edited by Marlooro~gnScnoo 250 S
Rossmow A d e n ~ e
LOS Angeles. CA 90004
An Invitation to Experiment with an Experiment Otto Phanstiel Jacksonville Episcopal High School. 4455 Atlantic Boulevard, Jacksonville. FL 32207
me Problem method itself. The following combination demonstration and exoeriment arose from mv exnerience at the Drevfus Institute. I dope it illustrates the excitement as well as t6e idea.
Too often the commercial lab manual we give to our students is a collection of "How To" reciues. In an effort to shorten the experiment and tomake thing's simple, the manual write-ups in effect remove the excitement and mystery that are involved in chemistry. The manual usually begins with a How To use lab equipment and almost certainly includes a How T o determine density, a How T o determine a melting noint. and a How T o find the ionization constant for a weak k i d . i n every case, the manual supplies the student with a auestion he or she never asked, a method that someone else created, a blank slot for the student's data, a series of questions with blanks for analysis, and then the student is asked to make a conclusion. Somehow, something gets lost, and yet we often wonder why students are not more excited by chemistry.
me Philosophy I suggest that a true experiment really does not have a conclusion: instead it is an exciting. dvnamic relatioushio between the question posed by the-individual student, thk method develoued . bv- the student toanswer the auestion. and the results the student finds, which often lead to a new question. A true exueriment does not have a definite answer nor can it he fittedinto a time slot. An experiment may last five minutes or a lifetime. I t is more like a dialog between the ohserver and the natural world around the observer, I use a diaeram to illustrate the parts of an experiment: in particular, iishows the connections and the unending sequence of the parts.
The Context Sometime durine the middle of the school vear. first-vear chemistry studenGbegin their study of acid-base chemistry. In the typical curriculum, the discussion of indicators comes a t the end of the acid-base section. I suggest turning this around and introducing the concepts of acid-base chemistry via a demonstration of various indirators. I encourage use of the vorahulary of acid-base chemtrtrv when explainina what the students &e seeing even though they may not he f s i l i a r with the terms. This is an excellent opportunity for the student to "hear" these terms for the first time while visually seeing the effects, rather than learning the vocabulary as isolated words to he used later. The purpose of this demonstration is to show the use of indicators, pH's, titration, huffers. acids and bases. It is not to heein an in-deuth discussion of any one of these terms since this is the initial exposure. In fact, the instructor's opening statement might he This is probably the most heaotifulor at leasr thr ma4 colurCul part ofchrnlisrry. Chemists use mdirntors to trll fhrrn what the pH of a solution is, and indicators are also used to determine the strengths of acids and bases. Often they are used to simply tell if something is acidic or basic. The demonstration uses five different sinnle indicators - soecies . and 15 small heakers arranged in five rows containing 5 ml water. The demonstration can be done on the stace of an overhead projector. The Demonstration Place a few drops of the first indicator in the first set of three beakers. Add five drops of base to the first heaker, leave the second as a control, and add five drops of acid to the third beaker. A 1M HCI and 1M NaOH solution will do fine. Repeat this procedure with the remaining indicators until all 15 beakers have an array of colors. Allow and encourage student comments about the various colors but do not go into an in-depth discussion. Move the discussion by pointing out
When students experience such a dynamic relationship, they learn more than facts and procedures. They have, in essence, been caught up in the excitement of the scientific
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The rolor of a sperific indicator is drpendrnt on the pH 181the sdutwn. One can mix the indlcatorc nnd produce colw changer at rweml different pH's. The demonstrator might actually mix the five single species indicators being used or display a universal indicator as an example. Continue the demonstration with titration. Set beaker with 5 mL of water, several drops of the universaiindicator, and three drops of 1M HCI. Now titrate the sample
. slowly using a weak hasic solution in a huret, a pipet, or simply
The Interaction If this is the first time the students have done this type of experiment, he ready for a barrage of questions. Portrayed below might he the worst case of helplessness the instructor will encounter.
an eyedropper, The students will see color changes, hut the changes will hequick. Chemists often use huffer solutions, which are solutions that remain at a fixed pH range.
Student (holding flower): How do I get the color out? Teachec I don't know, but isn't that what a solvent does? Student: What solvent should I use? Teacher: What solvents do you know? Student: Water. Teacher: Try it. Student: Nothing happens. Teacher I hope you wrote that in your lab hook, hut that makes sense, since rain would wash the color out and as far as I know that doesn't happen on the planet earth. Student: Well, then what should I use? Teacher What other solvents are there? Student: Alcohol. Teacher. Try it. Student: It's turning color. Teachec Write it down. Student: How do I remove the solvent from the petals? Teachec I don't know. Student: Well, you're no help. Teacher: Right!
