CHAPTER IX SOLUTIONS 56. 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? 57. 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. 58. Supersaturated solutions. To 20 cc. of water in a beaker add 20 g. of hydrated sodium sulfate and gently warm until solution is complete. Consult the solubility curve of sodium sulfate. How high is it advisable to heat the solution? Is the solution mobile or viscous ? Pour the solution into two test tubes and set it aside to cool. If no crystallization occurs at room temperature, the solution should be supersaturated. Add a very small crystal of the solid sodium sulfate. Why do crystals form? In what condition is the remaining solution? Define supersaturation. A concentrated solution of ferric nitrate in dilute nitric acid saturated at about 60° is more certain than the hydrated sodium sulfate to work well. 59. Solubility of liquids. Pour about 5 cc. of water into a test tube, add an equal volume of kerosene, and mix the liquids by shaking the test tube. Set the tube aside for a few minutes and note the result. Repeat the experiment, substituting alcohol for kerosene. Note the result. 60. Fractional distillation. 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 separation of the two liquids? By the above method a mixture of liquids boiling at different temperatures may often be separated more or less perfectly. What name is given to this process? 61. Constant-boiling solutions. To 1 or 2 cc. of dilute hydrochloric acid add a drop or two of a solution of silver nitrate. In this case the formation of a white precipitate (silver chloride) shows the presence of hydrochloric acid in the solution. Pour about 75 cc. of dilute hydrochloric acid into a distilling-flask connected with a Liebig condenser and slowly distill, catching the distillate in a test tube. When about 5 cc. has been collected, test it with 1 or 2 drops of silver nitrate solution. Is there an appreciable precipitate? Continue the distillation, testing each portion of 5 cc. as it is collected. Does the quantity of the precipitate formed increase gradually or suddenly? When a decided precipitate is obtained, collect about 2 cc. of |