CHAPTER XVI

SULFUR AND ITS COMPOUNDS

91. Properties of sulfur. a. Examine the physical properties of a piece of brimstone. Pour 2 or 3 cc. of carbon disulfide over a little powdered brimstone in a test tube (keep. away from flame). Cover the mouth of the tube with the thumb and shake the contents gently until the sulfur is dissolved. Pour the clear solution into an evaporating-dish, cover it loosely with a filter paper, and set it aside in the hood. The carbon disulfide soon evaporates, the sulfur being deposited in crystals. Examine these with a magnifying-glass.

b. Half fill a test tube with powdered brimstone and apply heat enough to just melt it. Note the properties of the liquid. Pour a little of the liquid into a beaker of water, dry the product on filter paper, and test its solubility in carbon disulfide. Now apply a stronger heat and observe that the liquid becomes darker and at a certain temperature (200°-250°) is so thick that the tube may be inverted without spilling it. Finally, increase the heat until the sulfur boils (445°), and pour the boiling liquid into a beaker of cold water. Examine the product. What name is given to this form of sulfur? Dry a small piece on filter paper and test its solubility in carbon disulfide. Expose it to the air for a day or two. Have its properties remained unchanged?

c. Fill a porcelain crucible with powdered brimstone and apply a very gentle heat until the sulfur is just melted. Withdraw the flame and watch the liquid carefully as it cools. Crystals soon begin to form on the surface, rapidly growing from the circumference toward the center. Before they reach the center quickly pour the remaining liquid into

a dish and examine the crystals adhering to the sides of the crucible. Compare them with those formed in a. In how many forms have you obtained the sulfur? Which is the stable form?

d. Burn a small piece of sulfur. What is the product of the combustion? Note the odor of it. Heat to boiling a little sulfur in the test tube used in b and drop a small strip of bright copper foil into the boiling liquid. Is there any visible evidence of a chemical reaction? What is formed? What is the product formed in § 11, b?

92. Transition point. The transition of monoclinic sulfur into rhombic is too slow to be observed readily. The following is a more striking experiment: Obtain a piece of thin-walled glass tubing about 15 cm. long and 5 mm. in diameter. Seal one end in the Bunsen flame, blowing the seal into rounded form like a test tube. Obtain about half a gram of cupromercuric iodide (side shelf) and put it into this test tube. Half fill a beaker with water, place it on a wire gauze on a ring stand, and suspend a thermometer in such a way that the bulb dips into the water. Heat the water until the temperature reaches about 60°, then turn the flame down so that the rise in temperature is very slow. Use the test tube containing the scarlet powder as a stirring-rod and watch for a change in color. What change occurs? At what temperature? Withdraw the flame, allowing the temperature to fall slowly. At what point does the reverse change occur? Repeat the experiment to ascertain within what limits of temperature the change occurs. Return the powder to the stock bottle. Cupromercuric iodide is sometimes used as a paint for bearings in machinery, since its change in color indicates that the bearing is becoming heated.

NOTE. If desired, the student may himself prepare the cupromercuric iodide as follows: Dissolve a few crystals of mercuric chloride in 2 or 3 cc. of hot water in one test tube, a few crystals of potassium iodide in another, about the same weight of copper sulfate in a third, and a similar quantity of sodium sulfite in a fourth. Add the potassium iodide, drop by drop, to the mercuric chloride until the bright scarlet