filtrcte~ & re~idue~
edited by ROBERT REEVES Marlborough School 250 S. Rossmore Avenue LOSAngeles, CA 90004
Charles's Law: Students Develop Their Own Procedure Diane Rose 2882 Ziegle. Cincinnati, OH 45208
High School chemistry students spend a great deal of time in the laboratory. They are told to follow the procedure precisely and to perform the experiment safely in order to achieve good results. How many students actually realize the significance of each step and its order in the experiments? The steps are often carried out without considering the effect of a specific chemical or the end result. Students sometimes fail to use the correct amount of a particular chemical because they do not think the precision is that important. Some students finish an entire experiment without even thinking about the purpose. This article gives teachers a novel approach to help their students appreciate the importance of each step given in the experimental procedure. An experiment involving Charles's Law has been chosen since the relationship between volume and temperature is easily understood and virtually no chemicals are necessary. After learning the purpose of the experiment and receiving a list of equipment and chemicals, the students are asked to write their own procedure and execute it in the lab. Prelab First, the students are taught the importance of an experimental procedure. Next they learn the meaning of variables
712
Journal of Chemical Education
and controls within an experiment. Then the students examine several simple experiments for validity by posing the following questions: (1) Are the controls adequate and the variables changed one at a
time? (2) Are enough dataeollected? (3) Is the purpose achieved?
In an experiment in which the order of the steps has been jumbled, the students should he able to arrange the procedure in the proper order. Charles's Law is then introduced. I t is critical that the students understand the volumetemperature relationship in order to develop a procedure. Have the students identify all the experimental variables related to Charles's Law and determine which of these are to be held constant by the experimentor. The students must he adequately prepared in order to write their own procedure for the first time. Providing them with the following handout was quite helpful. Handout The purpose of this lab is to investigate the relationship expressed in Charles's Law. Yon are to develop a procedure
Temperature versus Calculated Volume'
5
Trial
1
2
3
4
Temperature. 'C Circurnference,cm Volume, cm3
20 28.5 391
48
64 30.5 479
82
99
31.0
32.9
29.4 429
Circumference = 2rr
503
601
(2)
circumference r= 2*
Tempsraturs I T 1
Plot of
temperature versus volume of a gas for student results.
using the following material that will examine this relationship. 1 Balloon-spherical shape String Metric ruler
Postlab
Allow the students time to work in small groups to discuss ideas for the procedure and summarize these ideas as a class before the end of the period. Stress that the students must have a completed procedure and data table before beginning the lab.
The next dav the students comoared their results with those of their ciassmates. Graphs oivolume versus temperature pinpointed experimental errors. In eeueral, volume rneas&ements a t temperatures above 90 'c-were inaccurate due to the difficulty of holding the string around the balloon at high temperatures. The volume-temperature line was extrapolated to zero volume so as to determine the experimental value of absolute zero. A class average of 15%error was obtained. The results did verify the direct relationship of the volume and temperature of a gas. After several measurements were taken, students began to anticipate the change in size of their balloon. Students noted pnhlcms that they had not anticipated in their exoeriment. Measuring the balloon'svolume was bvfar the most difficult obstacle.'k suggestion was made to &e a taoe measure instead of a string and ruler to measure the ci&nference of the balloon. &hers felt that finding the balloon's volume by water disolacement would be more accurate. A debate ensued. Several groups found that when their balloons were allowed to expand in the hot water bath, they became too large for the beaker. One group used a very small balloon and discovered their percent error was much greater than the class average. An approximate size range for the balloon was sueeested. A few students wondered how oressure was keot constant. The discussion was quite stimulating because some students did not acceot that the balloon's elasticitv would keep the pressure essktially the same.
Lab Noles
Concluslon
The experiment is easily executed in 50 min. Students made four to six volume measurements from el00 OC. Very few students thought to use the room temperature volume of the gas as data! Many students had difficulty measuring the balloon's circumference as it must be submerged in the water bath to get an accurate reading. Some held the string around the balloon with their hands, others used tongs. Creativity abounded! Samnle data is presented in the table and the results are plotted in the figure.
The students modified their procedure based on what they had learned from the experhent and class discussion. Their comments indicated that they learned a areat deal. They were more apt to follow a prockdure carefuily the rest of the year since they had learned that there is a reason for the detail in each step. I believe that this experiment is well worth the time involved. Not only does i t demonstrate Charles's Law, hut it also gives the students an appreciation of laboratory procedures.
Ice
Bunsen burner Thermometer 2-L beaker Ring stand Ring clamp Wire gauze Tongs When designing a procedure for this experiment i t is important tocontrolvariables suchas pressure and the number of molecules so that the volume-temperature relationship can be studied. Here are some questions that will help you get started. 1. What container will hold the gas malecules? 2. How will the number of molecules of gas he held constant? 3. How can the volume of the gas he measured? Recall that the
formula for the volume of a sphere is
4. Haw will the temperature of the gas be varied? 5.
How many volume-temperature measurementsshould be made?
Volume 64
Number 8
August 1987
713
