EXERCISE 27

SODIUM; SODIUM HYDROXIDE

Apparatus. Evaporating-dish with glass-plate cover; burner. Materials. Bit of sodium, half as large as a pea; red litmus paper.

Recall experiment a, Exercise 8. Obtain from your instructor a bit of sodium. Cut it and note the rapidity with which the freshly cut surface is tarnished. Half fill your evaporating-dish with water; then drop the sodium into this and quickly cover the dish with a glass plate (R). Is the sodium heavier or lighter than water? After the action has ceased, place a drop of the resulting liquid on a piece of red litmus paper. Contrast its action with that of hydrochloric acid on litmus paper (Exercise 26). Mix 1 or 2 drops of the solution with 5 cc. of water and taste a drop of the resulting solution. Contrast with the taste of hydrochloric acid (Exercise 26). Evaporate the solution to dryness (?).

EXERCISE 28

THE PROPERTIES OF ACIDS, BASES, AND SALTS

Apparatus. 3 small beakers; stirring-rod; evaporating-dish: ring stand and burner; apparatus shown in Fig. 34.

Materials. A few drops of each of the following acids: hydrochloric, sulfuric, nitric, acetic (R. S.); solutions of the following bases: sodium hydroxide, potassium hydroxide (R. S.), calcium hydroxide (R.S.); strips of blue and of red litmus paper; *10 cc. solution of hydrogen chloride in benzene.

a. Recall the properties of hydrochloric acid (Exercise 26). Prepare a dilute solution of each of the following acids by adding 1 or 2 drops of the acid to 10 cc. of water: hydrochloric, sulfuric, nitric, acetic.

By means of a clean glass rod transfer a drop of each to a piece of blue litmus paper. Note the result. In a similar

way try their effect on red litmus paper. Taste one drop of the dilute solutions (rinse the mouth with water after tasting). Determine whether or not the solutions conduct the electric

current, using the method employed in Exercise 24.

es) Compare the formulas of the acids. In what respect are the acids similar in composition?

b. In a similar way try the effect on red litmus paper of a solution of each of the following bases: sodium hydroxide (recall Exercise 27), potassium hydroxide, calcium hydroxide. on a Do they affect the blue litmus paper? Taste a drop of the calcium hydroxide solution. Determine whether or not the solutions conduct the electric current, using the method employed in Exercise 24.

Compare the formulas of the bases. In what respect are the bases similar in composition?

c. Dilute 5 cc. of the ordinary laboratory solution of sodium hydroxide (1 part of the hydroxide to 10 parts of water) with an equal volume of water. To this solution add 4 or 5 drops of hydrochloric acid (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 ?

Now continue to add the acid drop by drop until the resulting solution is neutral (that is, has no effect on either blue or red litmus paper) or is, at most, slightly acid. Pour the solution into an evaporating-dish and evaporate to dryness. What compound remains? Taste it.

What is the name given to the compounds formed by the interaction of acids and bases?

*d. Obtain from your instructor 10 cc. of a solution of hydrogen chloride in benzene. Does it conduct the electric current (?)? Try its action on litmus paper (?); on zinc (?). Account for the results.

e. Characterize acids and bases (1) as to composition, (2) as to their action on litmus, (3) as to taste, (4) as to their conductivity, (5) as to their interaction with each other.

EXERCISE 29

THE RATIO OF ACID TO BASE IN NEUTRALIZATION

(QUANTITATIVE)

Apparatus. 2 burettes and supports, as shown in Fig. 37; small beaker and stirring-rod.

Materials. Sodium hydroxide solution prepared by adding 20 cc. of the laboratory reagent to 100 cc. water; 1 cc. sulfuric acid added to 100 cc. water; a few drops of a phenolphthalein solution (R. S.).

(Two students may work together.)

Rinse out a burette, first with distilled water and then with a little of the solution of sodium hydroxide. Support the burette (Fig. 37), and pour into it the hydroxide solution until the level of the liquid is 1 or 2 cm. above the zero mark. Turn the stopcock and let the solution flow out until the bottom of the curved surface (meniscus) of the liquid in the burette is on a level with the zero mark. In a similar way fill a second burette with the acid solution.

FIG. 37

Now let exactly 15 cc. of the acid solution flow into a small beaker, add two drops of phenolphthalein solution, and run in 2 or 3 cc. of the hydroxide 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, and then a drop at a time until the entire liquid, on stirring, remains colored faintly pink. This marks approximately the point of neutralization. Note the number of cubic centimeters of the hydroxide 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 hydroxide solution required to neutralize 1 cc. of the acid solution. What do the results prove?

EXERCISE 30

THE DISPLACEMENT OF METALS FROM THEIR
COMPOUNDS

Apparatus. 4 test tubes; test-tube rack.

Materials. 4 strips each of zinc and copper (1 cm. x 10 cm.); 0.5 g. lead nitrate dissolved in 10 cc. water; 0.5 g. copper nitrate dissolved in 10 cc. water; 0.5 g. mercuric nitrate dissolved in 10 cc. water; 3 cc. sulfuric acid dissolved in 10 cc. water.

a. Pour into separate test tubes to a depth of 4 or 5 cm. solutions of the following compounds: (1) lead nitrate, (2) dilute sulfuric acid, (3) copper nitrate, (4) mercuric nitrate. Set the tubes in a rack in the order given above and label them "A," "B," " C," and "D" respectively.

Now place in each tube a strip of zinc. (It is convenient to have a strong thread attached to the upper part of each strip so that the strip may easily be withdrawn from the tube.) Note any change taking place in the appearance of the zinc.

After from twenty to thirty minutes withdraw the strips and wipe them carefully on a piece of white paper. Note any evidence tending to show that the zinc has displaced the lead, hydrogen (p. 191 of text), copper, and mercury from their salts. Has the solution of copper nitrate faded in color? (Metals in a very finely divided form are black, as a rule.)

b. Repeat experiment a, substituting for the zinc a strip of copper. Contrast the results obtained with those obtained in a. How do you account for the change in the color of the solution of mercuric nitrate after the addition of the copper strip? Are your results in accord with the displacementseries table given on page 191 of text?

EXERCISE 31

*THE DETERMINATION OF THE AMOUNT OF HYDROGEN DISPLACED BY A DEFINITE WEIGHT OF DIFFERENT METALS (QUANTITATIVE)

Apparatus. Apparatus shown in Fig. 38 (the bottle B and fittings are the same as used in Exercise 12 (Fig. 25)).

Materials. About 1 g. granulated zinc; dilute sulfuric acid prepared by adding 10 cc. concentrated acid to 30 cc. water; dilute hydrochloric acid prepared by adding 20 cc. of the concentrated acid to 20 cc. water.

(Two students work together. Some should use sulfuric acid as the solvent and others hydrochloric acid, so that the results may be compared. The apparatus is not difficult to prepare and gives excellent results.)

nected to the funnel H by means of a rubber tube which can be closed by the screw clamp I. The bottles A and B are joined by a rubber tube K.

Disconnect the bottles A and B at K and fill the bottle B and the exit tube C E with water, as in Exercise 12, and tightly close the screw clamp D.