CHAPTER VIII

ACIDS; BASES; SALTS; NEUTRALIZATION

66. Acids. a. Prepare dilute solutions of each of the following acids: hydrochloric, nitric, sulfuric, acetic. Determine whether or not these solutions conduct the electric current. 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? Add one drop of hydrochloric acid solution to 10 cc. of water. Stir thoroughly and taste the solution. Repeat, using acetic acid. How do the two compare? Compare the formulas of the acids. In what respect are the acids similar in composition?

b. Ask the instructor for a solution of dry hydrogen chloride in benzene or toluene. Determine whether it conducts the electric current (care must be taken to exclude all moisture). Pour a little of the solution on a clean, dry iron nail. Repeat, using the ordinary aqueous solution. Account for the difference in action between the two solutions. From your results characterize the properties of acids on the basis that the above acids are typical ones and that all others act like them.

67. Bases. In a similar way test the conducting power of dilute solutions of each of the following bases: sodium hydroxide, potassium hydroxide, calcium hydroxide. Likewise 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.

68. 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 (that is, has no effect on either blue or red litmus paper) or, at most, slightly acid. Determine whether or not the resulting solution is an electrical conductor. 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.

69. 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

FIG. 30

a little of the alkaline solution. Support the burette (Fig. 30) 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 cubic centimeters of the alkaline solution required to neutralize 1 cc. of the acid solution. What do the results prove?

70. Electromotive series. a. Dissolve about 1 g. of copper sulfate in a test tube two thirds full of water and dip an iron nail into the solution, first rubbing the nail bright with sandpaper (R). Reverse the experiment by dipping a bright piece of copper foil into a solution of ferrous sulfate. Is the reaction reversible? Why?

b. Suspend a piece of mossy zinc or tin by a thread in a solution of lead acetate, and let it stay undisturbed, noting the appearance of the metal from time to time. Would you expect a piece of silver or bismuth to give similar results?

c. Polish a piece of aluminium foil and dip it for a time in pure water. Is hydrogen evolved? Would you expect it to be? Now dip the foil into a dilute solution of mercuric chloride and watch for evidences of evolution of hydrogen. Is mercury deposited on the foil? Would it evolve hydrogen ? A deposit of mercury prevents any oxide or hydroxide from sticking to the aluminium. Does this help in explaining the evolution of hydrogen?

71. Relative strengths of acids. a. Upon what does the relative strength of two acids in equimolecular concentration depend? How will this affect the rate at which the two acids will act upon a third substance? Will the two acids, when treated with a metal, evolve hydrogen at the same rate? Will the volume of hydrogen ultimately evolved be the same?

b. Arrange a hydrogen generator A (Fig. 31) and fit it with a delivery tube B. Fill a test tube C with water and invert it in a beaker of water, placing a slender rubber band