The next topic to he demonstrated is a buffered system. Begin by stating that these samples of commercial buffers are at different p H ranges. Obtain six new beakers each containing different huffer solutions. Buffers around pH 2,4,6,8,10, and 12 are ideal. Label each beaker with the p H number. If necessary, the buffers can be prepared (such information is found in "The Merrk Indrx,"puhlished by Mercke& Co., Inc.. 10th d l . Add a few droos of universal indicator to each heaker and display the array of colors. At this point, try to avoid an indeoth discussion of huffers since the demonstration is iust an introduction. The time frame for this demonstration can ranee from 20 to 40 minutes depending on the student responses. i f the class can oroceed immediatelv to the lab. the interest eenerated hv the demonstration is channeled into the 6xperiment, an ide2 situation. The Experiment
The teacher input is an opening statement andvery simple instructions: You may know that all flowers have unique and often very unusual indicators. Or if you like the sense of the dramatic: What makes a red rose red? The instructions are to take out your lab book and date the page. Your task is to go out and ehwse a flower-isolate the indicator and determine the pH where your chosen indicator changes color.
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Note: If no flowers are available outside, florists, funeral parlors and graveyards could he a free source of old flowers. Make sure each student has hisfher own flower or flowers. The style of the teacher now becomes Socratic; that is, answer a question with another question. The instructor's role is to guide, encourage, share enthusiasm, hut to try not to answer questions. The Write-up Instead of writing up the experiment in a formal maimer, trv these three headines: Question-Method-Results. If the students are not usedto t i i s methodology, clarify what each term means A Question: is what "pops" into your mind as you proceed with your experiment. A Method: is whatever you try to do to answer the question. A Result: is what you observed, measured,or found out using this method. The next instruction is vital If the results you find cause you to think of a new question, then write down the question. When you think of a method to answer theqnestion. write down the method. When vou .. eet results. make sure to remeriher L O w r m d m n t h re~ults. ~ R c m ~ ~ m b ~ r i I ~ ~ ~ihtng r e i ~ as a u rrmq r r a ~ l r Whnt . youohrerve,a~strange as it seems, is really what happened
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The Equipment Equipment requirements will vary depending on how creative an aonroach the students take to the oroblem. Often. achemistry iab student drawer hasall theequ'ipment needed: The instructur'h jul~ufren is to supply the chemicals requested by the itudents. Of course, watch for unsafe conditions. In smaller classes. students work individuallv: however if there are 20 or more; pairs of students can obtain satisfactory results.
Most students should he hevond that. and anv student with minimum lab skills should h e able to extract the color, filter the solution, test with a few drops of base and few drops of acid, and set up a series of test tubes with different huffers to determine where changes occur (all without instruction). Note: If student's indicator solution is faint, the instructor might sugaest concentrating.i t by evaporating the solvent on a hot (no flames). If the student a t this point says, "That's i t I ' m done," the instructor might remind him he is to isolate the indicator, and ask "How do you know you have just one substance in your solution?" At this point, a very strange event occurs: The student is authentically involved in hisher experiment and has a need for a How-To lab orocedure. An excellent tool to have in the lab is a file of ~ o d - T O lab procedures taken from old lab manuals. I have seen students go through four or five procedures on their own time to resolve a question they were trvine to answer. Ironicallv. I have seen the same students c o k p k n when given a ~ o w - ; r olab to do during their assigned lab time. The How-To file should have a large selection on How-To seoarate substances. One folder should he devoted t o chromktugraphy. In the srudents'searches, they could try several mrrhud\ hefore they find the right solvent to redlsret a g o d separation. Be suie the folder contains n-hutinol-ionc. NHs(aq) (41) solvent, and n-hutanol-glacial acetic acid (41) solvents with appropriate cautions and instructions on hazard avoidance. The extraction should he left overnight for maximum separation. Most students will reach this point by the end of a double period lab (90 min). On the following day, the teacher suggests a black (UV) light t o see the different lines of separation. A spectacular display of colors may be observed. Now hand the students a pair of ~ scissors ~ ~ ~ c hand tell them to separate the fractions and find which one is the indicator.
