EXERCISES IN CHEMISTRY EXERCISE 1 THE METRIC SYSTEM (See Appendix B.) Apparatus. Graduated test tube; balance sensitive to 1 cg. and set of weights from 50 g. to 1 cg.; watch glass; 100 cc. beaker. Materials. Teaspoonful of common salt; test tube full of distilled water; nickel five-cent piece. a. Length. By means of the scale (see Appendix), measure the length (in centimeters) of various pieces of apparatus included in your outfit, as a test tube, file, and blowpipe. What is the diameter of your filter paper? Finally, estimate the lengths of various objects, as a pencil, a test tube; then measure. Continue until you can approximate the lengths of small objects. b. Volume. By means of a graduated test tube or cylinder, measure (in cubic centimeters) the volumes of various test tubes, beakers, and flasks included in your outfit. (In reading off the amount of the liquid in a graduated tube, always read from the lower part of the meniscus; that is, the curved surface of the liquid.) c. Weight. Ask the assistant for instruction in regard to the use of the balance; then weigh various small objects, as a porcelain crucible, a watch glass. What is the approximate weight of a nickel five-cent piece? Accurately balance a watch glass on the scalepan (using either the weights or a small pill box and fine shot) and weigh out on this exactly 5.2 g. of common salt. Balance a small (clean and dry) beaker on the scalepan; then remove it from the pan and pour into it, as nearly as possible, 10 cc. of distilled water (measured in the graduated tube included in your laboratory outfit). Reweigh, and note the weight of the water. How do you account for the fact that it does not weigh exactly 10 g.? EXERCISE 2 THE BUNSEN BURNER; MANIPULATION OF Apparatus. Bunsen burner and wing-top attachment, as shown in Fig. 4; hard-glass test tube with good cork to fit; set of cork-borers; triangular file; round file. Materials. Wooden splint (1 cm. × 12 cm. is a convenient size); white cardboard (10 cm. square); soft glass tubing 15 cm. in length; glass rod, 25 cm. in length, a. The Bunsen burner. The Bunsen burner is a form of apparatus used for producing heat and is commonly employed in the laboratory. It consists of the tube A (Fig. 1), screwed into the base C. The tube has two small openings near its lower part. A small band B, provided with similar openings, fits around the lower part of the tube in such a way that the openings of the tube may be closed or kept open by turning the band. Gas is admitted through D by means of rubber tubing. A B FIG. 1 Unscrew the tube and examine the different parts of the burner; then put them together again and light the gas by holding a lighted match 4 or 5 cm. above the tube and turning on the gas. The supply should be adjusted so as to give a flame about 10 cm. high. The gas flowing through the tube mixes with the air drawn in through the openings in the lower part of the tube and burns with an almost nonluminous flame. If the band is adjusted so as to close the openings, the flame becomes luminous. Always use the nonluminous flame unless directed otherwise, B Hold a small wooden splint horizontally in the base of the Bunsen flame for two or three seconds and note the results. In the same way determine the relative temperatures of various parts of the flame. Turn the gas down until the flame is 7 or 8 cm. in height, then quickly thrust a piece of white cardboard, about 10 cm. square, vertically through the center of the flame, the lower end of the cardboard resting against the top of the burner. Remove the cardboard before it is ignited and from the scorched portions note the relative temperatures of different parts of the flame. Draw a diagram to illustrate your results. FIG. 2 b. To fit a tube with stopper and glass tube as shown in Fig. 2. In all operations requiring the application of a strong heat to glass, the heat must be applied gently at first. Highly heated glass must be cooled slowly; otherwise it is easily broken. FIG. 3 From one of the lengths of soft-glass tubing cut a piece about 15 cm. in length. To do this, place the tubing on the desk and draw the edge of a triangular file across the point at which you wish to cut the glass. After the glass has been scratched, take the tube in the hands with the thumbs placed near together just back of the scratch (Fig. 3), and gently pull the glass apart, at the same time exerting a slight pressure with the thumbs. If the tube does not yield readily to a gentle pressure, a deeper scratch must be made. In the case of large tubing it may be found necessary to file a groove entirely around the glass. The edges of the cut tube will be sharp, and should be rounded by being rotated in the tip of the Bunsen flame. To bend the glass tubing, first heat it at the point where you wish to bend it, in the luminous Bunsen flame spread out by means of the socalled "wing-top" burner FIG. 4 (Fig. 4). Hold the tube lengthwise in the flame, gently rotating it so that all sides may be equally heated. Continue the heating until the glass bends easily, then remove it from the flame and quickly bend it to a right angle B (Fig. 2). Great care must be taken to heat the tube uniformly, otherwise the bore of the tube will be contracted (A, B, Fig. 5), forming a bend which is not only unsightly but is easily broken. Next select a cork of such a size that the smaller end will just enter the hard-glass test tube A (Fig. 2). Soften the דר FIG. 5 FIG. 6 cork by rolling it between the desk and a block of wood. Now insert into the cork the glass tube prepared as directed above. To do this, select a borer slightly smaller than the tube. Place the cork on the desk and cut half through it with the borer, not by punching but by rotating the borer under gentle pressure (Fig. 6); then reverse the cork and bore through from the other end. Care must be taken to keep the borer at a right angle to the top and base of the cork. The hole should be straight and smooth. The glass tube, rounded at the edges, is now inserted in the cork by a gentle screwlike motion. If the hole is too small to admit the tube when a gentle pressure is applied, it may be slightly enlarged with a round file. Now put the cork in the test tube and set the apparatus aside for use in preparing oxygen (Exercise 7). c. To make glass stirring-rods. Make two of them, one about 10 cm. FIG. 7 B and one about 15 cm. in length. Cut the rods to the proper length and round the ends by heating in the Bunsen flame. Place the finished rods in the desk for future use. *d. To make a wash bottle. The student often will find a wash bottle useful in laboratory work. It may be readily made as represented in Fig. 7. A 500 cc. flask is used. A and B represent soft glass tubes bent as shown in the figure. B is connected with the glass jet D by a piece of rubber tubing C. The glass jet D is made as follows: Heat a piece of soft glass tubing 12 or 15 cm. in length in the Bunsen flame until the walls of A B FIG. 8 the heated portion thicken and the size of the bore diminishes (Fig. 8, 4). The tube must be constantly rotated, to prevent the softened portion from sagging. Now quickly remove the tube from the flame, and, holding it in a vertical |