The "Chemical Fountain" An old experiment in a new setting Ruth Ben-Zvi and Judith Silberstein The Science Teaching Department, The Weizmann Institute of Science, Rehovot, Israel
". . .the most striking cases of euriosity-arousal are those concerning the strange, the unusual, the puzzling" ( I 1 For many high school students, chemistry is a boring and unexciting suhiect that they have to learn and somehow master. he reasons for this attitude are numerous (1)many of the concepts are abstract and difficult to grasp, especially by young students; (2) the methods of presentation are not geared to capture the students' interest; (3) usually comparatively little effort is made to motivate students to overcome the problems facing them. A new teaching sequence was developed for an introductory high school chemistry course. In this teaching sequence use is made of experiments designed so as to capture students' interest, to enhance their motivation, and to get them involved in the learnine process. Ten such expkriments were introduced during the year, half of them dealinr with qualitative a s ~ e c t sand the other half with An example of the use of a quantitative experiment, tocether with some results of a comparative evaluation study of this teaching sequence was published before (2). The present article will demonstrate the use of a well-known experiment-"the fountain experiment" (3)- suggested by Faradav in 1827. in a new settine desiened esneciallv to enhance the students' motivation and involvement in learning the acid-base concept. It is one of a series of learning situations that we have developed in order to overcome many of the difficulties mentioned in the context of a hieh school cbemistry curriculum (4). I n the "chemical fountain" experiment, use is made of the high solubility of NHs(,) (or HClkj) in water so that when a bulb full of the gas is brought into contact with water, the liquid bursts into the bulb because of the rapid "disappearance" of the gas as it is absorbed into the water' (see Fig.
- .,
1).
The experiment is done as a teacher's demonstration. It is repeated twice-first with HCl(,) and then with NH3(,,. Afew drops of phenolphthalein are added to the water before the exoeriment and without notifvine the students. The students are not told beforehand whatthe experiment is. All they see is two " e m ~ t v "bulbs with Pasteur pinets passine through the corks2 andtwo glasses of clear colo;l&s liquids. ?he &dents are given a questionnaire and asked to: 1) note all they observe. 2) write all the relevant questions they can think of. 3) try to explain what they saw. 4) suggest methods, either theoretical or experimental in whichtheir
theories can be tested.
At 25°C 100 ml of HzO dissolves 82.3 g of HC1 or 89.9 g of NHs. This experiment succeeds only with gases with high solubility in water.
in the second-a medicine dropper containing water. The reaction is started by squeezing the water from the dropper. 68 / Journal of Chemical Education
The following elements characterize this method of presentation. 1) The "chemical fountain" is strange, unusual, and puzzling and as such arouses curiosity and interest in the students. 2) The written questionnaire forces every student to pay attention to the lesson and to try to figure out what goes on. In this way all students, even the "silent majority" have to respond. 3) Since the learning situation is novel, the students cannot draw directly from pre-learned knowledge. Some of the students may indeed have seen acid-base indicators in their biology class or learned about gas pressure in physics lessons, but that does not, in fact, help them in understanding what happens in the fountain. Moreover, it may add to the feelings of conflict as can be seen by questions asked by them:
.
How does the liquid "overcome" the pressure in the bulb? Why is there a change in color in only one bulb? Why does the water riwin spite of the forces of gravity?
The formation of cognitive conflict has long ago been recognized as a good method for enhancing students motivation to learn ( I ) and indeed the student who is puzzled by the apparent contradiction between his assumptions and observations becomes eager to understand t h e contradiction and thereby resolve the conflict. 4 ) In the third part of the questionnaire, the students are asked to give an explanation of what they have seen. Examples of their answers translated from the students'own words are quoted below: Ithink that the reaction between the gases causes the formation of material with a volume smaller than that of the resctants and that is why the water "jumps" inside. A more plausible explanation is that in the water there is a material that causes the gases to dissolve and then their "affinity"ta dissolve
is greater than their "affinity" to keep atmospheric pres~ sure. Different gases react in a different manner with the same liquid because of their different properties. The water reacts with the gases in a differentmanner as was shown by the different color. The ohenomenon that water rises is ~ r o b a hcommon l~ ..-.-~ to all the
.
stances behave in a similar way. If possible, their suggestions should then be followed up and a variety of acids and bases tested. The students are then told what the original gases were and the following discussion carried out. First a comparison of the role of water in the two reactions.
pasea.
N~OH(S) %~a+(,)
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ferent, shows that the phenomenon does not happen just because of structure of the gas hut because of what was added to the water.. .. The teacher who collects the questionnaires after the experiment can use the students questions and suggested t h ories as a basis for discussion in the following lessons. I n tli!-s manner students get an immediate response and positive feedback, which also encourages their involvement in the subject matter and their feeling of satisfaction. In order to form a basis for introducing the topic of acids and bases, it is suggested that the teacher should not present the experiment by just demonstrating the two fountains-one colorless (HCllgI)and the other colored ( N H d The following stages are suggested: 1) Before the experiment, the teacher should test the conductivity
of the two liquids (in each case-water containing phenol~hthalein). 2) kfter the experiment, repetition of conductivity measurement shows that ions exist in each liquid (althoughthe NH3solution is n Door conductor).The difference in colors indicates that the ions
+ OH-*,)
(1)
(2) N H x ~+I HzO NHI+~wI + OH-iw) leading to the definition of a base according to Bronsted and ~owry; The students studying the curriculum "Chemistry for High School" (3) are familiar with the reaction
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(3) NHxp)+ H C ~ I NH&%s) having encountered it during their studies of properties of gases. This reaction is now brought up again and compared with reaction (2) (the role of NHJ as a hydrogen ion acceptor) and with the neutralization of NH3(,,] by HCl(,,) leading to the net reaction
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Htlaq1 + 0H-c,,1 Hz0 Summary Motivation to learn is difficult to define and even harder to measure. A teacher once said: "Motivation is that sparkle in the eyes of students which tells me I have succeeded in my joh." We suggest that the "chemical fountain" experiment used as suggested here may bring that sparkle to the eyes of many students. Literature Cited (I) Borlyne,D. E.,ATheoryofH"rnanCunosity,Britlah J o u r ~ l o / P ~ y ~ h a l o g 45.18a-191 y, 11954).
had reacted and somehow disappeared
(2) Bsn-Zvi. R.andSithsrslein.J.,JCHEM. EDUC.,57,792 (1980). (31 Fowies. C.. Lecture Experiments in Chemistry. 6th Ed., G. Bell & Son% London.
I i this part
(4 '1Chsmi8rryfoiHlphSchor,l."ExperimentalEdition, Rehovot, Weirmannlnntitufe of
incorpuntted i n t u the original experitne~~r. t r \ ru lind what other u h students ttauiillysuggert rhnr tr
1963.
Science, 1972,3 Vols. lin Hebrew).
Volume 58, Number 1, January 1981 / 69