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The Lab Book The student's lab hook by this time could have as few as five or as manv as 25 seauential Question-Method-Result sections. I t is feasible to stop the experiment here, but by now the student may have hecome so involved that he or she will want to continue-often with questions such as "What is the structure of this unique indicator?"or "What is i t made of?" Now both the instructor and the student will need help unless the lab has a gas chromatograph or an infrared spectrophoVolume 62 Number 6 June 1985
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tometer. T h e student has reached fithe.limits of what can he done in a high school chemistry lab. Don't miss the golden opportunity t o emphasize: 1) Often questions are unanswered because of the limitations of the tools we have. 2) A red experiment never has a conclusion. The Time Frame
T h e average class of 20 chemistry students with minimum lab skills should he able to d o both demonstration and lab experiment in three class periods (one double lab period and one regular class period). An excellent follow-up experiment is a determination of the molarity of a n unknown basic solution using a known acid and a student's own indicator extracted from the flowers. Warning: Be ready t o discuss why i t worked or why it did not work.
The Personal Experience Chemistry teachers should try the students' experiments before assigning them. This experiment was developed a t the Camille and Henry Dreyfus Institute held a t Princeton and shows t h a t even old and iaded chemistrv teachers can eet "~ excited from this methoddloyy. T h e initial idea camp from a lecture civen b\f Iloris KoIh who mentioned the unusual nature of naturaiindicators extracted from flowers. My initial reaction was that it might be worth a trv even thoueh I was not expecting much t o happen. ~
My initial Questions: Would this make a good experiment far high school chemistry students? How good will the indicator extracted from flowers he? Method: Go out and pick the purple wisteria flowers growing on the walls of Frick Auditorium. Results: (In an attempt to he as inconspicuous as possible, I started to grab the petals and place them in a beaker.) My fmper was immediately stung by a bee who was also utilizhg the flowers, which proves that all exoeriments can be dangerous. Question: What should he used for extraction? Method: Try denatured ethyl alcohol 95% and mix with petals. Results: A grayish-colored alcohol solution (very uninteresting). Question: Since the solution appears very diluted, will concentrating the solution help? Method: Use hot plate and evaporate. Results: A more concentrated grayish liquid. Question: What effects do acid and base have on solution? Method: Place two small beakers on white paper. Add 5 ml of 1M NaOH to the first and 5 mL of 1 M HC1 to the second. Then add a few drops of the concentrated wisteria indicator,
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Hraulls nasr rurnrd hrghr red (the q ~ ~ i v s l e~~f~)hmolphthnlein~ nt nnd acid turned hright yrllw (wr). imprecsnr~. Question; If hase side is red and the acid side is yellow, where is the blue to create a purple flower? Method: Try titrating acid solution with hase. Results: Yellow solution turned green-blue-red. Question: In what pH range will this wisteria indicator change? Method: Set up a series of test tubes with different buffers to see color sequences. Results: An entire array of different colors resulted. This indicator is almost equivalent to a universal indicator. (This is hecoming interesting.) Question: How many indicators are in this sample? Method: Try to separate fractions using paper chromatography. Trv alcohol. Results: separation occurs hut not very good (solvent too volatile). Question: Need a solvent system that will separate fractions slowlv. Method: A& experts. Results: N-hutanol and concentrated NHJ were suggested by visiting professor, and n-hutanol and glacial acetic acid were suggested by lab assistant who had worked on plant extractions for 10 years. Question: What is best system to use? Method: Trv both. Results: N-butanol and concentrated NHa(aq)(41) showed excellent separation and had distinct fraction lines. Question: While separation lines were visible, a better way to show the different fractionsis needed so student could cut out the different fractions and separate components. Method: Try UV light. Results: Brilliant display of at least 15 different fractions. Many shades of blue fluorescence.(Spectacular display was final surprise; I'm sold!) In terms of my original question: "Would this make a good experimrnr for high schuol c h m i s r r y studrnrs?" The answer was obviously " ~ r s "tor the following reastms: 1) Each step involved new surprises. 2) Experiment easily holds the interest of the student. 3) Procedure involves use af pH, titrations, indicators. buffers.
chro~narorr~phy. and orheibasic veparorlon terhniq&a. I I Exprrimrnt isupen-ended: rhor is, rhe rrudcnt could continue thls exorrimen1 will1 more rauiommt. . 5) Mast important, the student is in constant dialog with unknown phenomena.
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Let m e encourage you t o try i t and enjoy yourself. Chemistry can and is fun! And, rest assured, the students will be quite attentive when pH, titrations, huffen, and Le Chaqier's principle are discussed.
