overhead projector demon1tratiow
edited by DORIS KOLB Bradley University Peo!ia, IL 61625
Some Ideas From The Past The following demonstrations are well known and have been described earlier in this Journal. some of them more than half a century ago. All of the demomtrations are simple to prepare and highly suitable for display on the stage of an. ordinary overhead projector. In some cases directions have been modified. Common ion Enect Fillinger, H. H. J. Chem. Educ. 1931, 8, 1852-1855
T o an aqueous solution of ammonia (about 1 M) add a little phenolphthalein indicator solution. The red color of the solution indicates the presence of hydroxide ion from the ionization of the ammonium hydroxide. Stir a little solid NH4C1into the solution, and the red color disappears, since the NHI+ ion suppresses the ionization of the aqueous ammonia. A similar demonstration can be carried out usine acetic acid. A solution of acetic acid (about 1M) containinga little methvl orange indicator is red because of the hvdroaen ion when a small amount of solid sodium acetate is stirred into the solution, the color changes to yellow, since the increased acetate ion concentration suppresses the ionization of the acetic acid. Silver Tree Walmn, J. H. J. Chem. Educ. 1931, 8.303-306.
Twist a few pieces of copper wire together, press the form flat, and place it in a Petri dish. Pour over it a solution of 2% silver nitrate. The copper displaces the silver ion in the solution, and crystals of silver metal start to form immediately dong the surface of the copper wire. In a similar manner a lead "tree" can be made by pouring 5% lead acetate solution over a frame made of thin strips of zinc. Crystal Formation from Supersaturated Solutions Bacon, E. K. J. Chem. Educ. 1948, 25, 251-252.
A tiny "seed" crystal of solute is dropped into a clear, supersaturated solution, and crystals "grow" in all directions starting from the point where the first crystal fell. Solutions that are supersaturated to a suitable degree, so that crystal growth is not too rapid, may he prepared as follows: Sodium tbiosulfate (66 g NazS2Oa.5H20+ 15 mL H20) or sodium acetate (35 g NaC2H302.3H20 20 mL HzO) is heated until all solid has been dissolved. The liquid is then cooled back to room temnerature (without crvstal formation) producing a supersaturated solution. (The solution can he prepared several days beforehand.) Adding a small crystal of solute initiates the crystallization of excess solute from the solution. (Although a crystallizing dish, Petri dish, or beaker can be used for this demonstration, an especially convenient container is a wide, flat-form weighing bottle, 5-7 cm in diameter, equipped with a cover. It can be used to dissolve the crystals on a hot plate, to allow the liquid to cool and form a supersaturated solution, to demonstrate the crystallization on the overhead projector, and finally to store the material until it is readv to he used aeain. The same " solution can he used over and over many times before a fresh solution needs to be prepared.)
+
Making Iron Passive with Nitric Acid Moeller.1. J. Chem. Educ 1941, 18, 296.
Place some 6 M HN03 in a beaker so that it is 1-2 cm deep. Take an old iron file and nlace i t in the acid solution. showing that the iron reacts k i t h the acid producing bubbles along its surface. Then take the file and dip i t into concentrated HNOs, which makes the surface of the iron passive. Bubbles no longer form on the iron when it is nlaced in dilute acid. If the file-is scratched to expose fresh-surface on the iron, its activity is restored. Optlcai Activity Nolier. C.
R. J. Chem. Educ. 1949, 26,269-270.
A pair of Polaroid sheets and a heaker containing a solution of an optiailly acti\.r rornpound ran be used tLiemnnstrntt: the prinriple ofthe polurimrter. One Polaroid sheet is laid dircrth. on the ~ r o i e r t ostare. r while the othrr is olared on top of the heacer,"so that ihk solution is sandwiched between the two Polaroid sheets. As the sheet on the too is slowly rotated (within the horizontal plane), the background area darkens, while the circular area of the solution remains light. The solution area will darken, hut a t a different degree of rotation, indicating that the solution has bent the plane of the polarized light passing through it. A recently prepared solution containing about 100 g- of sucrose dissolved in 100 mL of water worksvery well. Carbon Dloxlde In Human Breath Dreisbach. 0. A. J. Chem Educ. 1959,36, A159.
