CHAPTER XVII THE CHLORINE FAMILY NOTE. The experiments on chlorine, hydrogen chloride, and hydrochloric acid have been given in Chapter X. The student should review these in connection with the present chapter. 99. Hydrogen fluoride; hydrofluoric acid. (The gas is very corrosive and must not be inhaled; its solution must not be brought in contact with the skin.) Warm gently over a small, luminous flame a glass plate on which some pieces of paraffin have been placed. When the paraffin is melted, tilt the plate about so as to cover it with a uniform layer of the wax. When the plate becomes cold, scratch your name through the wax with a pin or other sharp point. Place in a lead dish 3 g. of powdered fluorite and add sufficient sulfuric acid to make a paste of it. Cover the dish tightly with the waxed side of the glass plate and set it in the hood for an hour; then remove the paraffin and examine the glass. Write the equations for all the reactions involved. In what kind of bottles is hydrofluoric acid stored? 100. Salts of hydrochloric acid: chlorides. Place in separate test tubes a few drops of a solution of each of the following compounds: sodium chloride, potassium chloride, calcium chloride. Add to each 1 or 2 drops of a solution of silver nitrate. Explain the results (R). Add a few drops of ammonium hydroxide to each test tube, and note the result. Now add nitric acid, drop by drop, to each test tube until the liquid is acid, and note the result. The formation with silver nitrate of a white precipitate which is soluble in ammonium hydroxide and insoluble in nitric acid serves as a good test for chlorides. Will this test also serve to detect the presence of hydrochloric acid? 101. Weight of 1 liter of hydrogen chloride. The weight of 1 liter of hydrogen chloride may be determined by the method employed in determining the weight of 1 liter of carbon dioxide (§ 52). Prepare the gas as described in § 64. The gas should be bubbled through concentrated sulfuric acid in bottle B (Fig. 34), in order to dry it. 102. Preparation and properties of bromine. (The vapor of bromine must not be inhaled. Perform the experiments in a hood.) a. Use the apparatus employed in the preparation of nitric acid (Fig. 40). Put into the retort a mixture of 2 g. of potassium bromide or sodium bromide and 4 g. of manganese dioxide, and add to this through a funnel a cold dilute solution of sulfuric acid, prepared by slowly adding 5 cc. of sulfuric acid to 10 cc. of water. Shake the retort so as to mix the contents thoroughly. The test-tube receiver should contain sufficient water to allow the end of the retort to dip just below its surface. Now heat the retort gently. The bromine is liberated and distills over (R). Continue the heating until all the bromine has been collected. b. Note the properties of the bromine collected in the bottom of the receiver. Has the water dissolved any of it? What property is implied in the name of the element? Test the bleaching conduct of the aqueous solution. How does it compare with chlorine as a bleaching agent? Try the effect of a little of the bromine water on starch solution. c. Pour 2 or 3 cc. of the solution of bromine into each of two test tubes. To the one add an equal volume of carbon disulfide and shake it vigorously. What is the distribution of the bromine between the two solvents? To the other test tube add carbon tetrachloride or chloroform and compare the results with those obtained with carbon disulfide. 103. Hydrobromic acid and the bromides. a. Add a few drops of sulfuric acid to some crystals of potassium bromide. Note the formation of white fumes, and also of a reddishbrown vapor. Give the composition of each and account for its formation. (Recall that concentrated sulfuric acid is an oxidizing agent and hydrobromic acid is a reducing agent.) b. Dissolve in water a crystal of potassium bromide or of sodium bromide and apply the silver nitrate test for chlorides (§ 100). Describe the results (R). c. To a solution of a crystal of a bromide add a little chlorine water. How do you account for the change in color? Shake the solution with 2 or 3 cc. of carbon tetrachloride. Does this make the test more delicate? 104. Preparation and properties of iodine. In a large test tube place a mixture of 2 g. of potassium iodide and 4 g. of manganese dioxide. Pour over this mixture 5 cc. of sulfuric acid, place an empty funnel in the tube to serve as a loose stopper, and apply a gentle heat (R). Note the vapor of the iodine and the grayish-black crystals which are soon deposited on the sides of the test tube. What property does the name of the element suggest? Half fill two test tubes with starch solution. Add to the first a few drops of a solution of iodine prepared by shaking a small crystal in water. To the second add a few drops of an aqueous solution of potassium iodide. Note the results. Now add to the second tube a little chlorine water (R). Does the chlorine water alone change the starch? Explain the results. Add about 2 cc. of carbon tetrachloride, and shake the mixture. In which solvent is the iodine more soluble? Dissolve a crystal of iodine in alcohol. This solution is called tincture of iodine. 105. Hydriodic acid and the iodides. a. Repeat the experiment of § 103, a and b, substituting potassium iodide for the bromide. Note and explain the results. b. Grind to a fine powder two or three crystals of iodine in a mortar and add a little water. Transfer the resulting mixture to a test tube, adding sufficient water to nearly fill the tube. Now pass a slow current of hydrogen sulfide through the liquid until the iodine all disappears. Filter off the white solid (explain) and boil the filtrate until the hydrogen sulfide is expelled (to test for the presence of hydrogen sulfide, hold in the vapor a strip of paper moistened with a solution of a salt of lead; if the acid is present in the vapor, the strip turns dark, owing to the formation of black lead |