Day 2 Ask the students to write a standard demonstration report. The following format has served my purposes well with demonstrations I. Obseruations-labelled and colored sketches of what is actually seen 11. Interpretation-including as much explanation, chemical and intuitive, as the student can handle 111. Questions-an opened-ended part that can be used to prevent and detect boredom and lack of attention (Where there is intellectual life, Were are questions.)
I evaluate the reports as follows. +2: very complete, careful work +1: a fair try, some comprehension
4: some effort, minimal or no comprehension
-1: no paper or no effort
In my grading system these points are added to the numerator for averaeine. -. but the denominator is not changed; the denominator comes from the number of letter marks earned on tests. auizzes. etc. I also hand out the fa& sheet shown in the box. Manv students can see the inconsistencv between the genera"lization of greater activity from l e ~ i right o in period 4 and how these three pairs of metals actually perform in water. This agar mix is essentially "semisolid" water. After the calculations (see item 8 on fact sheet) are completed, it all fits a pattern, a satisfying model to fit the observations.
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This demonstration is very popular with my classes and successful in meeting its goals of involving the students. Students eniov actuallv secine evidence that the electrons have migratki to the s l " ~ a reolored ~ l ~ areas. It is very exhilarating for them when the calculations are completed and the volume per proton patterns are seen. The look of pride on their faces at their achievment is so satisfvine that I always feel that the work involved in the materials is well worth the llttle effi~rt! The students get that "eureka" feeling, as if to say Wow I understand hGw electrochemistry with some metals works!" This is great. Literature Cited
Demonstrating The Conservation of Matter A Trilogy of Experiments Submitted by:
David Martin, Randy D. Russell, and Nicholas C. ~homas' Auburn University at Montgomery Montgomery, AL 36117 Checked by:
Galen Meil University of Montana Missoula. MT 59812
The law of conservation of matter is one of the first principles introduced in many science courses, including general chemistry. Students learn that during a chemical change there is no detectable increase or decrease in the total amount of matter in a system. Because the amount of matter can be determined easily by measuring its mass, this law can be demonstrated and, therefore, verified by
following the total mass of a system during the course of a chemical change. The following three short demonstrations, which should be presented in succession, utilize the reaction between two common reagents, dilute hydrochloric acid and calcium carbonate, and illustrate the validity of this important law. Demonstration 1: The Loss of Mass During a Chemical Change One hundred milliliters of 1 M hydrochloric acid is placed in a 250-mL Erlenmeyer flask. Powdered calcium carbonate (2.0 g) is weighed into a small plastic or glass vial. The vial should be small enough to fit into the flask and large enough to hold 2 g of the carbonate. The vial is then carefully lowered into the flask so that it does not tip over in the acid. (It may be necessary to place a marble, or some other heavy, nonmetallic object, in the vial ta prevent it tipping prematurely.) The flask is then placed on an electronic pan balance that is tared to read zero. Very accurate balances are not essential, and a readability of 0.1 g is satisfactory. The flask is gently shaken, causing the vial to fall, and the acid and carbonate to mix, and quickly relaced on the balance. There are no surorises here. In ;bout two minutes all the carbonate dissoives (with occasional eentle shaking) and a mass reduction of about 0.6 e is obseked due to l& of carbon dioxide, as expected. 1; large classes, the mass change can be observed by a student who can inform the rest of the class of the mass loss. If available, a video camera and television would be ideal to project the digital balance readout for the entire class to follow. At this point students can be mvited tucalculatc the theoretical m o u n t of carbon dioxide that should be oroduced during the reaction (0.88 g) and to explain the h a t i o n from the observed mass loss. Because carbon dioxide is soluble in water, some gas remains dissolved in solution. Demonstration 2: The Conservation of Mass Observed -Or is it? The above experiment is repeated with fresh acid and carbonate, but this time the carbonate is placed inside a rubber balloon (using a powder funnel). The balloon is then carefully stretched over the neck of the flask making sure that none of the carbonate falls into the acid. The flask is placed on the balance that is tared to zero. At this point the class can be informed that your intention is to pour the carbonate directly from the balloon into the acid. However- because the carbon dioxide cannot escapewhat will happen to the total mass of the system? The most popular response will be that no mass loss should be observed, because the gas is now contained in the balloon-no surprises here either, right? Wrong! Amass loss of about 0.5 g is obsewed in this experiment. Most students will be puzzled by this observation and scratch their heads as they endeavor to produce an explanation for the apparent breakdown in the conservation of matter law. A hint to resolve this dilemma can be offered by suggesting that students calculate the mass of air dis~lacedfrom the atmosphere as the balloon expands. To do this, they will need the density of air, approximately 0.0012 g/mL at 20 'C and 1atm, and the volume of the balloon, about 400 mL, which can be measured by determining the volume of water displaced when the balloon is hubmerged in a large beaker full of water. Students will find that the calculated mass of displaced air is equal to the mass loss observed during the reaction, about half a gram. Is this a coincidence? The mystery finally may be solved by reference to Archimedes' s principle: "A body immersed in a fluid will
' Author to whom colrespondence should be addressed. Volume 69 Number 11 November 1992
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