CHAPTER XI SODIUM; SODIUM HYDROXIDE 67. Sodium; sodium hydroxide. a. Recall § 20. Obtain a small piece of sodium from the instructor. Cut it and note the rapidity with which the freshly cut surface is tarnished. Half fill an evaporating-dish with water, then drop the sodium into it and quickly cover the dish with a glass plate. After the action (R) has entirely ceased, test the liquid with red litmus paper. What compound is dissolved in the water? b. Weigh out about 5 g. of good quicklime and moisten it with about an equal weight of water in an evaporating-dish, warming it, if necessary, to make it slake (R). When it has ceased slaking, dilute to 100 cc. and add the required weight of sodium carbonate to produce complete double decomposition (R). Boil for a few minutes to render filtration easier, and then filter. What is the filtrate? How is this obtained commercially in solid form? Test a little of the filtrate to see if an excess of carbonate has been added (how will you do this?), and if an excess is found, devise a way to remove it. Add 2 drops of the filtrate to 5 cc. of water and taste the solution (?). Dip a piece of red litmus paper into the filtrate (?). CHAPTER XII ACIDS; BASES; SALTS; NEUTRALIZATION 68. Acids. a. Recall the properties of hydrochloric acid (§ 66). Prepare dilute solutions of each of the following acids by adding 2 cc. of each of the concentrated acids to 10 cc. of water and thoroughly mixing: hydrochloric, nitric, sulfuric, acetic. By means of a clean glass rod transfer a drop of each to a piece of blue litmus paper and then to a piece of red. What changes do you notice? b. Recall the action of hydrochloric, sulfuric, and nitric acids on zinc (§§ 22, 23). Nitric acid is a strong oxidizing agent. Might this account for the fact that hydrogen is not evolved when this acid reacts with zinc? c. Add one drop of hydrochloric acid solution to 10 cc. of water. Stir thoroughly and taste the solution. Repeat, using acetic acid (?). Compare the formulas of the acids. In what respect are the acids similar in composition? 69. Bases. Prepare dilute solutions of each of the following bases: sodium hydroxide, potassium hydroxide, calcium hydroxide. Try the effect of each of these solutions on blue and on red litmus paper. Taste a drop of the calcium hydroxide solution. Compare the formulas of the above bases. In what respects are the bases similar in composition? From your results characterize the properties of bases on the supposition that the above compounds are typical bases. 70. Salts. Dilute 5 cc. of the ordinary laboratory solution of sodium hydroxide (1 part of the solid to 10 parts of water) with an equal volume of water. To this solution add a few drops of hydrochloric acid; the two react with evolution of heat (R). Stir the resulting solution with a glass rod and test its action on blue and on red litmus paper. Has it acid or basic properties? Continue to add the acid, drop by drop, with stirring, till the resulting solution is neutral or, at most, slightly acid. Pour the solution into an evaporating-dish and evaporate to dryness. What compound remains? What is the name given to the compounds formed by the action of acids with bases? Characterize these compounds. MHHIM WHHOTH H 71. Ratio of acid to base in neutralization. Prepare a dilute solution of sodium hydroxide by diluting 20 cc. of the laboratory reagent to 100 cc.; also a dilute solution of sulfuric acid by adding 1 cc. of the concentrated acid to 100 cc. of water. Rinse out a burette (storeroom), first with distilled water, then with a little of the alkaline solution. Support the burette (Fig. 35) and pour the alkaline solution into it until the level of the liquid is 2 or 3 cm. above the zero mark. Turn the stopcock and let the solution flow out until the bottom of the curved surface (meniscus) is on a level with the zero. mark. In a similar way fill a second burette with the acid solution. Now let exactly 15 cc. of the acid solution flow into a small beaker, add 2 drops of a solution of phenolphthalein and run in 2 or 3 cc. of the alkaline solution. Notice that where the liquids come in contact, a reddish color is produced, which disappears quickly on stirring. Run in more of the solution, a little at a time, until the color fades slowly, then a drop at a time until the entire liquid, on stirring, remains colored faintly red. This marks approximately the point of neutralization. Note the number of cubic centimeters of the alkaline solution used. Repeat the experiment, using different volumes of acid, say 10 cc. and 20 cc. Calculate in each case the number of cc. of the alkaline solution required to neutralize 1 cc. of the acid solution. What do the results prove? FIG. 35 |