Chemical investigations for high-school and junior college students

HIGH-SCHOOL CHEMISTRY * ... for High-School and Junior College Students .... Add to each in when two substances react, such as an acid and a car-...
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Chemical Investigations for High-School and Junior College Students CHARLES H . STONE 14 Marion Terrace, Brookline, Massachusetts

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N ITS generally accepted sense, research is a term applied to an investigation of problems, the answers to which are not known, with the end of determining those answers and thus increasing the sum of knowledge. Such work is obviously beyond the abilities of the beginning student in chemistry. But there are many little investigative problems, the answers to which are known generally, but which present intriguing challenges to the student beginning the subject. A few such follow: 1. Mercuric oxide, when heated, decomposes into mercury and oxygen. Do other simple oxides decompose similarly to produce oxygen and the metal? Let pupils heat small samples of the oxides of lead, copper, cadmium, zinc, and magnesium, respectively, in separate test tubes. Is oxygen given offin any case? What conclusion can be made? Where is mercury in the displacement series? What bearing does the position of a metal in this series seem to have upon the stability of its oxide? 2. Manganese dioxide is a catalyst when used in the preparation of oxygen from potassium chlorate. Are other dioxides also catalysts with potassium chlorate? In separate test tubes heat 5-g. samples of potassium chlorate until they just begin to give off oxygen. Remove external heat. Drop into the tubes barium dioxide, lead dioxide, silicon dioxide, and tin dioxide. Is the rate of evolution of oxygen influenced in any case? What conclusion can be reached? 2a. Is manganese dioxide a catalyst in other reactions? Add.a little manganese dioxide to heated mercury oxide, warmed hydrogen peroxide solution, and a reacting mixture of zinc and hydrochloric acid, in turn. What conclusion can be drawn? 3. The reaction between zinc and sulfuric acid produces hydrogen. How about other acids, such as oxalic acid solution, ammonium chloride solution, acetic acid, nitric acid, or hydrochloric acid, both dilute and concentrated? Try each experiment. In which cases is the action slow? In which case is a gas given off which is obviously not hydrogen? What are the conclusions? 3a. Continuing part 3, do other metals act as well as zinc? Place dilute warm sulfuric acid in several test tubes in a rack. Now add to each tube in turn small pieces of iron, copper, tin, magnesium, and lead. In what case is hydrogen evolved? Which metals fail to react? What is the position of the nomeacting metals in the displacement series? What metals act most vigorously, and where are these metals located in the

displacement series? Drop a bright d i e into 5 ml. of concentrated hydrochloric acid contained in a large test tube or small beaker. What results are noted? What happens if a tiny bit of sodium is dropped into warm sulfuric acid? (This experiment may be tried only with EXTREME CAUTION and protection of the experimenter, never without direct supervision by the instructor.) State some conclusions about the activity of various metals with acids. 4. Manganese dioxide acts as an oxidizing agent with concentrated hydrochloric acid to liberate chlorine. Will other oxidizing agents serve? In separate test tubes place a very little potassium dichromate, potassium chlorate, lead dioxide, barium dioxide, mercury oxide, hydrogen peroxide solution, and potassium permanganate crystals. Add one or two drops of concentrated hydrochloric acid to each and warm gently. Note whether chlorine is liberated in any case. What conclusion may be drawn? 5 . Carbon dioxide is produced by the reaction between marble chips (calcium carbonate) and hydrochloric acid. Will other carbonates act similarly? In separate test tubes place copper carbonate, lead carbonate, zinc carbonate, magnesium carbonate, sodium carbonate, and barium carbonate, and treat each with dilute hydrochloric acid. Test for the presence of carbon dioxide gas above each solid. In any case does the action seem to slow down gradually? What conclusions can be drawn? 6. Lime (calcium oxide) is prepared by heating calcium carbonate. Do other carbonates act similarly? In separate small crucibles or evaporating dishes heat 5-g. samples each of lead carbonate, copper carbonate, cadmium carbonate, zinc carbonate, and other carbonates. What happens in each case? Describe the results. What happens when mercuric carbonate is heated? Explain it. Find out whether carbon dioxide can be made by heating dry sodium carbonate or dry sodium bicarbonate. Which carbonates are most readily decomposed by heat? Locate the metals in these carbonates in the displacement series. What can be said about the decomposition of the carbonates of the alkali inetals? Where are these metals in the displacement series? Which carbonates when heated decompose into the metal as one product? Where are these metals in the displacement series? Make a general statement about the effect of heat on carbonates. 7. Insolubility of product. In three test tubes (Continued on Nac 617)


place 5 g. of marble chips, respectively. Add to each in order 10 ml. of dilute sulfuric, nitric, and hydrochloric acids. Compare the rates of the reactions. How is the difference in rates explained? Look up the solubilities of the sulfates of lead and calcium. Compare these solubilities with those of the chlorides and nitrates of the same metals. Refer to the reaction between lead carbonate and hydrochloric acid in part 5. What exnlanation can be made? P r e ~ a r ea " eeneral statement regarding the effect of an insoluble product

when two substances react, such as an acid and a carbonate. ~ h resourceful , teacher win readily think of various other little investigations which may be assigned to the better students. The conclusions reached will remain with those students as fundamental ideas concerning certain chemical reactions. It may be remarked that the underlying general principle is vastly more important than the specific case.