot more accidents outside of chemistry. Even in sales and service, the accident rate per 100 person years is higher!


    Bonny Bonobo alias Brat
    You sound like a great chemistry teacher Enrico! I really miss my school day chemistry lessons, they were such fun and we had a great chemistry teacher, Mrs Roe. 

    Some of the experiments went horribly wrong which made them even more interesting, especially as we didn't have safety gas chambers in those days. 

    I will never forget the out of control nitrous oxide laughing gas experiment, I don't think I have ever seen so many people laughing together for so long, except for once maybe at a Dylan Moran comedy performance.

    My latest forum article 'Australian Researchers Discover Potential Blue Green Algae Cause & Treatment of Motor Neuron Disease (MND)&(ALS)' Parkinsons's and Alzheimer's can be found at http://www.science20.com/forums/medicine
    UvaE
    I will never forget the out of control nitrous oxide laughing gas experiment ..
    At least he produced N2O and didn't explode the ammonium nitrate while preparing it! (Assuming of course, he derived the the laughing gas from NH4NO3.)

    You sound like a great chemistry teacher Enrico! 
    Thanks. But there's always room for improvement and a fear of running out of energy!