Place about 30 mL of alcohol in a heaker and add some alkali blue indicator. The solution should be red. If it is not, add a few drops of dilute base. Set the heaker on the projector stage, and ask a student to blow into the solution either directly, while the solution is being stirred, or using a straw. The color changes from red to blue (the acid color of the indicator), since human breath contains C02, which is a weak acid. If phenolphthalein is used as the indicator, the initial red color simply disappears. Amphoteric Hydroxides Alyea, H. N. J. Chem Educ. 1956, 33, A377.
Pour about 10 mL of 1M P b ( N 0 3 ) ~solution into each of two beakers. Add a few drops of 6 M NaOH to each one, forming precipitates of Pb(OH)2. (These will appear as black spots on the projector screen.) When 6 M HNO, is added to the first heaker, the precipitate dissolves. Pb(OH), + 2 HNO,
-
+
Pb2+ 2 NOs-
+ 2 H,O
Edltor's Note: On page 350 of fie April 1987 issue of This Journal (column 1, paragraph 5) Ihe reduction product of the dichromate ion should be chromic ion. Cr3+, not Cr2+. We regret the error. The box at the upper leR of that same page contains several misprintedversions of lhe thiocyanate Ion, all of which should be SCN-.
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When 6 M NaOH is added to the second beaker, that precipitate also dissolves. Ph(OH)2+ 2 NaOH
-
2 Nst
+ Pb0:- + 2 H,O
The P b salt can he replaced by the salts of other elements, such as Al or Zn, that also form amphoteric hydroxides. Surface Tenslon-Mercury
"Beatlng Heart"
Campbell, J. A. J. Chem. Educ. 1957,34. A105 and 362.
Place a dropperful of clean mercury in the center of a watch glass. Add 6 M HzSOa until it just covers the mercury. Then add a dropperful of K2Cr207solution (about 0.1 M). Place an iron nail, or a piece of iron wire, so that it points toward the center of the watch glass, with the tip just touching the mercury. (It helps if the iron is attached to a support outside the watch glass so that it is held firmly in place.) T o initiate the demonstration drop concentrated HySO4 onto the mercury pool, one drop at a time, until a rhythmic motion begins. Because the mercury pool often assumes a triangular shape, i t somewhat resembles a heating heart. The surface tension of Hg depends on its degree of electrical charge. When the Hg touches the nail, the Hg picks up electrons from the Fe, thus increasing the surface tension of the Hg and causing it to become more spherical in shape. When the dichromate oxidizing agent removes electrons from the Hg, the surface tension goes down and the Hg pool
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Journal of Chemical Education
spreads out. But as it spreads out, it touches the Fe and picks up more electrons, so that the cycle is repeated. Natural Acld-Base Indicators Srabadvary, F. (Oesper, R. E., trans.) J. Chsm Edoc. 1964, 41, 285-287.
The fact that many natural plant juices can be used as acid-base indicators has been known for several hundred years. Grape juice, herry juices, and extracts of flower petals have long been known to change color in acid and hase. In the 17th century Robert Boyle often used "syrup of violets" to identify acids and bases, although he also used litmus and extracts from roses and various other plants. More recently Mehane and Rybolt [J.Chem. Educ. 1985, 62, 2851 have examined the color changes of indicators made from 15 different edible fruits and vegetables. Demonstrations using such common natural materials as acid-base indicators are very effective when shown on the overhead projector. For example, pairs of beakers containing samples of plant juices might be treated with acid and then with hase to demonstrate their indicator action. Or perhaps an indicator such as grape juice might be used to test a group of household materials such as vinegar, ammonia, lemonade, white wine, soda water, or a water solution of sugar, borax, baking soda, or detergent. The liquids should he transparent, of course, and colorless for best results.
