Maybe Just One Point of Light?

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  • Maybe Just One Point of Light? Gary L. Pool Western Carolina University, Cullowhee, NC 28723

    As has been advocated, discovery by students enhances their interest and accomplishments in the chemistry labo- ratory (1, 2). Discovery through classroom exercises to challenee students' reasonine. rather than s im~le recall. - -. also is exciting to them and to teachers who can vicariously experience the excitement of the discovery. The exercise must draw on the reasonably expected experiences and current knowledge base of the students. The formulation of the goal must be obvious and uncomplicated in order not to obscure the quest. I have used the following activity with groups ranging in experience from high school science stu- dents to college faculty, and the discussions generally fol- - . ~ - low a similar sequence.

    The Problem and the Illuminating Question The problem is stated simply:

    I have been invited on a space flight, but I am afraid of the dark and will take along a candle in case the lights in the spacecraft fail.

    The question is,

    Will the candle burn in the spaceship while in flight in space?

    The charge is to formulate an answer and a rationale to defend that answer. The class is divided into discussion groups deliberately situated so they can overhear each other. Each group is given a candle, and I step back to mon- itor. I agree to answer questions of clarification, hut I min- imize my intervention in the flow of ideas. I am always entertained, enlightened, and excited by observing the dis- covery process in action.

    The groups begin their deliberations with the usual ini- tial random comments such as:

    "Of course not!" "Take a flash light." "The shuttle would explode." "There's no gravity." "There's no air."

    Eventually they light the candles and begin to focus on the real parameters suggested by their observations. It is evident that they begin to recall from their previous sci- ence that burning requires the essentials of combustible fuel and oxygen. The exchanges go something like this:

    "The wax of the candle is here and on the spaceship so there are no fuel pmhlems."

    "But there's no gravity." "So what?" "The hot wax would float around the cabin and bum you." "But that presumes the candle would bum." "What about the oxygen?" " h e seen pictures of astronauts in the shuttle. They breath

    without air tanks, so there must be oxygen in the cabin." "The smoke would fill the cabin, and we would suffwate." "But first the candle would have to bum."

    Soon a consensus begins to emerge concluding that the major identifiable difference between earth and the space- craft is that there is no significant gravity on the space- craft in space. They will submit this as the final answer, but i t cannot be allowed without a rationale, a reason, a

    defense. In other words, how would the absence of gravity make a difference, if any, in the candle burning?

    Shedding Light on the Question - - In the ensuing discussions you hear the conversation

    switch to considering the flame and gravity, and comments focus on their observation that theflame-points up. (One comment overheard was, "There's no up in space so the flame will not know where to go and will just get con- fused.") Then one of the students usually brings up the subject of the flame heating the air and the hot air rising. (Now or later their use of "air" must be modified to include the hot ~roduct eases. carbon dioxide and water.) Some- one typi'cally w i f s h k e that this is why you are supposed to crawl on the floor to escape from a bumine buildine. because the hot, smoke-filled air rises. Many Goups wiTi explain this by the statement that the hot gases are lighter than the cooler surrounding air. Ifthey do not modify "ligh- ter" to "less dense" themselves, then prompting questions from the monitor are necessary. Excitement rises as the students sense that they are getting close to an answer.

    Typically one of the students will then state that it is obvious that. in the absence of eravitv. the gases have no density or that hot and cool gas& have the &me densities. Thls s~imulatcs an interesting round of discussion. Even- tually, recalling that density i s mass per unit volume, they realize that the property of densitv does persist in space and that the productgase still wodd havea lower density than the cooler surrounding air, but in the absence of grav- ity a difference in density is of no consequence. Hot and cool gases alike would experience weightlessness. The pic- ture then evolves that on earth when a candle burns the hot product gases are less dense than the cooler surmund- ine air so the hot eases rise and are redaced bv cooler. - oxygen-rich air (convection) that sustainsthe combustion: If the spacecraft candle begins to bum in the absence of gravity, the product gases depleted of oxygen would re- main surroundine the candle and the flame would be smothered. perhaps a candle would not be of much use on a dark spaceship.

    Other Bright Ideas from the Students - The exact flow of the student deliberations depends on

    their backmounds and is not totally predictable. Other in- teresting aspects of this topic mayakse spontaneously or may be coaxed out by prompting questions from the moni- tor during a wrap-up where each group presents their analysis. One group discussed whether or not the product gases would form a perfect sphere around the wick in the absence of gravity. One gmup knew that carbon dioxide eas was more dense than air because someone had seen ;arbon dioxide poured down a trough to extinguish can- dles. but as a hot ~roduct gas here it is less dense. Another group discussed h a t a b&ane lighter or Bunsen burner does not depend on gravity. The rush of pressurized fuel aspirates in fresh oxygen (i.e., Bernoulli effect). An inven- tive m u p wanted to run around the spaceship with the candk in-hand to get away from the gases and to keepa fresh supply ufoxygen available. They rcplsccd that idea bv usinrr a fan instead. Also amsidered was the aues- tion tgat if tce earth had the high gravity of hp i t e r Gould

    Volume 70 Number 2 February 1993 145

  • candles flare up more or would the wind of inrushing air just blow them out?

    Some group will go beyond your expectations and visit the library as one of my gmups did. They reported to me that sin= 1972 NASA has had the technology to create an environment that will support life but is not conducive to sustaining burning. When compounds such as CF4, C2Fs, and CsFa are added to the atmosphere, burning is virtually impossible. The chemistrv behind the combustion retard- ing and extinguishing properties of these halogenated compounds (3) forms the basis for another interesting dis- - covery session.

    Durine the course of their discussions in emuns or in a - z wrap-u