II.

Properties

All forms of sulfur are insoluble in water, but the crystalline forms are soluble in carbon disulfide. When ordinary sulfur melts, it forms a straw-colored liquid that turns darker with additional heating and then finally boils. When molten sulfur is slowly cooled, its physical properties change in accordance with the temperature, pressure, and method of crust formation. Sulfur thus exists in a variety of forms called allotropes, which consist of the liquids S_λ, and S_µ, and several solid varieties, of which the most familiar are rhombic sulfur and monoclinic sulfur (see Crystal). The most stable variety of the element is rhombic sulfur, a yellow, crystalline solid with a density of 2.06 g/cm³ at 20°C (68°F). Rhombic sulfur is slightly soluble in alcohol and ether, moderately soluble in oils and extremely soluble in carbon disulfide. When kept at temperatures above 94.5°C (202.1°F) but below 120°C (248°F) the rhombic form changes into monoclinic sulfur consisting of elongated, transparent, needlelike structures with a density of 1.96 g/cm³ at 20°C (68°F). The temperature at which rhombic and monoclinic sulfur are in equilibrium, 94.5°C (202.1°F), is known as the transition temperature. When ordinary rhombic sulfur is melted at 115.21°C (239.38°F), it forms the mobile, pale yellow liquid S_λ, which becomes dark and viscous at 160°C (320°F) to form S_µ. If sulfur is heated almost to its boiling point of 444.6°C (832.3°F) and is then poured rapidly into cold water, it does not have time to crystallize into the rhombic or monoclinic state, but forms a transparent, sticky, elastic substance known as amorphous, or plastic, sulfur, which consists for the most part of supercooled S_µ.

Sulfur has valences of two, four, and six, as evidenced by the compounds ferrous sulfide, FeS; sulfur dioxide, SO₂; and barium sulfate, BaSO₄, respectively. It combines with hydrogen and the metallic elements in the presence of heat to form sulfides. The most common sulfide is hydrogen sulfide, H₂S, a colorless, poisonous gas with the odor of rotten eggs. Sulfur combines also with chlorine in several proportions to produce sulfur monochloride, S₂Cl₂, and sulfur dichloride, SCl₂. When burned in air, sulfur combines with oxygen to form sulfur dioxide, SO₂, a heavy, colorless gas with a characteristic, suffocating odor. In moist air it is slowly oxidized to sulfuric acid and is a basic constituent of other acids, such as thiosulfuric acid H₂S₂O₃, and sulfurous acid H₂SO₃. The latter has two replaceable hydrogens and forms two series of salts: normal and acid sulfites. When in solution, the acid sulfites, or bisulfites, of the alkali metals, such as sodium bisulfite, NaHSO₃, are acid in reaction. Solutions of the normal sulfites, such as sodium sulfite, Na₂SO₃, and potassium sulfite, K₂SO₃, are slightly alkaline.

Sulfur dioxide is released into the atmosphere in the combustion of fossil fuels, such as gas, petroleum, and coal, and constitutes one of the most troublesome air pollutants. The concentration of sulfur dioxide in air may range from 0.01 to several parts per million, and it may be responsible for the decay of buildings and monuments, for acid rain, and for human discomfort and disability. See Air Pollution.