both the glass rod and the tube extend just through the small end of the stopper. Now close the rubber tube tightly with a screw clamp B. Disconnect the test tube and remove the glass rod from the stopper, preparatory to performing the experiment. Prepare an alkaline solution of pyrogallic acid, as follows: Dissolve 5 g. of potassium hydroxide in 5 cc. of water and cool the solution to room temperature. Add this to a solution of 4 g. of pyrogallic acid in 10 cc. of water, and at once pour the resulting liquid into the funnel 4. Quickly open the screw clamp until both the rubber tube and the glass tube are filled with the liquid, then close tightly. Connect the test tube, holding it by the rim to avoid heating the contained air, and insert the glass rod in the cork. The air inclosed in the tube is now at the same temperature and pressure as the surrounding air. Now open the screw clamp. The liquid flows in, absorbing the oxygen. When the liquid ceases to enter, grasp the tube by the rim and invert it as shown in the dotted lines of the figure, adjusting it so that the level of the liquid is the same in both the tube and the funnel (why?). Then clamp the rubber tube and return the test tube to its original position. Mark the volume of the air originally inclosed in the tube by placing a narrow strip of gummed paper about the tube at the lower end of the stopper; also mark by a strip of paper placed at the level of the liquid in the tube, the volume of the oxygen absorbed. Disconnect the tube and rinse it. Measure the volume of the tube to each strip of paper by pouring in water from a graduated cylinder. From these data calculate the volume of oxygen and of nitrogen in 100 volumes of air.

NOTE. This experiment disregards the presence in air of all constituents other than oxygen and nitrogen. The volume of such constituents, however, in the volume of air taken, is so small that it may be neglected.

52. Other constituents of the air. a. Expose a piece of calcium chloride to the air for two or more hours. Explain.

b. Expose a few cubic centimeters of limewater to the air for a half hour. Explain the results.

CHAPTER VII

SOLUTIONS AND IONIZATION

53. Rate of solution. a. Drop a crystal of potassium permanganate into a test tube partly filled with water and shake the liquid until the solid is dissolved. Note the color of the solution.

b. Place a test tube filled with water in a test-tube rack and drop into it a crystal of potassium permanganate. Allow the liquid to remain (without shaking it) until near the close of the laboratory period. Note the result. What does the experiment show?

54. Solubility. a. To a test tube two thirds full of boiling water add common salt, small portions at a time, as long as any dissolves. If an excess remains undissolved after boiling a short time, add just enough water to bring it into solution. Immerse the test tube in a beaker of cold water. Is salt more soluble in cold or in hot water? Repeat the experiment, using powdered potassium chloride or potassium nitrate instead of salt, immersing the test tube in the same beaker as before. When the two tubes have cooled, note the relative quantities of the solids that have separated. What inference do you draw?

b. Filter the salt solution through a dry filter into a dry test tube and note the temperature. Weigh a small evaporating-dish and watch-glass cover; then pour 10 or 15 cc. of the clear salt solution into the dish and reweigh. Now evaporate to dryness, taking note of the precautions given in § 46. When the dish is cool, reweigh. From your results calculate the solubility of common salt at the indicated temperature.

c. In a similar way determine the solubility of the potassium nitrate or of potassium dichromate.