Tested Overhead ProjectionSeries Compiled by HUBERT N. ALYEA Princeion University
16. CARBON AND ITS INORGANIC COMPOUNDS
C. Carbon Dioxide
Note: Toward the end of the experiment, a tiny swirl of CaC03 precipitate will be noticed in cell I . Ask class why. Answer: COz, escaping from cell 2, sinks into cell I (COz heavier than air) and reacts with the Ca(OH)2 there forming CaC03. Dem. 505-School
3. PHYSICAL PROPERTIES OF Con (continued) Dem. 503-Rising
Mothballs with COz
Experiment developed by August J . Colo To show: Effect of gaseous pressure on the size of bubbles and their effect on rate of rise-and-fall of a moth ball in a COz solution. Materials: moth-ball, NaHCOs HOAc solution (300 ml H20 10 ml glacial acetic acid), Mp tank with vacuum source, such as an aspirator bulb.
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Procedure: Three-quarters fill Mp tank with dilute HOAc. Add 1 g NaHCOa and stir well. (a) Drop in a moth-ball. (b) Stopper and shut valve. (c) Evacuate. Observations: (a) At atmospheric pressure the mothball collects COX,rises t o the surface of the liquid, then discharges the COz and sinks. This cycle repeats many times. (b) Upon evacuation, COz gas escapes more readily; the moth-ball stays on the surface.
Colors: COz Changing pH
To show: The colors of your rival school turning into those of your school. Materials: C-2, dry ice, a pair of indicators which in basic solution are the colors of your rival school, but which in acid are those of your school. See choice of indicators Dems. 117-122. Preparation before projection: Fill each cell of C 2 three-quarters full of water, add a drop of HC1-aq, then indicator (chosen from Dems. 117-22) to each cell t o give the colors of your school. To each cell add NaOH-aq dropwise with stirring until the colors turn t o those of your rival school. (For longer delay b e fore your own school colors appear, add more NaOHas). Procedure: Project C-2 prepared as above; the first colors the class will see will be those of their rival school. (Say on the eve of the football game). Now drop a lump of dry ice into each cell. Observations: Violent smoking and bubbling of GOz. Suddenly the colors of the rival school turn into those of your school. Explain the pH change involved.
4. CHEMICAL REACTIONS OF COZ
Dem. S O b D r y k e inlo Limewafer
D. Carbonates and Bicarbonates
Experiment developed by Evangeline B. Klug. To show: When C02 is bubbled through limewater, insoluble CaC03 precipitates; with more COz, this CaC03dissolves to form soluble Ca(HC03)z. Material:
C-3, clear saturated limewater, dry ice.
Procedure: (a) Project G 3 with its cells containing 5 ml sat. limewater 10 ml H,O. Into cell 2 drop one 0.2-ml piece of dry ice. Into cell S drop four such pieces. (b) To cells 2 and S add, dropwise, 5 ml dil. HCI.
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Observations: Cell 1 remains clear [Ca(OH)J Cell 2 precipitates CaC08 as granular brownish streaks which (macro) are actually white. Cell S a t first precipitates CaCO,, but with excess COz redissolves forming soluble Ca(HCO&. Reactions: (b) The solution fizz; the precipitate in Cell 2 dissolves. Reactions: cox cot Ca(0H). CaCO,
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CaCOs 1
+ 2HC1-
CsCL
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Ca(HCO& H20
+ COZ t
Dem. 5064nsoluble Corbonafes
Experiment developed by August J . Colo. To show: Formation of various insoluble carbonates. (For the analytical carbonate group (Ca, Sr, Ba) see Dem. 455). Materials: CuClz-aq, NiCL-aq, CoC12-aq, CaCl,aq, C-4, Alka-Seltzer tablet or dry ice. Procedure: Project C-4 three-quarters full of water. Separately add 4 drops of the salt solutions t o the four cells. Crush one Alka-Seltzer tablet into a dozen small pieces, and drop one or two pieces into each cell. Observations: As the solid dissolves, COz is released and precipitates form in the bottom of the tube. Notes: 1. Dry ice can be used but is not so dramatic. 2. Or, a dropperful of Na&03-aq in SD-133 can be used. Immerse the dropper in the solution in each cell and let the CO3- ion diffuse downwards and form precipitates. Volume 45, Number 6, June 1968
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