32 



SMITH S INTERMEDIATE CHEMISTRY 



and can be started and stopped at will. The sodium hydroxide 

 produced is very soluble in water and remains dissolved. 



Catalytic or Contact Action. The influence of manganese 

 dioxide in causing the potassium chlorate to decompose more 

 easily (p. 31) well deserves n6tice. The effect is very striking if 

 some pure potassium chlorate is melted carefully, to avoid super- 

 heating, in a wide-mouth flask (Fig. 20). The flask is provided 

 with a wide exit tube, from which a rubber tube may lead to a 

 bottle inverted in a trough filled with water as in Fig. 18. A 

 little manganese dioxide is contained in the upper, closed tube. 

 No effervescence of the chlorate can be seen at its melting-point 

 (357) only a little air, expanded by the heating, issues from 

 the tube. When, however, the closed tube containing the man- 

 ganese dioxide is rotated into a vertical position (see dotted lines), 

 and the black powder falls into the chlorate, the oxygen comes 

 off in torrents, in consequence of the enormous acceleration of the 



n decomposition. As a pre- 



caution against injury from 

 an explosion of the flask, it is 

 advisable to wrap the latter in 

 a towel before turning the tube. 

 It must also be noted that 

 the manganese dioxide is not 

 itself permanently altered. If 

 the material left after the 

 action is shaken with water, 

 the potassium chloride dis- 

 solves, while the dioxide does 

 not. Filtration (p. 13) thus 

 enables us to recover the latter, and to ascertain that it has 

 been changed neither in quantity nor in properties. 



The only effect of the dioxide is to hasten the decomposition of 

 the chlorate, which would otherwise be too slow at 200 (p. 31), 



FIG. 20 



