OXIDES AND OXYGEN ACIDS OF SULPHUR 259 



This method takes advantage of the facts that sulphurous acid 

 is only slightly ionized in solution, which renders reaction 1 prac- 

 tically complete (see p. 241), and that this acid is unstable and 

 decomposes (equation 2) when there is not a large excess of water 

 present. 



Physical Properties. The usual six physical properties may 

 be noted : The gas is colorless, but has a characteristic taste and 

 odor. It has a density considerably greater than that of air (SC>2 

 =~1)4 against J38J15) . It can be liquefied below 156 (the crit. 

 temp.) and the liquid boils at 8. As the pressure required 

 at 20 is only 3J atmospheres, the liquid can be kept in bottles 

 like syphons, or in sealed tin cans. It is extremely soluble JIL 

 water (about 40 vols. ijp 1 vol. water at 15). The solution is 

 sulplujmus acid, /^^f^^ . 



Chemical Properties and Uses. Sulphur dioxide is very 

 stable. It combines with water giving a solution of sulphurous 

 acid. The gas is used in bleaching straw, silk, and wool (compare 

 p. 225). The bleaching action seems largely to consist in combi- 

 nation with the coloring matter, to give a colorless compound. 

 Hence straw hats recover the yellow color of straw by exposure 

 to light, which slowly reverses the reaction and liberates the 

 sulphur dioxide. 



To prevent the growth of fungi or other organisms, wine casks 

 are fumigated with sulphur dioxide before being filled. Dried 

 peaches and apples are prepared by exposing slices of the fruit 

 on trays to sulphur dioxide. The sulphurous acid produced 

 bleaches the fruit, keeps insects away, and prevents the formation 

 of dark-colored substances during the subsequent drying. 



Enormous quantities of sulphur dioxide are employed in the 

 manufacture of sulphuric acid and of sulphites. 



Properties of Sulphurous Acid H 2 SO 3 . Sulphurous acid, 

 in [aqueous solution, shows all the properties of a transition acid. 



