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Tested Overhead Projection Series Cornp~led by HUBERT N. ALYEA Princeton Universitv 9. ATOMIC STRUCTURE. RADIOACTIVITY (continued) C. Planetary Dem. 395-Covalent Bonds, Cu(NH3),++ To show: Formation of the complex Cu(NH&++ as an example of coordiriate covalcnt binding. Materials: NHraq, C-3, stirrer, CuC12-aq. Procedure: (a) Project C-3 with cach cell two-thirds full of CuCl,aq diluted to pale but distinct blue. (b) Add NHraq dropwise with stirring to Cell 2 until permanent precipitate forms. (c) Add NHa-aq drop- wise, with stirring, to tube 3 until the precipitate which first forms, redissolves. Observations: (a) Pale blue hydrated Cu++. (b) Pale blue (show macro) Cu(OH)% which projects a dirty green. (c) Deep purple cupric tetra-ammine ion, Cu(NH&++. In this, the N of cach NH3 furnishes two electrons to the cupric ion to give coordinate covalent binding. Dem. 396-Hydrogen Bonding To show: Hydrogen bonding as evidenced by viscosity. Procedure: Nearly fill three round plastic boxes, 3-in. dia. X 1-in. high, with glycerine, water, and ethanol: on each liquid float a match-stick. Place the boxes on top of one another on the horizontal stage and project. Momentarily swirl them simultaneously. Observation: Hydrogen bonding, evidenced by high viscosity is greatest in glycerine, least in ethanol; the match ceases swirling sooner in the most viscous. Reference: CAMPBELL, J. A., J. Chem. Educ. 34, A105 (1957). 10. GROUP ZERO. THE ATMOSPHERE A. Atmospheric Pressure See Dems. 7 89, 190. B. Substances in the Air 1. OXYGEN Dem. 397-Air + Pyrogallol Experiment developed by W. H. Slabaugh. To show: Analysis of a gas, such as air, for oxygen content. Materials: Soh. A. 30 g NaOH + 30 ml H,O. Soh. B. 3 g pyrogallol + 10 ml HzO. Mix equal volumes of A + B just before using. A 20-ml hypo- dermic syringe. Procedure: (CARE: alkaline pyrogallol solutions is extremely caustic and also stains the skin.) Avoid warming syringe with hands. Fill syringe with 20 ml air. Project to show volume of gas. Now draw in about 2 ml alkaline pyrogallol solution. Stoppering nipple end, shake solution for two full minutes. RTow push plunger t o just eject all liquid, hut leaving all gas in the syringe. Project and read new volume. Observation: In two runs 10 ml of air reduced to 8.3 and 8.8 ml correspond to 17 and 12% respect,ively. Dem. 398-Paper Burned in Air To show: A fallacious belief in what occurs when paper is burned in air in a glass inverted over water. Materials: H-3 plastic tray, M-p square box, pieces of paper. Procedure: Fill the box full of water, and pour it into the plastic tray on the projector. Set fire to a small crumpled piece of paper, hold it for a few seconds at the mouth of the inverted box, then quickly lower the box and burning paper into the water. Observation: The water rises the way up. Why? Discussion: Refer the class to Dem. 22 where steelwool rusted, and Dem. 23 where P burned; in these cases the water rose also '/a because the 21% oxygen in the air was consumed, creating a vacuum. But with burn- ing paper the water should not have risen because (a) there is no volume change when 1 volume of O2 is replaced by 1 volume of COz and (b) paper does not burn in under 16% oxygen in the air, so only a quarter of the oxygen was consumed. Answer: (a) the water rose up because heat expanded the air in the box and, upon cooling the gas it contracted; (b) it could not rise more than because that is the volume of water poured into the tray. 2. CARBON DIOXIDE IN THE AIR Dem. 3 9 e A i r + Limewafer Experiment developed by Evangeline B. Klug, To show: COzof the air can be trapped by limewater. Materials: Clear, saturated limewater; C-2, adhesive tape, microscope slide. Procedure: Three-quarters fill G 2 with limewater. Tape a microscope slide over cell 1 to shut out the air. After two hours, project. Stir cell 2. Volume 44, Number 7, July 7967 / A599
