EXERCISE 13

A STUDY OF THE PROCESS OF DISTILLATION

Apparatus. Flask (250-cc.); condenser and connections as shown in Fig. 26, or apparatus shown in Fig. 27; ring stands; wire gauze 12 cm. square; watch glass.

Materials. 10 cc. alcohol.

a. (Two or more students may work together if the apparatus is not available for each.) Connect a Liebig condenser B with a 250-cc. flask A, as represented in Fig. 26. The flask is set on a

the outer tube of the condenser, escaping through the tube D. (Why is cold water forced in at C rather than at D?)

Fill the flask one-fourth full of hydrant or well water and boil until 50 cc. or more of liquid has collected in the receiver E. (In case condensers are not available, the apparatus shown in Fig. 27 may be used. When this is used the steam from the boiling water in flask A is condensed by conducting it through B into the test tube C, which is kept cold by the ice water in the beaker D.)

Compare the distillate (distilled water) with the hydrant water in appearance and in taste (?).

Place 4 or 5 drops of the distilled water on a watch glass and evaporate, holding the watch glass 10 or 15 cm. above the tip of the flame. Is there any residue? Repeat, using hydrant water. Why is

distilled water used in the laboratory?

b. Repeat the distillation, using a sample of muddy water in A (?).

c. Pour 1 or 2 cc. of alcohol into a porcelain dish and bring a flame in contact with it. Is the alcohol inflammable? Now distill a mixture of 10 cc. of alcohol (boiling point 78.3°) and 30 cc. of water. Collect the first

1 or 2 cc. of the distillate in an evaporating dish and test with a flame. In the same way test successive portions of the distillate. Does there seem to be a partial separation of the two liquids? In this way a mixture of liquids boiling at different temperatures may generally be separated more or less perfectly. The process is termed fractional distillation.

EXERCISE 14

THE COMPOSITION OF WATER

Apparatus. Hydrogen generator and tubes as shown in Fig. 28 and described below; balance; apparatus shown in Fig. 25, but without test tube and stopper.

Materials. 10 g. granulated zinc; 2 g. black powdered copper oxide; sulfuric acid; granular calcium chloride, sufficient to fill tubes B and D.

a. Recall the experiments included under Exercises 9 and 10. What do the results show in reference to the

composition of water? Is the process employed in Exercise 10 one of synthesis or analysis? Are the results qualitative or quantitative in character?

*b. The experiments included under Exercise 10, may be modified in the following way so as to give quantitative results. Arrange an apparatus as shown in Fig. 28, in which A represents the hydrogen generator, and B and D are tubes filled with dry calcium chloride. The hard-glass tube

C and the porcelain boat E are obtained from the storeroom. The tube is about 35 cm. in length. Introduce about 2 g. of black oxide of copper into the boat and weigh accurately to milligrams. Introduce the boat into the glass tube so that the end of the boat is about 8 cm. from the end of the tube connected with D. Close the ends of tube D with short pieces of rubber tubing, one end of each being closed with a small glass rod; then weigh the tube accurately. Remove the rubber tubes and glass rods, carefully preserving them for use when the tube is again weighed. Now connect the apparatus as shown in the figure, taking care to render it air-tight. How is this determined? Generate hydrogen slowly, and when the apparatus is free from air, heat the boat very gently, using the wing-top burner. Gradually increase the heat, all the time maintaining the slow current of hydrogen. When the copper oxide is reduced,

or nearly so, withdraw the heat but maintain the current of hydrogen until the apparatus is cool. If any of the water formed remains condensed in the end of tube C, a very gentle heat is cautiously applied (the flame must not strike the tube) until it is driven into the tube D.

When the apparatus has acquired the room temperature, disconnect A from the remainder of the apparatus and attach D (Fig. 28) by a short piece of rubber tubing to the short bent glass tube in bottle B of Fig. 25. The bottle is filled with water, and a portion of it is slowly siphoned over through C, D (Fig. 25). In this way a current of air is drawn through the apparatus, displacing the hydrogen. Finally, disconnect the apparatus, and at once close the ends of the tube D (Fig. 28) with the rubber tubes provided with glass rods. Weigh the boat and contents; also the tube D. From your results calculate the composition of water. Compare your results with those obtained by other members of the class. Calculate the general average of all the results. How does this result compare with the actual values (p. 77 of text). What sources of error are included in the experiment?

EXERCISE 15

THE PREPARATION AND PROPERTIES OF HYDROGEN

PEROXIDE

Apparatus. Test tube; 200-cc. beaker; glass rod; funnel and filter paper.

Materials. 10 cc. of ordinary hydrogen peroxide solution; wooden splint; 1 g. manganese dioxide (powder); starch paste (R.S.); crystal potassium iodide; 6 cc. ether; potassium dichromate (R.S.); 1 g. sodium peroxide; ice water (100 cc.); piece of blue litmus paper; sulfuric acid.

a. What is the strength of the hydrogen peroxide solutions sold by the druggist (p. 83 of text)? Pour 3 cc. of the solution into a large test tube and add

1 g. of finely

powdered manganese dioxide. Test the gas evolved with a glowing splint (?). What kind of an agent is hydrogen peroxide ?

Filter the mixture remaining in the tube. The solid is the unchanged manganese dioxide. In what other experiment has manganese dioxide been used to assist in bringing about a chemical action, the dioxide apparently at least undergoing no change?

b. To 1 cc. of starch paste, add 4 cc. of water. Dissolve in this a small crystal of potassium iodide and then add a few drops of a solution of hydrogen peroxide. The peroxide liberates the iodine from the potassium iodide and the resulting free iodine colors the starch paste. Note the results.

c. Pour 3 cc. of the solution of hydrogen peroxide into a test tube and add an equal volume of ether (CAUTION: ether is very inflammable). Shake the solution vigorously and notice that the ether quickly rises to the top of the tube when the tube is set aside. Now add 1 drop of a solution of potassium dichromate and again shake the mixture. Note the color of the layer of ether now. A blue color in the ether constitutes a delicate test for hydrogen peroxide.

d. When sodium peroxide is added to water, hydrogen peroxide is formed, but this decomposes even at ordinary temperatures into water and oxygen (p. 27 of text). If the temperature is kept low however, the hydrogen peroxide remains unchanged and it is possible in this way to prepare the compound. To prepare the peroxide proceed as follows: Add 1 g. of sodium peroxide, a little at a time, to 100 cc. of ice water. Now add dilute sulfuric acid, a few drops at a time, to the liquid (stir thoroughly with a glass rod) until a drop of the resulting liquid placed on a piece of blue litmus paper changes the blue to a red color. Then test the liquid for the presence of hydrogen peroxide in accordance with the methods given above under b and c.