overhead projector demon/ tratiow
edited by KOLB
DORIS
Bradley University Pearis. IL 61625
Simple Experiment With Unknown Liquids MaT. and M. Martin Sanchez University of Madrid Madrid 28010, Spain
The only chemical materials needed for this experiment are four smallunlabelled bottles containing colorless liquids: water, 1 M HCI, 1 M Na2C03, and phenolphthalein (1% alcoholic solution). The object of the experiment is to determine which bottle contains which liquid. Students should participate in the process of deciding what steps to take and explaining any observations. Small beakers or Petri dishes can be used for mixing samples of the liquids. The HC1 produces bubbles with the Na2C03; the Na2C03 makes the phenolphthalein turn red; and excess HC1 makes the red color disappear. The water does not react with any of the other liquids.
Figure 1. Clear plastic outer storage box.
[ E d i t o r ' s Note: In presenting this kind of classroom experiment, the plastic spot plate mentioned next might he very useful.]
A Free Plastic Spot Plate for Overhead Demonstrations Carol Gwdgame Marsh Judron Senior High School 9142 Farm Road 78 Converse, TX 78109
The nearest hospital or clinical lahoratory is a potential source of many disposable items that can he useful to chemistry teachers. For example, there are plastic reagent kits used in connection with clinical tests for such things as triglycerides, uric acid, and glucose. They consist of two parts: (1) a clear plastic outer box suitable for storing small items or demonstrations (Fig. 1) and (2) an inner transparent container with 15 or 20 small wells, suitable as a projectable spot plate for overhead demonstrations (Fig. 2). The individual wells are round cups about an inch in diameter (the size of a quarter) and flat a t the bottom. Each cup in the 20-well container holds about 20 mL. The transparent unit can be placed on the overhead projector and used like a set of plastic beakers for demonstrating many kinds of chemicalreactions. The wells can even he connected with strips of filter paper (as salt bridges) and used to demonstrate electrolvsis reactions. The containers can also be used in the laborkory as individual student spot plates. The backeround color can be chanced as needed by placing laminated construction paper o f a n y desired color under the plates. Clean-up is quick and easy, and storage space is minimal, since many units can be stacked together. The clinical reagent kits mentioned here are products of CIBA-CORNING, Gilford Systems, 132 Artino Street, Oherlin, OH 44074. The empty packaging units can usually be obtained free from a local hospital by contacting the administrator and requesting assistance from the laboratory staff. A collection box should he provided, if necessary, and a pick-up time should be scheduled. Because of storage proh964
Journal of Chemlcal Education
Figure 2. Twenty-welled clear plastic container.
lems and health regulations, hospitals cannot let these throw-away items accumulate for extended periods of time.
pH Measurements on the Overhead Projector Suehn~mosv
Sally Solomon and Nancy Hlgh
Orexel Universly Philadelphia, PA is104 CHECKED BY
Mel Mosher Misso~riSouthern State College Joplln, MO 64801
Universal indicator solution' and a corresponding pH indicator chart2 are used to make pH measurements on an overhead projector where the results are visible to all students in a large lecture hall. The pH of each sample solution is found by adding universal indicator and comparing the color obtained with the colors of 12 transparent, colored
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Fischer Scientific, 191 South Gulph Road, King of Prussia, PA 19406: cataiaa number SO-1-60. ~iicher~Eientific,191 South Gulph Road, King of Prussia, PA 19406; catalog number 1-180. ,
squares of the chart. T h e indicator chart is mounted on a transparency slide along with the appropriate values of p H for each of the colored sauares. a s shown in t h e figure. T o perform demonstrations on t h e overhead projectoFa blank transnarencv slide is laced on tor, of the one used to mount the color chirt. ~ e a k e r containi;lg s test solutions are positioned on the slide near t h e color chart, allowing room t o write ionization equations or t o d o calculations. Several demonstrations t h a t can b e done using this colorful technique are described below. Demonstrations Equipment and Chemicals Universal indiratw solution Universal pll indicator chnrr 50-mL beakers 20 mL 1 M NHaCl solution 20 mL 1M NaAc solution 20 mL 0.5 M HAcIO.5 M NaAc buffer solution 0.1 M NaOH solution IdentifyingAcids and Bases Qualitative determinations of solution pH are used to identify solutions as acidic or basic. Students can be asked to predict what they expect to happen when indicator is added to each test solution. Including NH&l and NeAc solutions helps to impress upon students that salt solutions are not always neutral. (Incidentally, most students will predict that NHGI solutions should be basic.) Tap water, snow, rain, colorless sodas, and water with COz (added by breathing through a straw) are among the solutions that can be used for all levels of general chemistry. Recall that the range of the indicator solution is 4-10 when choosing sample solutions. Demonstratingihe Action of Buffers A 20-mL volume of water is put in one 50-mL beaker and 20 mL of equimalar HAc-NaAe buffer solution in another. Toeach is added 1 mL universal indicator solution. Calculated and measured pH values for the buffer solution may he compared at this point. Then a 1mL volume of 0.1 M NaOH is added to both solutions, and the results are noted. Thecapacity of the buffer solution can be demonstrated by continuing to add the NaOH solution to the buffer, 1mL at a time.
3eakers containing t e s t solution! Space for equations
The pH indicator chart is mounted on a transparency slide leaving room for test solutions and equations or calculations. The gray area is cardboard. squares labelled with colors are filled with colored cellophane, and those with pH values are empty.
Volume 64
Number 11
November 1987
965
